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32 results

drm_fops.c

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  • filemap.c 105.62 KiB
    // SPDX-License-Identifier: GPL-2.0-only
    /*
     *	linux/mm/filemap.c
     *
     * Copyright (C) 1994-1999  Linus Torvalds
     */
    
    /*
     * This file handles the generic file mmap semantics used by
     * most "normal" filesystems (but you don't /have/ to use this:
     * the NFS filesystem used to do this differently, for example)
     */
    #include <linux/export.h>
    #include <linux/compiler.h>
    #include <linux/dax.h>
    #include <linux/fs.h>
    #include <linux/sched/signal.h>
    #include <linux/uaccess.h>
    #include <linux/capability.h>
    #include <linux/kernel_stat.h>
    #include <linux/gfp.h>
    #include <linux/mm.h>
    #include <linux/swap.h>
    #include <linux/mman.h>
    #include <linux/pagemap.h>
    #include <linux/file.h>
    #include <linux/uio.h>
    #include <linux/error-injection.h>
    #include <linux/hash.h>
    #include <linux/writeback.h>
    #include <linux/backing-dev.h>
    #include <linux/pagevec.h>
    #include <linux/blkdev.h>
    #include <linux/security.h>
    #include <linux/cpuset.h>
    #include <linux/hugetlb.h>
    #include <linux/memcontrol.h>
    #include <linux/cleancache.h>
    #include <linux/shmem_fs.h>
    #include <linux/rmap.h>
    #include <linux/delayacct.h>
    #include <linux/psi.h>
    #include <linux/ramfs.h>
    #include <linux/page_idle.h>
    #include <asm/pgalloc.h>
    #include <asm/tlbflush.h>
    #include "internal.h"
    
    #define CREATE_TRACE_POINTS
    #include <trace/events/filemap.h>
    
    /*
     * FIXME: remove all knowledge of the buffer layer from the core VM
     */
    #include <linux/buffer_head.h> /* for try_to_free_buffers */
    
    #include <asm/mman.h>
    
    /*
     * Shared mappings implemented 30.11.1994. It's not fully working yet,
     * though.
     *
     * Shared mappings now work. 15.8.1995  Bruno.
     *
     * finished 'unifying' the page and buffer cache and SMP-threaded the
     * page-cache, 21.05.1999, Ingo Molnar <mingo@redhat.com>
     *
     * SMP-threaded pagemap-LRU 1999, Andrea Arcangeli <andrea@suse.de>
     */
    
    /*
     * Lock ordering:
     *
     *  ->i_mmap_rwsem		(truncate_pagecache)
     *    ->private_lock		(__free_pte->__set_page_dirty_buffers)
     *      ->swap_lock		(exclusive_swap_page, others)
     *        ->i_pages lock
     *
     *  ->i_mutex
     *    ->i_mmap_rwsem		(truncate->unmap_mapping_range)
     *
     *  ->mmap_lock
     *    ->i_mmap_rwsem
     *      ->page_table_lock or pte_lock	(various, mainly in memory.c)
     *        ->i_pages lock	(arch-dependent flush_dcache_mmap_lock)
     *
     *  ->mmap_lock
     *    ->lock_page		(access_process_vm)
     *
     *  ->i_mutex			(generic_perform_write)
     *    ->mmap_lock		(fault_in_pages_readable->do_page_fault)
     *
     *  bdi->wb.list_lock
     *    sb_lock			(fs/fs-writeback.c)
     *    ->i_pages lock		(__sync_single_inode)
     *
     *  ->i_mmap_rwsem
     *    ->anon_vma.lock		(vma_adjust)
     *
     *  ->anon_vma.lock
     *    ->page_table_lock or pte_lock	(anon_vma_prepare and various)
     *
     *  ->page_table_lock or pte_lock
     *    ->swap_lock		(try_to_unmap_one)
     *    ->private_lock		(try_to_unmap_one)
     *    ->i_pages lock		(try_to_unmap_one)
     *    ->lruvec->lru_lock	(follow_page->mark_page_accessed)
     *    ->lruvec->lru_lock	(check_pte_range->isolate_lru_page)
     *    ->private_lock		(page_remove_rmap->set_page_dirty)
     *    ->i_pages lock		(page_remove_rmap->set_page_dirty)
     *    bdi.wb->list_lock		(page_remove_rmap->set_page_dirty)
     *    ->inode->i_lock		(page_remove_rmap->set_page_dirty)
     *    ->memcg->move_lock	(page_remove_rmap->lock_page_memcg)
     *    bdi.wb->list_lock		(zap_pte_range->set_page_dirty)
     *    ->inode->i_lock		(zap_pte_range->set_page_dirty)
     *    ->private_lock		(zap_pte_range->__set_page_dirty_buffers)
     *
     * ->i_mmap_rwsem
     *   ->tasklist_lock            (memory_failure, collect_procs_ao)
     */
    
    static void page_cache_delete(struct address_space *mapping,
    				   struct page *page, void *shadow)
    {
    	XA_STATE(xas, &mapping->i_pages, page->index);
    	unsigned int nr = 1;
    
    	mapping_set_update(&xas, mapping);
    
    	/* hugetlb pages are represented by a single entry in the xarray */
    	if (!PageHuge(page)) {
    		xas_set_order(&xas, page->index, compound_order(page));
    		nr = compound_nr(page);
    	}
    
    	VM_BUG_ON_PAGE(!PageLocked(page), page);
    	VM_BUG_ON_PAGE(PageTail(page), page);
    	VM_BUG_ON_PAGE(nr != 1 && shadow, page);
    
    	xas_store(&xas, shadow);
    	xas_init_marks(&xas);
    
    	page->mapping = NULL;
    	/* Leave page->index set: truncation lookup relies upon it */
    
    	if (shadow) {
    		mapping->nrexceptional += nr;
    		/*
    		 * Make sure the nrexceptional update is committed before
    		 * the nrpages update so that final truncate racing
    		 * with reclaim does not see both counters 0 at the
    		 * same time and miss a shadow entry.
    		 */
    		smp_wmb();
    	}
    	mapping->nrpages -= nr;
    }
    
    static void unaccount_page_cache_page(struct address_space *mapping,
    				      struct page *page)
    {
    	int nr;
    
    	/*
    	 * if we're uptodate, flush out into the cleancache, otherwise
    	 * invalidate any existing cleancache entries.  We can't leave
    	 * stale data around in the cleancache once our page is gone
    	 */
    	if (PageUptodate(page) && PageMappedToDisk(page))
    		cleancache_put_page(page);
    	else
    		cleancache_invalidate_page(mapping, page);
    
    	VM_BUG_ON_PAGE(PageTail(page), page);
    	VM_BUG_ON_PAGE(page_mapped(page), page);
    	if (!IS_ENABLED(CONFIG_DEBUG_VM) && unlikely(page_mapped(page))) {
    		int mapcount;
    
    		pr_alert("BUG: Bad page cache in process %s  pfn:%05lx\n",
    			 current->comm, page_to_pfn(page));
    		dump_page(page, "still mapped when deleted");
    		dump_stack();
    		add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
    
    		mapcount = page_mapcount(page);
    		if (mapping_exiting(mapping) &&
    		    page_count(page) >= mapcount + 2) {
    			/*
    			 * All vmas have already been torn down, so it's
    			 * a good bet that actually the page is unmapped,
    			 * and we'd prefer not to leak it: if we're wrong,
    			 * some other bad page check should catch it later.
    			 */
    			page_mapcount_reset(page);
    			page_ref_sub(page, mapcount);
    		}
    	}
    
    	/* hugetlb pages do not participate in page cache accounting. */
    	if (PageHuge(page))
    		return;
    
    	nr = thp_nr_pages(page);
    
    	__mod_lruvec_page_state(page, NR_FILE_PAGES, -nr);
    	if (PageSwapBacked(page)) {
    		__mod_lruvec_page_state(page, NR_SHMEM, -nr);
    		if (PageTransHuge(page))
    			__mod_lruvec_page_state(page, NR_SHMEM_THPS, -nr);
    	} else if (PageTransHuge(page)) {
    		__mod_lruvec_page_state(page, NR_FILE_THPS, -nr);
    		filemap_nr_thps_dec(mapping);
    	}
    
    	/*
    	 * At this point page must be either written or cleaned by
    	 * truncate.  Dirty page here signals a bug and loss of
    	 * unwritten data.
    	 *
    	 * This fixes dirty accounting after removing the page entirely
    	 * but leaves PageDirty set: it has no effect for truncated
    	 * page and anyway will be cleared before returning page into
    	 * buddy allocator.
    	 */
    	if (WARN_ON_ONCE(PageDirty(page)))
    		account_page_cleaned(page, mapping, inode_to_wb(mapping->host));
    }
    
    /*
     * Delete a page from the page cache and free it. Caller has to make
     * sure the page is locked and that nobody else uses it - or that usage
     * is safe.  The caller must hold the i_pages lock.
     */
    void __delete_from_page_cache(struct page *page, void *shadow)
    {
    	struct address_space *mapping = page->mapping;
    
    	trace_mm_filemap_delete_from_page_cache(page);
    
    	unaccount_page_cache_page(mapping, page);
    	page_cache_delete(mapping, page, shadow);
    }
    
    static void page_cache_free_page(struct address_space *mapping,
    				struct page *page)
    {
    	void (*freepage)(struct page *);
    
    	freepage = mapping->a_ops->freepage;
    	if (freepage)
    		freepage(page);
    
    	if (PageTransHuge(page) && !PageHuge(page)) {
    		page_ref_sub(page, thp_nr_pages(page));
    		VM_BUG_ON_PAGE(page_count(page) <= 0, page);
    	} else {
    		put_page(page);
    	}
    }
    
    /**
     * delete_from_page_cache - delete page from page cache
     * @page: the page which the kernel is trying to remove from page cache
     *
     * This must be called only on pages that have been verified to be in the page
     * cache and locked.  It will never put the page into the free list, the caller
     * has a reference on the page.
     */
    void delete_from_page_cache(struct page *page)
    {
    	struct address_space *mapping = page_mapping(page);
    	unsigned long flags;
    
    	BUG_ON(!PageLocked(page));
    	xa_lock_irqsave(&mapping->i_pages, flags);
    	__delete_from_page_cache(page, NULL);
    	xa_unlock_irqrestore(&mapping->i_pages, flags);
    
    	page_cache_free_page(mapping, page);
    }
    EXPORT_SYMBOL(delete_from_page_cache);
    
    /*
     * page_cache_delete_batch - delete several pages from page cache
     * @mapping: the mapping to which pages belong
     * @pvec: pagevec with pages to delete
     *
     * The function walks over mapping->i_pages and removes pages passed in @pvec
     * from the mapping. The function expects @pvec to be sorted by page index
     * and is optimised for it to be dense.
     * It tolerates holes in @pvec (mapping entries at those indices are not
     * modified). The function expects only THP head pages to be present in the
     * @pvec.
     *
     * The function expects the i_pages lock to be held.
     */
    static void page_cache_delete_batch(struct address_space *mapping,
    			     struct pagevec *pvec)
    {
    	XA_STATE(xas, &mapping->i_pages, pvec->pages[0]->index);
    	int total_pages = 0;
    	int i = 0;
    	struct page *page;
    
    	mapping_set_update(&xas, mapping);
    	xas_for_each(&xas, page, ULONG_MAX) {
    		if (i >= pagevec_count(pvec))
    			break;
    
    		/* A swap/dax/shadow entry got inserted? Skip it. */
    		if (xa_is_value(page))
    			continue;
    		/*
    		 * A page got inserted in our range? Skip it. We have our
    		 * pages locked so they are protected from being removed.
    		 * If we see a page whose index is higher than ours, it
    		 * means our page has been removed, which shouldn't be
    		 * possible because we're holding the PageLock.
    		 */
    		if (page != pvec->pages[i]) {
    			VM_BUG_ON_PAGE(page->index > pvec->pages[i]->index,
    					page);
    			continue;
    		}
    
    		WARN_ON_ONCE(!PageLocked(page));
    
    		if (page->index == xas.xa_index)
    			page->mapping = NULL;
    		/* Leave page->index set: truncation lookup relies on it */
    
    		/*
    		 * Move to the next page in the vector if this is a regular
    		 * page or the index is of the last sub-page of this compound
    		 * page.
    		 */
    		if (page->index + compound_nr(page) - 1 == xas.xa_index)
    			i++;
    		xas_store(&xas, NULL);
    		total_pages++;
    	}
    	mapping->nrpages -= total_pages;
    }
    
    void delete_from_page_cache_batch(struct address_space *mapping,
    				  struct pagevec *pvec)
    {
    	int i;
    	unsigned long flags;
    
    	if (!pagevec_count(pvec))
    		return;
    
    	xa_lock_irqsave(&mapping->i_pages, flags);
    	for (i = 0; i < pagevec_count(pvec); i++) {
    		trace_mm_filemap_delete_from_page_cache(pvec->pages[i]);
    
    		unaccount_page_cache_page(mapping, pvec->pages[i]);
    	}
    	page_cache_delete_batch(mapping, pvec);
    	xa_unlock_irqrestore(&mapping->i_pages, flags);
    
    	for (i = 0; i < pagevec_count(pvec); i++)
    		page_cache_free_page(mapping, pvec->pages[i]);
    }
    
    int filemap_check_errors(struct address_space *mapping)
    {
    	int ret = 0;
    	/* Check for outstanding write errors */
    	if (test_bit(AS_ENOSPC, &mapping->flags) &&
    	    test_and_clear_bit(AS_ENOSPC, &mapping->flags))
    		ret = -ENOSPC;
    	if (test_bit(AS_EIO, &mapping->flags) &&
    	    test_and_clear_bit(AS_EIO, &mapping->flags))
    		ret = -EIO;
    	return ret;
    }
    EXPORT_SYMBOL(filemap_check_errors);
    
    static int filemap_check_and_keep_errors(struct address_space *mapping)
    {
    	/* Check for outstanding write errors */
    	if (test_bit(AS_EIO, &mapping->flags))
    		return -EIO;
    	if (test_bit(AS_ENOSPC, &mapping->flags))
    		return -ENOSPC;
    	return 0;
    }
    
    /**
     * __filemap_fdatawrite_range - start writeback on mapping dirty pages in range
     * @mapping:	address space structure to write
     * @start:	offset in bytes where the range starts
     * @end:	offset in bytes where the range ends (inclusive)
     * @sync_mode:	enable synchronous operation
     *
     * Start writeback against all of a mapping's dirty pages that lie
     * within the byte offsets <start, end> inclusive.
     *
     * If sync_mode is WB_SYNC_ALL then this is a "data integrity" operation, as
     * opposed to a regular memory cleansing writeback.  The difference between
     * these two operations is that if a dirty page/buffer is encountered, it must
     * be waited upon, and not just skipped over.
     *
     * Return: %0 on success, negative error code otherwise.
     */
    int __filemap_fdatawrite_range(struct address_space *mapping, loff_t start,
    				loff_t end, int sync_mode)
    {
    	int ret;
    	struct writeback_control wbc = {
    		.sync_mode = sync_mode,
    		.nr_to_write = LONG_MAX,
    		.range_start = start,
    		.range_end = end,
    	};
    
    	if (!mapping_can_writeback(mapping) ||
    	    !mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
    		return 0;
    
    	wbc_attach_fdatawrite_inode(&wbc, mapping->host);
    	ret = do_writepages(mapping, &wbc);
    	wbc_detach_inode(&wbc);
    	return ret;
    }
    
    static inline int __filemap_fdatawrite(struct address_space *mapping,
    	int sync_mode)
    {
    	return __filemap_fdatawrite_range(mapping, 0, LLONG_MAX, sync_mode);
    }
    
    int filemap_fdatawrite(struct address_space *mapping)
    {
    	return __filemap_fdatawrite(mapping, WB_SYNC_ALL);
    }
    EXPORT_SYMBOL(filemap_fdatawrite);
    
    int filemap_fdatawrite_range(struct address_space *mapping, loff_t start,
    				loff_t end)
    {
    	return __filemap_fdatawrite_range(mapping, start, end, WB_SYNC_ALL);
    }
    EXPORT_SYMBOL(filemap_fdatawrite_range);
    
    /**
     * filemap_flush - mostly a non-blocking flush
     * @mapping:	target address_space
     *
     * This is a mostly non-blocking flush.  Not suitable for data-integrity
     * purposes - I/O may not be started against all dirty pages.
     *
     * Return: %0 on success, negative error code otherwise.
     */
    int filemap_flush(struct address_space *mapping)
    {
    	return __filemap_fdatawrite(mapping, WB_SYNC_NONE);
    }
    EXPORT_SYMBOL(filemap_flush);
    
    /**
     * filemap_range_has_page - check if a page exists in range.
     * @mapping:           address space within which to check
     * @start_byte:        offset in bytes where the range starts
     * @end_byte:          offset in bytes where the range ends (inclusive)
     *
     * Find at least one page in the range supplied, usually used to check if
     * direct writing in this range will trigger a writeback.
     *
     * Return: %true if at least one page exists in the specified range,
     * %false otherwise.
     */
    bool filemap_range_has_page(struct address_space *mapping,
    			   loff_t start_byte, loff_t end_byte)
    {
    	struct page *page;
    	XA_STATE(xas, &mapping->i_pages, start_byte >> PAGE_SHIFT);
    	pgoff_t max = end_byte >> PAGE_SHIFT;
    
    	if (end_byte < start_byte)
    		return false;
    
    	rcu_read_lock();
    	for (;;) {
    		page = xas_find(&xas, max);
    		if (xas_retry(&xas, page))
    			continue;
    		/* Shadow entries don't count */
    		if (xa_is_value(page))
    			continue;
    		/*
    		 * We don't need to try to pin this page; we're about to
    		 * release the RCU lock anyway.  It is enough to know that
    		 * there was a page here recently.
    		 */
    		break;
    	}
    	rcu_read_unlock();
    
    	return page != NULL;
    }
    EXPORT_SYMBOL(filemap_range_has_page);
    
    static void __filemap_fdatawait_range(struct address_space *mapping,
    				     loff_t start_byte, loff_t end_byte)
    {
    	pgoff_t index = start_byte >> PAGE_SHIFT;
    	pgoff_t end = end_byte >> PAGE_SHIFT;
    	struct pagevec pvec;
    	int nr_pages;
    
    	if (end_byte < start_byte)
    		return;
    
    	pagevec_init(&pvec);
    	while (index <= end) {
    		unsigned i;
    
    		nr_pages = pagevec_lookup_range_tag(&pvec, mapping, &index,
    				end, PAGECACHE_TAG_WRITEBACK);
    		if (!nr_pages)
    			break;
    
    		for (i = 0; i < nr_pages; i++) {
    			struct page *page = pvec.pages[i];
    
    			wait_on_page_writeback(page);
    			ClearPageError(page);
    		}
    		pagevec_release(&pvec);
    		cond_resched();
    	}
    }
    
    /**
     * filemap_fdatawait_range - wait for writeback to complete
     * @mapping:		address space structure to wait for
     * @start_byte:		offset in bytes where the range starts
     * @end_byte:		offset in bytes where the range ends (inclusive)
     *
     * Walk the list of under-writeback pages of the given address space
     * in the given range and wait for all of them.  Check error status of
     * the address space and return it.
     *
     * Since the error status of the address space is cleared by this function,
     * callers are responsible for checking the return value and handling and/or
     * reporting the error.
     *
     * Return: error status of the address space.
     */
    int filemap_fdatawait_range(struct address_space *mapping, loff_t start_byte,
    			    loff_t end_byte)
    {
    	__filemap_fdatawait_range(mapping, start_byte, end_byte);
    	return filemap_check_errors(mapping);
    }
    EXPORT_SYMBOL(filemap_fdatawait_range);
    
    /**
     * filemap_fdatawait_range_keep_errors - wait for writeback to complete
     * @mapping:		address space structure to wait for
     * @start_byte:		offset in bytes where the range starts
     * @end_byte:		offset in bytes where the range ends (inclusive)
     *
     * Walk the list of under-writeback pages of the given address space in the
     * given range and wait for all of them.  Unlike filemap_fdatawait_range(),
     * this function does not clear error status of the address space.
     *
     * Use this function if callers don't handle errors themselves.  Expected
     * call sites are system-wide / filesystem-wide data flushers: e.g. sync(2),
     * fsfreeze(8)
     */
    int filemap_fdatawait_range_keep_errors(struct address_space *mapping,
    		loff_t start_byte, loff_t end_byte)
    {
    	__filemap_fdatawait_range(mapping, start_byte, end_byte);
    	return filemap_check_and_keep_errors(mapping);
    }
    EXPORT_SYMBOL(filemap_fdatawait_range_keep_errors);
    
    /**
     * file_fdatawait_range - wait for writeback to complete
     * @file:		file pointing to address space structure to wait for
     * @start_byte:		offset in bytes where the range starts
     * @end_byte:		offset in bytes where the range ends (inclusive)
     *
     * Walk the list of under-writeback pages of the address space that file
     * refers to, in the given range and wait for all of them.  Check error
     * status of the address space vs. the file->f_wb_err cursor and return it.
     *
     * Since the error status of the file is advanced by this function,
     * callers are responsible for checking the return value and handling and/or
     * reporting the error.
     *
     * Return: error status of the address space vs. the file->f_wb_err cursor.
     */
    int file_fdatawait_range(struct file *file, loff_t start_byte, loff_t end_byte)
    {
    	struct address_space *mapping = file->f_mapping;
    
    	__filemap_fdatawait_range(mapping, start_byte, end_byte);
    	return file_check_and_advance_wb_err(file);
    }
    EXPORT_SYMBOL(file_fdatawait_range);
    
    /**
     * filemap_fdatawait_keep_errors - wait for writeback without clearing errors
     * @mapping: address space structure to wait for
     *
     * Walk the list of under-writeback pages of the given address space
     * and wait for all of them.  Unlike filemap_fdatawait(), this function
     * does not clear error status of the address space.
     *
     * Use this function if callers don't handle errors themselves.  Expected
     * call sites are system-wide / filesystem-wide data flushers: e.g. sync(2),
     * fsfreeze(8)
     *
     * Return: error status of the address space.
     */
    int filemap_fdatawait_keep_errors(struct address_space *mapping)
    {
    	__filemap_fdatawait_range(mapping, 0, LLONG_MAX);
    	return filemap_check_and_keep_errors(mapping);
    }
    EXPORT_SYMBOL(filemap_fdatawait_keep_errors);
    
    /* Returns true if writeback might be needed or already in progress. */
    static bool mapping_needs_writeback(struct address_space *mapping)
    {
    	if (dax_mapping(mapping))
    		return mapping->nrexceptional;
    
    	return mapping->nrpages;
    }
    
    /**
     * filemap_write_and_wait_range - write out & wait on a file range
     * @mapping:	the address_space for the pages
     * @lstart:	offset in bytes where the range starts
     * @lend:	offset in bytes where the range ends (inclusive)
     *
     * Write out and wait upon file offsets lstart->lend, inclusive.
     *
     * Note that @lend is inclusive (describes the last byte to be written) so
     * that this function can be used to write to the very end-of-file (end = -1).
     *
     * Return: error status of the address space.
     */
    int filemap_write_and_wait_range(struct address_space *mapping,
    				 loff_t lstart, loff_t lend)
    {
    	int err = 0;
    
    	if (mapping_needs_writeback(mapping)) {
    		err = __filemap_fdatawrite_range(mapping, lstart, lend,
    						 WB_SYNC_ALL);
    		/*
    		 * Even if the above returned error, the pages may be
    		 * written partially (e.g. -ENOSPC), so we wait for it.
    		 * But the -EIO is special case, it may indicate the worst
    		 * thing (e.g. bug) happened, so we avoid waiting for it.
    		 */
    		if (err != -EIO) {
    			int err2 = filemap_fdatawait_range(mapping,
    						lstart, lend);
    			if (!err)
    				err = err2;
    		} else {
    			/* Clear any previously stored errors */
    			filemap_check_errors(mapping);
    		}
    	} else {
    		err = filemap_check_errors(mapping);
    	}
    	return err;
    }
    EXPORT_SYMBOL(filemap_write_and_wait_range);
    
    void __filemap_set_wb_err(struct address_space *mapping, int err)
    {
    	errseq_t eseq = errseq_set(&mapping->wb_err, err);
    
    	trace_filemap_set_wb_err(mapping, eseq);
    }
    EXPORT_SYMBOL(__filemap_set_wb_err);
    
    /**
     * file_check_and_advance_wb_err - report wb error (if any) that was previously
     * 				   and advance wb_err to current one
     * @file: struct file on which the error is being reported
     *
     * When userland calls fsync (or something like nfsd does the equivalent), we
     * want to report any writeback errors that occurred since the last fsync (or
     * since the file was opened if there haven't been any).
     *
     * Grab the wb_err from the mapping. If it matches what we have in the file,
     * then just quickly return 0. The file is all caught up.
     *
     * If it doesn't match, then take the mapping value, set the "seen" flag in
     * it and try to swap it into place. If it works, or another task beat us
     * to it with the new value, then update the f_wb_err and return the error
     * portion. The error at this point must be reported via proper channels
     * (a'la fsync, or NFS COMMIT operation, etc.).
     *
     * While we handle mapping->wb_err with atomic operations, the f_wb_err
     * value is protected by the f_lock since we must ensure that it reflects
     * the latest value swapped in for this file descriptor.
     *
     * Return: %0 on success, negative error code otherwise.
     */
    int file_check_and_advance_wb_err(struct file *file)
    {
    	int err = 0;
    	errseq_t old = READ_ONCE(file->f_wb_err);
    	struct address_space *mapping = file->f_mapping;
    
    	/* Locklessly handle the common case where nothing has changed */
    	if (errseq_check(&mapping->wb_err, old)) {
    		/* Something changed, must use slow path */
    		spin_lock(&file->f_lock);
    		old = file->f_wb_err;
    		err = errseq_check_and_advance(&mapping->wb_err,
    						&file->f_wb_err);
    		trace_file_check_and_advance_wb_err(file, old);
    		spin_unlock(&file->f_lock);
    	}
    
    	/*
    	 * We're mostly using this function as a drop in replacement for
    	 * filemap_check_errors. Clear AS_EIO/AS_ENOSPC to emulate the effect
    	 * that the legacy code would have had on these flags.
    	 */
    	clear_bit(AS_EIO, &mapping->flags);
    	clear_bit(AS_ENOSPC, &mapping->flags);
    	return err;
    }
    EXPORT_SYMBOL(file_check_and_advance_wb_err);
    
    /**
     * file_write_and_wait_range - write out & wait on a file range
     * @file:	file pointing to address_space with pages
     * @lstart:	offset in bytes where the range starts
     * @lend:	offset in bytes where the range ends (inclusive)
     *
     * Write out and wait upon file offsets lstart->lend, inclusive.
     *
     * Note that @lend is inclusive (describes the last byte to be written) so
     * that this function can be used to write to the very end-of-file (end = -1).
     *
     * After writing out and waiting on the data, we check and advance the
     * f_wb_err cursor to the latest value, and return any errors detected there.
     *
     * Return: %0 on success, negative error code otherwise.
     */
    int file_write_and_wait_range(struct file *file, loff_t lstart, loff_t lend)
    {
    	int err = 0, err2;
    	struct address_space *mapping = file->f_mapping;
    
    	if (mapping_needs_writeback(mapping)) {
    		err = __filemap_fdatawrite_range(mapping, lstart, lend,
    						 WB_SYNC_ALL);
    		/* See comment of filemap_write_and_wait() */
    		if (err != -EIO)
    			__filemap_fdatawait_range(mapping, lstart, lend);
    	}
    	err2 = file_check_and_advance_wb_err(file);
    	if (!err)
    		err = err2;
    	return err;
    }
    EXPORT_SYMBOL(file_write_and_wait_range);
    
    /**
     * replace_page_cache_page - replace a pagecache page with a new one
     * @old:	page to be replaced
     * @new:	page to replace with
     *
     * This function replaces a page in the pagecache with a new one.  On
     * success it acquires the pagecache reference for the new page and
     * drops it for the old page.  Both the old and new pages must be
     * locked.  This function does not add the new page to the LRU, the
     * caller must do that.
     *
     * The remove + add is atomic.  This function cannot fail.
     */
    void replace_page_cache_page(struct page *old, struct page *new)
    {
    	struct address_space *mapping = old->mapping;
    	void (*freepage)(struct page *) = mapping->a_ops->freepage;
    	pgoff_t offset = old->index;
    	XA_STATE(xas, &mapping->i_pages, offset);
    	unsigned long flags;
    
    	VM_BUG_ON_PAGE(!PageLocked(old), old);
    	VM_BUG_ON_PAGE(!PageLocked(new), new);
    	VM_BUG_ON_PAGE(new->mapping, new);
    
    	get_page(new);
    	new->mapping = mapping;
    	new->index = offset;
    
    	mem_cgroup_migrate(old, new);
    
    	xas_lock_irqsave(&xas, flags);
    	xas_store(&xas, new);
    
    	old->mapping = NULL;
    	/* hugetlb pages do not participate in page cache accounting. */
    	if (!PageHuge(old))
    		__dec_lruvec_page_state(old, NR_FILE_PAGES);
    	if (!PageHuge(new))
    		__inc_lruvec_page_state(new, NR_FILE_PAGES);
    	if (PageSwapBacked(old))
    		__dec_lruvec_page_state(old, NR_SHMEM);
    	if (PageSwapBacked(new))
    		__inc_lruvec_page_state(new, NR_SHMEM);
    	xas_unlock_irqrestore(&xas, flags);
    	if (freepage)
    		freepage(old);
    	put_page(old);
    }
    EXPORT_SYMBOL_GPL(replace_page_cache_page);
    
    noinline int __add_to_page_cache_locked(struct page *page,
    					struct address_space *mapping,
    					pgoff_t offset, gfp_t gfp,
    					void **shadowp)
    {
    	XA_STATE(xas, &mapping->i_pages, offset);
    	int huge = PageHuge(page);
    	int error;
    	bool charged = false;
    
    	VM_BUG_ON_PAGE(!PageLocked(page), page);
    	VM_BUG_ON_PAGE(PageSwapBacked(page), page);
    	mapping_set_update(&xas, mapping);
    
    	get_page(page);
    	page->mapping = mapping;
    	page->index = offset;
    
    	if (!huge) {
    		error = mem_cgroup_charge(page, current->mm, gfp);
    		if (error)
    			goto error;
    		charged = true;
    	}
    
    	gfp &= GFP_RECLAIM_MASK;
    
    	do {
    		unsigned int order = xa_get_order(xas.xa, xas.xa_index);
    		void *entry, *old = NULL;
    
    		if (order > thp_order(page))
    			xas_split_alloc(&xas, xa_load(xas.xa, xas.xa_index),
    					order, gfp);
    		xas_lock_irq(&xas);
    		xas_for_each_conflict(&xas, entry) {
    			old = entry;
    			if (!xa_is_value(entry)) {
    				xas_set_err(&xas, -EEXIST);
    				goto unlock;
    			}
    		}
    
    		if (old) {
    			if (shadowp)
    				*shadowp = old;
    			/* entry may have been split before we acquired lock */
    			order = xa_get_order(xas.xa, xas.xa_index);
    			if (order > thp_order(page)) {
    				xas_split(&xas, old, order);
    				xas_reset(&xas);
    			}
    		}
    
    		xas_store(&xas, page);
    		if (xas_error(&xas))
    			goto unlock;
    
    		if (old)
    			mapping->nrexceptional--;
    		mapping->nrpages++;
    
    		/* hugetlb pages do not participate in page cache accounting */
    		if (!huge)
    			__inc_lruvec_page_state(page, NR_FILE_PAGES);
    unlock:
    		xas_unlock_irq(&xas);
    	} while (xas_nomem(&xas, gfp));
    
    	if (xas_error(&xas)) {
    		error = xas_error(&xas);
    		if (charged)
    			mem_cgroup_uncharge(page);
    		goto error;
    	}
    
    	trace_mm_filemap_add_to_page_cache(page);
    	return 0;
    error:
    	page->mapping = NULL;
    	/* Leave page->index set: truncation relies upon it */
    	put_page(page);
    	return error;
    }
    ALLOW_ERROR_INJECTION(__add_to_page_cache_locked, ERRNO);
    
    /**
     * add_to_page_cache_locked - add a locked page to the pagecache
     * @page:	page to add
     * @mapping:	the page's address_space
     * @offset:	page index
     * @gfp_mask:	page allocation mode
     *
     * This function is used to add a page to the pagecache. It must be locked.
     * This function does not add the page to the LRU.  The caller must do that.
     *
     * Return: %0 on success, negative error code otherwise.
     */
    int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
    		pgoff_t offset, gfp_t gfp_mask)
    {
    	return __add_to_page_cache_locked(page, mapping, offset,
    					  gfp_mask, NULL);
    }
    EXPORT_SYMBOL(add_to_page_cache_locked);
    
    int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
    				pgoff_t offset, gfp_t gfp_mask)
    {
    	void *shadow = NULL;
    	int ret;
    
    	__SetPageLocked(page);
    	ret = __add_to_page_cache_locked(page, mapping, offset,
    					 gfp_mask, &shadow);
    	if (unlikely(ret))
    		__ClearPageLocked(page);
    	else {
    		/*
    		 * The page might have been evicted from cache only
    		 * recently, in which case it should be activated like
    		 * any other repeatedly accessed page.
    		 * The exception is pages getting rewritten; evicting other
    		 * data from the working set, only to cache data that will
    		 * get overwritten with something else, is a waste of memory.
    		 */
    		WARN_ON_ONCE(PageActive(page));
    		if (!(gfp_mask & __GFP_WRITE) && shadow)
    			workingset_refault(page, shadow);
    		lru_cache_add(page);
    	}
    	return ret;
    }
    EXPORT_SYMBOL_GPL(add_to_page_cache_lru);
    
    #ifdef CONFIG_NUMA
    struct page *__page_cache_alloc(gfp_t gfp)
    {
    	int n;
    	struct page *page;
    
    	if (cpuset_do_page_mem_spread()) {
    		unsigned int cpuset_mems_cookie;
    		do {
    			cpuset_mems_cookie = read_mems_allowed_begin();
    			n = cpuset_mem_spread_node();
    			page = __alloc_pages_node(n, gfp, 0);
    		} while (!page && read_mems_allowed_retry(cpuset_mems_cookie));
    
    		return page;
    	}
    	return alloc_pages(gfp, 0);
    }
    EXPORT_SYMBOL(__page_cache_alloc);
    #endif
    
    /*
     * In order to wait for pages to become available there must be
     * waitqueues associated with pages. By using a hash table of
     * waitqueues where the bucket discipline is to maintain all
     * waiters on the same queue and wake all when any of the pages
     * become available, and for the woken contexts to check to be
     * sure the appropriate page became available, this saves space
     * at a cost of "thundering herd" phenomena during rare hash
     * collisions.
     */
    #define PAGE_WAIT_TABLE_BITS 8
    #define PAGE_WAIT_TABLE_SIZE (1 << PAGE_WAIT_TABLE_BITS)
    static wait_queue_head_t page_wait_table[PAGE_WAIT_TABLE_SIZE] __cacheline_aligned;
    
    static wait_queue_head_t *page_waitqueue(struct page *page)
    {
    	return &page_wait_table[hash_ptr(page, PAGE_WAIT_TABLE_BITS)];
    }
    
    void __init pagecache_init(void)
    {
    	int i;
    
    	for (i = 0; i < PAGE_WAIT_TABLE_SIZE; i++)
    		init_waitqueue_head(&page_wait_table[i]);
    
    	page_writeback_init();
    }
    
    /*
     * The page wait code treats the "wait->flags" somewhat unusually, because
     * we have multiple different kinds of waits, not just the usual "exclusive"
     * one.
     *
     * We have:
     *
     *  (a) no special bits set:
     *
     *	We're just waiting for the bit to be released, and when a waker
     *	calls the wakeup function, we set WQ_FLAG_WOKEN and wake it up,
     *	and remove it from the wait queue.
     *
     *	Simple and straightforward.
     *
     *  (b) WQ_FLAG_EXCLUSIVE:
     *
     *	The waiter is waiting to get the lock, and only one waiter should
     *	be woken up to avoid any thundering herd behavior. We'll set the
     *	WQ_FLAG_WOKEN bit, wake it up, and remove it from the wait queue.
     *
     *	This is the traditional exclusive wait.
     *
     *  (c) WQ_FLAG_EXCLUSIVE | WQ_FLAG_CUSTOM:
     *
     *	The waiter is waiting to get the bit, and additionally wants the
     *	lock to be transferred to it for fair lock behavior. If the lock
     *	cannot be taken, we stop walking the wait queue without waking
     *	the waiter.
     *
     *	This is the "fair lock handoff" case, and in addition to setting
     *	WQ_FLAG_WOKEN, we set WQ_FLAG_DONE to let the waiter easily see
     *	that it now has the lock.
     */
    static int wake_page_function(wait_queue_entry_t *wait, unsigned mode, int sync, void *arg)
    {
    	unsigned int flags;
    	struct wait_page_key *key = arg;
    	struct wait_page_queue *wait_page
    		= container_of(wait, struct wait_page_queue, wait);
    
    	if (!wake_page_match(wait_page, key))
    		return 0;
    
    	/*
    	 * If it's a lock handoff wait, we get the bit for it, and
    	 * stop walking (and do not wake it up) if we can't.
    	 */
    	flags = wait->flags;
    	if (flags & WQ_FLAG_EXCLUSIVE) {
    		if (test_bit(key->bit_nr, &key->page->flags))
    			return -1;
    		if (flags & WQ_FLAG_CUSTOM) {
    			if (test_and_set_bit(key->bit_nr, &key->page->flags))
    				return -1;
    			flags |= WQ_FLAG_DONE;
    		}
    	}
    
    	/*
    	 * We are holding the wait-queue lock, but the waiter that
    	 * is waiting for this will be checking the flags without
    	 * any locking.
    	 *
    	 * So update the flags atomically, and wake up the waiter
    	 * afterwards to avoid any races. This store-release pairs
    	 * with the load-acquire in wait_on_page_bit_common().
    	 */
    	smp_store_release(&wait->flags, flags | WQ_FLAG_WOKEN);
    	wake_up_state(wait->private, mode);
    
    	/*
    	 * Ok, we have successfully done what we're waiting for,
    	 * and we can unconditionally remove the wait entry.
    	 *
    	 * Note that this pairs with the "finish_wait()" in the
    	 * waiter, and has to be the absolute last thing we do.
    	 * After this list_del_init(&wait->entry) the wait entry
    	 * might be de-allocated and the process might even have
    	 * exited.
    	 */
    	list_del_init_careful(&wait->entry);
    	return (flags & WQ_FLAG_EXCLUSIVE) != 0;
    }
    
    static void wake_up_page_bit(struct page *page, int bit_nr)
    {
    	wait_queue_head_t *q = page_waitqueue(page);
    	struct wait_page_key key;
    	unsigned long flags;
    	wait_queue_entry_t bookmark;
    
    	key.page = page;
    	key.bit_nr = bit_nr;
    	key.page_match = 0;
    
    	bookmark.flags = 0;
    	bookmark.private = NULL;
    	bookmark.func = NULL;
    	INIT_LIST_HEAD(&bookmark.entry);
    
    	spin_lock_irqsave(&q->lock, flags);
    	__wake_up_locked_key_bookmark(q, TASK_NORMAL, &key, &bookmark);
    
    	while (bookmark.flags & WQ_FLAG_BOOKMARK) {
    		/*
    		 * Take a breather from holding the lock,
    		 * allow pages that finish wake up asynchronously
    		 * to acquire the lock and remove themselves
    		 * from wait queue
    		 */
    		spin_unlock_irqrestore(&q->lock, flags);
    		cpu_relax();
    		spin_lock_irqsave(&q->lock, flags);
    		__wake_up_locked_key_bookmark(q, TASK_NORMAL, &key, &bookmark);
    	}
    
    	/*
    	 * It is possible for other pages to have collided on the waitqueue
    	 * hash, so in that case check for a page match. That prevents a long-
    	 * term waiter
    	 *
    	 * It is still possible to miss a case here, when we woke page waiters
    	 * and removed them from the waitqueue, but there are still other
    	 * page waiters.
    	 */
    	if (!waitqueue_active(q) || !key.page_match) {
    		ClearPageWaiters(page);
    		/*
    		 * It's possible to miss clearing Waiters here, when we woke
    		 * our page waiters, but the hashed waitqueue has waiters for
    		 * other pages on it.
    		 *
    		 * That's okay, it's a rare case. The next waker will clear it.
    		 */
    	}
    	spin_unlock_irqrestore(&q->lock, flags);
    }
    
    static void wake_up_page(struct page *page, int bit)
    {
    	if (!PageWaiters(page))
    		return;
    	wake_up_page_bit(page, bit);
    }
    
    /*
     * A choice of three behaviors for wait_on_page_bit_common():
     */
    enum behavior {
    	EXCLUSIVE,	/* Hold ref to page and take the bit when woken, like
    			 * __lock_page() waiting on then setting PG_locked.
    			 */
    	SHARED,		/* Hold ref to page and check the bit when woken, like
    			 * wait_on_page_writeback() waiting on PG_writeback.
    			 */
    	DROP,		/* Drop ref to page before wait, no check when woken,
    			 * like put_and_wait_on_page_locked() on PG_locked.
    			 */
    };
    
    /*
     * Attempt to check (or get) the page bit, and mark us done
     * if successful.
     */
    static inline bool trylock_page_bit_common(struct page *page, int bit_nr,
    					struct wait_queue_entry *wait)
    {
    	if (wait->flags & WQ_FLAG_EXCLUSIVE) {
    		if (test_and_set_bit(bit_nr, &page->flags))
    			return false;
    	} else if (test_bit(bit_nr, &page->flags))
    		return false;
    
    	wait->flags |= WQ_FLAG_WOKEN | WQ_FLAG_DONE;
    	return true;
    }
    
    /* How many times do we accept lock stealing from under a waiter? */
    int sysctl_page_lock_unfairness = 5;
    
    static inline int wait_on_page_bit_common(wait_queue_head_t *q,
    	struct page *page, int bit_nr, int state, enum behavior behavior)
    {
    	int unfairness = sysctl_page_lock_unfairness;
    	struct wait_page_queue wait_page;
    	wait_queue_entry_t *wait = &wait_page.wait;
    	bool thrashing = false;
    	bool delayacct = false;
    	unsigned long pflags;
    
    	if (bit_nr == PG_locked &&
    	    !PageUptodate(page) && PageWorkingset(page)) {
    		if (!PageSwapBacked(page)) {
    			delayacct_thrashing_start();
    			delayacct = true;
    		}
    		psi_memstall_enter(&pflags);
    		thrashing = true;
    	}
    
    	init_wait(wait);
    	wait->func = wake_page_function;
    	wait_page.page = page;
    	wait_page.bit_nr = bit_nr;
    
    repeat:
    	wait->flags = 0;
    	if (behavior == EXCLUSIVE) {
    		wait->flags = WQ_FLAG_EXCLUSIVE;
    		if (--unfairness < 0)
    			wait->flags |= WQ_FLAG_CUSTOM;
    	}
    
    	/*
    	 * Do one last check whether we can get the
    	 * page bit synchronously.
    	 *
    	 * Do the SetPageWaiters() marking before that
    	 * to let any waker we _just_ missed know they
    	 * need to wake us up (otherwise they'll never
    	 * even go to the slow case that looks at the
    	 * page queue), and add ourselves to the wait
    	 * queue if we need to sleep.
    	 *
    	 * This part needs to be done under the queue
    	 * lock to avoid races.
    	 */
    	spin_lock_irq(&q->lock);
    	SetPageWaiters(page);
    	if (!trylock_page_bit_common(page, bit_nr, wait))
    		__add_wait_queue_entry_tail(q, wait);
    	spin_unlock_irq(&q->lock);
    
    	/*
    	 * From now on, all the logic will be based on
    	 * the WQ_FLAG_WOKEN and WQ_FLAG_DONE flag, to
    	 * see whether the page bit testing has already
    	 * been done by the wake function.
    	 *
    	 * We can drop our reference to the page.
    	 */
    	if (behavior == DROP)
    		put_page(page);
    
    	/*
    	 * Note that until the "finish_wait()", or until
    	 * we see the WQ_FLAG_WOKEN flag, we need to
    	 * be very careful with the 'wait->flags', because
    	 * we may race with a waker that sets them.
    	 */
    	for (;;) {
    		unsigned int flags;
    
    		set_current_state(state);
    
    		/* Loop until we've been woken or interrupted */
    		flags = smp_load_acquire(&wait->flags);
    		if (!(flags & WQ_FLAG_WOKEN)) {
    			if (signal_pending_state(state, current))
    				break;
    
    			io_schedule();
    			continue;
    		}
    
    		/* If we were non-exclusive, we're done */
    		if (behavior != EXCLUSIVE)
    			break;
    
    		/* If the waker got the lock for us, we're done */
    		if (flags & WQ_FLAG_DONE)
    			break;
    
    		/*
    		 * Otherwise, if we're getting the lock, we need to
    		 * try to get it ourselves.
    		 *
    		 * And if that fails, we'll have to retry this all.
    		 */
    		if (unlikely(test_and_set_bit(bit_nr, &page->flags)))
    			goto repeat;
    
    		wait->flags |= WQ_FLAG_DONE;
    		break;
    	}
    
    	/*
    	 * If a signal happened, this 'finish_wait()' may remove the last
    	 * waiter from the wait-queues, but the PageWaiters bit will remain
    	 * set. That's ok. The next wakeup will take care of it, and trying
    	 * to do it here would be difficult and prone to races.
    	 */
    	finish_wait(q, wait);
    
    	if (thrashing) {
    		if (delayacct)
    			delayacct_thrashing_end();
    		psi_memstall_leave(&pflags);
    	}
    
    	/*
    	 * NOTE! The wait->flags weren't stable until we've done the
    	 * 'finish_wait()', and we could have exited the loop above due
    	 * to a signal, and had a wakeup event happen after the signal
    	 * test but before the 'finish_wait()'.
    	 *
    	 * So only after the finish_wait() can we reliably determine
    	 * if we got woken up or not, so we can now figure out the final
    	 * return value based on that state without races.
    	 *
    	 * Also note that WQ_FLAG_WOKEN is sufficient for a non-exclusive
    	 * waiter, but an exclusive one requires WQ_FLAG_DONE.
    	 */
    	if (behavior == EXCLUSIVE)
    		return wait->flags & WQ_FLAG_DONE ? 0 : -EINTR;
    
    	return wait->flags & WQ_FLAG_WOKEN ? 0 : -EINTR;
    }
    
    void wait_on_page_bit(struct page *page, int bit_nr)
    {
    	wait_queue_head_t *q = page_waitqueue(page);
    	wait_on_page_bit_common(q, page, bit_nr, TASK_UNINTERRUPTIBLE, SHARED);
    }
    EXPORT_SYMBOL(wait_on_page_bit);
    
    int wait_on_page_bit_killable(struct page *page, int bit_nr)
    {
    	wait_queue_head_t *q = page_waitqueue(page);
    	return wait_on_page_bit_common(q, page, bit_nr, TASK_KILLABLE, SHARED);
    }
    EXPORT_SYMBOL(wait_on_page_bit_killable);
    
    /**
     * put_and_wait_on_page_locked - Drop a reference and wait for it to be unlocked
     * @page: The page to wait for.
     * @state: The sleep state (TASK_KILLABLE, TASK_UNINTERRUPTIBLE, etc).
     *
     * The caller should hold a reference on @page.  They expect the page to
     * become unlocked relatively soon, but do not wish to hold up migration
     * (for example) by holding the reference while waiting for the page to
     * come unlocked.  After this function returns, the caller should not
     * dereference @page.
     *
     * Return: 0 if the page was unlocked or -EINTR if interrupted by a signal.
     */
    int put_and_wait_on_page_locked(struct page *page, int state)
    {
    	wait_queue_head_t *q;
    
    	page = compound_head(page);
    	q = page_waitqueue(page);
    	return wait_on_page_bit_common(q, page, PG_locked, state, DROP);
    }
    
    /**
     * add_page_wait_queue - Add an arbitrary waiter to a page's wait queue
     * @page: Page defining the wait queue of interest
     * @waiter: Waiter to add to the queue
     *
     * Add an arbitrary @waiter to the wait queue for the nominated @page.
     */
    void add_page_wait_queue(struct page *page, wait_queue_entry_t *waiter)
    {
    	wait_queue_head_t *q = page_waitqueue(page);
    	unsigned long flags;
    
    	spin_lock_irqsave(&q->lock, flags);
    	__add_wait_queue_entry_tail(q, waiter);
    	SetPageWaiters(page);
    	spin_unlock_irqrestore(&q->lock, flags);
    }
    EXPORT_SYMBOL_GPL(add_page_wait_queue);
    
    #ifndef clear_bit_unlock_is_negative_byte
    
    /*
     * PG_waiters is the high bit in the same byte as PG_lock.
     *
     * On x86 (and on many other architectures), we can clear PG_lock and
     * test the sign bit at the same time. But if the architecture does
     * not support that special operation, we just do this all by hand
     * instead.
     *
     * The read of PG_waiters has to be after (or concurrently with) PG_locked
     * being cleared, but a memory barrier should be unnecessary since it is
     * in the same byte as PG_locked.
     */
    static inline bool clear_bit_unlock_is_negative_byte(long nr, volatile void *mem)
    {
    	clear_bit_unlock(nr, mem);
    	/* smp_mb__after_atomic(); */
    	return test_bit(PG_waiters, mem);
    }
    
    #endif
    
    /**
     * unlock_page - unlock a locked page
     * @page: the page
     *
     * Unlocks the page and wakes up sleepers in wait_on_page_locked().
     * Also wakes sleepers in wait_on_page_writeback() because the wakeup
     * mechanism between PageLocked pages and PageWriteback pages is shared.
     * But that's OK - sleepers in wait_on_page_writeback() just go back to sleep.
     *
     * Note that this depends on PG_waiters being the sign bit in the byte
     * that contains PG_locked - thus the BUILD_BUG_ON(). That allows us to
     * clear the PG_locked bit and test PG_waiters at the same time fairly
     * portably (architectures that do LL/SC can test any bit, while x86 can
     * test the sign bit).
     */
    void unlock_page(struct page *page)
    {
    	BUILD_BUG_ON(PG_waiters != 7);
    	page = compound_head(page);
    	VM_BUG_ON_PAGE(!PageLocked(page), page);
    	if (clear_bit_unlock_is_negative_byte(PG_locked, &page->flags))
    		wake_up_page_bit(page, PG_locked);
    }
    EXPORT_SYMBOL(unlock_page);
    
    /**
     * end_page_writeback - end writeback against a page
     * @page: the page
     */
    void end_page_writeback(struct page *page)
    {
    	/*
    	 * TestClearPageReclaim could be used here but it is an atomic
    	 * operation and overkill in this particular case. Failing to
    	 * shuffle a page marked for immediate reclaim is too mild to
    	 * justify taking an atomic operation penalty at the end of
    	 * ever page writeback.
    	 */
    	if (PageReclaim(page)) {
    		ClearPageReclaim(page);
    		rotate_reclaimable_page(page);
    	}
    
    	/*
    	 * Writeback does not hold a page reference of its own, relying
    	 * on truncation to wait for the clearing of PG_writeback.
    	 * But here we must make sure that the page is not freed and
    	 * reused before the wake_up_page().
    	 */
    	get_page(page);
    	if (!test_clear_page_writeback(page))
    		BUG();
    
    	smp_mb__after_atomic();
    	wake_up_page(page, PG_writeback);
    	put_page(page);
    }
    EXPORT_SYMBOL(end_page_writeback);
    
    /*
     * After completing I/O on a page, call this routine to update the page
     * flags appropriately
     */
    void page_endio(struct page *page, bool is_write, int err)
    {
    	if (!is_write) {
    		if (!err) {
    			SetPageUptodate(page);
    		} else {
    			ClearPageUptodate(page);
    			SetPageError(page);
    		}
    		unlock_page(page);
    	} else {
    		if (err) {
    			struct address_space *mapping;
    
    			SetPageError(page);
    			mapping = page_mapping(page);
    			if (mapping)
    				mapping_set_error(mapping, err);
    		}
    		end_page_writeback(page);
    	}
    }
    EXPORT_SYMBOL_GPL(page_endio);
    
    /**
     * __lock_page - get a lock on the page, assuming we need to sleep to get it
     * @__page: the page to lock
     */
    void __lock_page(struct page *__page)
    {
    	struct page *page = compound_head(__page);
    	wait_queue_head_t *q = page_waitqueue(page);
    	wait_on_page_bit_common(q, page, PG_locked, TASK_UNINTERRUPTIBLE,
    				EXCLUSIVE);
    }
    EXPORT_SYMBOL(__lock_page);
    
    int __lock_page_killable(struct page *__page)
    {
    	struct page *page = compound_head(__page);
    	wait_queue_head_t *q = page_waitqueue(page);
    	return wait_on_page_bit_common(q, page, PG_locked, TASK_KILLABLE,
    					EXCLUSIVE);
    }
    EXPORT_SYMBOL_GPL(__lock_page_killable);
    
    int __lock_page_async(struct page *page, struct wait_page_queue *wait)
    {
    	struct wait_queue_head *q = page_waitqueue(page);
    	int ret = 0;
    
    	wait->page = page;
    	wait->bit_nr = PG_locked;
    
    	spin_lock_irq(&q->lock);
    	__add_wait_queue_entry_tail(q, &wait->wait);
    	SetPageWaiters(page);
    	ret = !trylock_page(page);
    	/*
    	 * If we were successful now, we know we're still on the
    	 * waitqueue as we're still under the lock. This means it's
    	 * safe to remove and return success, we know the callback
    	 * isn't going to trigger.
    	 */
    	if (!ret)
    		__remove_wait_queue(q, &wait->wait);
    	else
    		ret = -EIOCBQUEUED;
    	spin_unlock_irq(&q->lock);
    	return ret;
    }
    
    /*
     * Return values:
     * 1 - page is locked; mmap_lock is still held.
     * 0 - page is not locked.
     *     mmap_lock has been released (mmap_read_unlock(), unless flags had both
     *     FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_RETRY_NOWAIT set, in
     *     which case mmap_lock is still held.
     *
     * If neither ALLOW_RETRY nor KILLABLE are set, will always return 1
     * with the page locked and the mmap_lock unperturbed.
     */
    int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
    			 unsigned int flags)
    {
    	if (fault_flag_allow_retry_first(flags)) {
    		/*
    		 * CAUTION! In this case, mmap_lock is not released
    		 * even though return 0.
    		 */
    		if (flags & FAULT_FLAG_RETRY_NOWAIT)
    			return 0;
    
    		mmap_read_unlock(mm);
    		if (flags & FAULT_FLAG_KILLABLE)
    			wait_on_page_locked_killable(page);
    		else
    			wait_on_page_locked(page);
    		return 0;
    	}
    	if (flags & FAULT_FLAG_KILLABLE) {
    		int ret;
    
    		ret = __lock_page_killable(page);
    		if (ret) {
    			mmap_read_unlock(mm);
    			return 0;
    		}
    	} else {
    		__lock_page(page);
    	}
    	return 1;
    
    }
    
    /**
     * page_cache_next_miss() - Find the next gap in the page cache.
     * @mapping: Mapping.
     * @index: Index.
     * @max_scan: Maximum range to search.
     *
     * Search the range [index, min(index + max_scan - 1, ULONG_MAX)] for the
     * gap with the lowest index.
     *
     * This function may be called under the rcu_read_lock.  However, this will
     * not atomically search a snapshot of the cache at a single point in time.
     * For example, if a gap is created at index 5, then subsequently a gap is
     * created at index 10, page_cache_next_miss covering both indices may
     * return 10 if called under the rcu_read_lock.
     *
     * Return: The index of the gap if found, otherwise an index outside the
     * range specified (in which case 'return - index >= max_scan' will be true).
     * In the rare case of index wrap-around, 0 will be returned.
     */
    pgoff_t page_cache_next_miss(struct address_space *mapping,
    			     pgoff_t index, unsigned long max_scan)
    {
    	XA_STATE(xas, &mapping->i_pages, index);
    
    	while (max_scan--) {
    		void *entry = xas_next(&xas);
    		if (!entry || xa_is_value(entry))
    			break;
    		if (xas.xa_index == 0)
    			break;
    	}
    
    	return xas.xa_index;
    }
    EXPORT_SYMBOL(page_cache_next_miss);
    
    /**
     * page_cache_prev_miss() - Find the previous gap in the page cache.
     * @mapping: Mapping.
     * @index: Index.
     * @max_scan: Maximum range to search.
     *
     * Search the range [max(index - max_scan + 1, 0), index] for the
     * gap with the highest index.
     *
     * This function may be called under the rcu_read_lock.  However, this will
     * not atomically search a snapshot of the cache at a single point in time.
     * For example, if a gap is created at index 10, then subsequently a gap is
     * created at index 5, page_cache_prev_miss() covering both indices may
     * return 5 if called under the rcu_read_lock.
     *
     * Return: The index of the gap if found, otherwise an index outside the
     * range specified (in which case 'index - return >= max_scan' will be true).
     * In the rare case of wrap-around, ULONG_MAX will be returned.
     */
    pgoff_t page_cache_prev_miss(struct address_space *mapping,
    			     pgoff_t index, unsigned long max_scan)
    {
    	XA_STATE(xas, &mapping->i_pages, index);
    
    	while (max_scan--) {
    		void *entry = xas_prev(&xas);
    		if (!entry || xa_is_value(entry))
    			break;
    		if (xas.xa_index == ULONG_MAX)
    			break;
    	}
    
    	return xas.xa_index;
    }
    EXPORT_SYMBOL(page_cache_prev_miss);
    
    /*
     * mapping_get_entry - Get a page cache entry.
     * @mapping: the address_space to search
     * @index: The page cache index.
     *
     * Looks up the page cache slot at @mapping & @offset.  If there is a
     * page cache page, the head page is returned with an increased refcount.
     *
     * If the slot holds a shadow entry of a previously evicted page, or a
     * swap entry from shmem/tmpfs, it is returned.
     *
     * Return: The head page or shadow entry, %NULL if nothing is found.
     */
    static struct page *mapping_get_entry(struct address_space *mapping,
    		pgoff_t index)
    {
    	XA_STATE(xas, &mapping->i_pages, index);
    	struct page *page;
    
    	rcu_read_lock();
    repeat:
    	xas_reset(&xas);
    	page = xas_load(&xas);
    	if (xas_retry(&xas, page))
    		goto repeat;
    	/*
    	 * A shadow entry of a recently evicted page, or a swap entry from
    	 * shmem/tmpfs.  Return it without attempting to raise page count.
    	 */
    	if (!page || xa_is_value(page))
    		goto out;
    
    	if (!page_cache_get_speculative(page))
    		goto repeat;
    
    	/*
    	 * Has the page moved or been split?
    	 * This is part of the lockless pagecache protocol. See
    	 * include/linux/pagemap.h for details.
    	 */
    	if (unlikely(page != xas_reload(&xas))) {
    		put_page(page);
    		goto repeat;
    	}
    out:
    	rcu_read_unlock();
    
    	return page;
    }
    
    /**
     * pagecache_get_page - Find and get a reference to a page.
     * @mapping: The address_space to search.
     * @index: The page index.
     * @fgp_flags: %FGP flags modify how the page is returned.
     * @gfp_mask: Memory allocation flags to use if %FGP_CREAT is specified.
     *
     * Looks up the page cache entry at @mapping & @index.
     *
     * @fgp_flags can be zero or more of these flags:
     *
     * * %FGP_ACCESSED - The page will be marked accessed.
     * * %FGP_LOCK - The page is returned locked.
     * * %FGP_HEAD - If the page is present and a THP, return the head page
     *   rather than the exact page specified by the index.
     * * %FGP_ENTRY - If there is a shadow / swap / DAX entry, return it
     *   instead of allocating a new page to replace it.
     * * %FGP_CREAT - If no page is present then a new page is allocated using
     *   @gfp_mask and added to the page cache and the VM's LRU list.
     *   The page is returned locked and with an increased refcount.
     * * %FGP_FOR_MMAP - The caller wants to do its own locking dance if the
     *   page is already in cache.  If the page was allocated, unlock it before
     *   returning so the caller can do the same dance.
     * * %FGP_WRITE - The page will be written
     * * %FGP_NOFS - __GFP_FS will get cleared in gfp mask
     * * %FGP_NOWAIT - Don't get blocked by page lock
     *
     * If %FGP_LOCK or %FGP_CREAT are specified then the function may sleep even
     * if the %GFP flags specified for %FGP_CREAT are atomic.
     *
     * If there is a page cache page, it is returned with an increased refcount.
     *
     * Return: The found page or %NULL otherwise.
     */
    struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
    		int fgp_flags, gfp_t gfp_mask)
    {
    	struct page *page;
    
    repeat:
    	page = mapping_get_entry(mapping, index);
    	if (xa_is_value(page)) {
    		if (fgp_flags & FGP_ENTRY)
    			return page;
    		page = NULL;
    	}
    	if (!page)
    		goto no_page;
    
    	if (fgp_flags & FGP_LOCK) {
    		if (fgp_flags & FGP_NOWAIT) {
    			if (!trylock_page(page)) {
    				put_page(page);
    				return NULL;
    			}
    		} else {
    			lock_page(page);
    		}
    
    		/* Has the page been truncated? */
    		if (unlikely(page->mapping != mapping)) {
    			unlock_page(page);
    			put_page(page);
    			goto repeat;
    		}
    		VM_BUG_ON_PAGE(!thp_contains(page, index), page);
    	}
    
    	if (fgp_flags & FGP_ACCESSED)
    		mark_page_accessed(page);
    	else if (fgp_flags & FGP_WRITE) {
    		/* Clear idle flag for buffer write */
    		if (page_is_idle(page))
    			clear_page_idle(page);
    	}
    	if (!(fgp_flags & FGP_HEAD))
    		page = find_subpage(page, index);
    
    no_page:
    	if (!page && (fgp_flags & FGP_CREAT)) {
    		int err;
    		if ((fgp_flags & FGP_WRITE) && mapping_can_writeback(mapping))
    			gfp_mask |= __GFP_WRITE;
    		if (fgp_flags & FGP_NOFS)
    			gfp_mask &= ~__GFP_FS;
    
    		page = __page_cache_alloc(gfp_mask);
    		if (!page)
    			return NULL;
    
    		if (WARN_ON_ONCE(!(fgp_flags & (FGP_LOCK | FGP_FOR_MMAP))))
    			fgp_flags |= FGP_LOCK;
    
    		/* Init accessed so avoid atomic mark_page_accessed later */
    		if (fgp_flags & FGP_ACCESSED)
    			__SetPageReferenced(page);
    
    		err = add_to_page_cache_lru(page, mapping, index, gfp_mask);
    		if (unlikely(err)) {
    			put_page(page);
    			page = NULL;
    			if (err == -EEXIST)
    				goto repeat;
    		}
    
    		/*
    		 * add_to_page_cache_lru locks the page, and for mmap we expect
    		 * an unlocked page.
    		 */
    		if (page && (fgp_flags & FGP_FOR_MMAP))
    			unlock_page(page);
    	}
    
    	return page;
    }
    EXPORT_SYMBOL(pagecache_get_page);
    
    static inline struct page *find_get_entry(struct xa_state *xas, pgoff_t max,
    		xa_mark_t mark)
    {
    	struct page *page;
    
    retry:
    	if (mark == XA_PRESENT)
    		page = xas_find(xas, max);
    	else
    		page = xas_find_marked(xas, max, mark);
    
    	if (xas_retry(xas, page))
    		goto retry;
    	/*
    	 * A shadow entry of a recently evicted page, a swap
    	 * entry from shmem/tmpfs or a DAX entry.  Return it
    	 * without attempting to raise page count.
    	 */
    	if (!page || xa_is_value(page))
    		return page;
    
    	if (!page_cache_get_speculative(page))
    		goto reset;
    
    	/* Has the page moved or been split? */
    	if (unlikely(page != xas_reload(xas))) {
    		put_page(page);
    		goto reset;
    	}
    
    	return page;
    reset:
    	xas_reset(xas);
    	goto retry;
    }
    
    /**
     * find_get_entries - gang pagecache lookup
     * @mapping:	The address_space to search
     * @start:	The starting page cache index
     * @end:	The final page index (inclusive).
     * @pvec:	Where the resulting entries are placed.
     * @indices:	The cache indices corresponding to the entries in @entries
     *
     * find_get_entries() will search for and return a batch of entries in
     * the mapping.  The entries are placed in @pvec.  find_get_entries()
     * takes a reference on any actual pages it returns.
     *
     * The search returns a group of mapping-contiguous page cache entries
     * with ascending indexes.  There may be holes in the indices due to
     * not-present pages.
     *
     * Any shadow entries of evicted pages, or swap entries from
     * shmem/tmpfs, are included in the returned array.
     *
     * If it finds a Transparent Huge Page, head or tail, find_get_entries()
     * stops at that page: the caller is likely to have a better way to handle
     * the compound page as a whole, and then skip its extent, than repeatedly
     * calling find_get_entries() to return all its tails.
     *
     * Return: the number of pages and shadow entries which were found.
     */
    unsigned find_get_entries(struct address_space *mapping, pgoff_t start,
    		pgoff_t end, struct pagevec *pvec, pgoff_t *indices)
    {
    	XA_STATE(xas, &mapping->i_pages, start);
    	struct page *page;
    	unsigned int ret = 0;
    	unsigned nr_entries = PAGEVEC_SIZE;
    
    	rcu_read_lock();
    	while ((page = find_get_entry(&xas, end, XA_PRESENT))) {
    		/*
    		 * Terminate early on finding a THP, to allow the caller to
    		 * handle it all at once; but continue if this is hugetlbfs.
    		 */
    		if (!xa_is_value(page) && PageTransHuge(page) &&
    				!PageHuge(page)) {
    			page = find_subpage(page, xas.xa_index);
    			nr_entries = ret + 1;
    		}
    
    		indices[ret] = xas.xa_index;
    		pvec->pages[ret] = page;
    		if (++ret == nr_entries)
    			break;
    	}
    	rcu_read_unlock();
    
    	pvec->nr = ret;
    	return ret;
    }
    
    /**
     * find_lock_entries - Find a batch of pagecache entries.
     * @mapping:	The address_space to search.
     * @start:	The starting page cache index.
     * @end:	The final page index (inclusive).
     * @pvec:	Where the resulting entries are placed.
     * @indices:	The cache indices of the entries in @pvec.
     *
     * find_lock_entries() will return a batch of entries from @mapping.
     * Swap, shadow and DAX entries are included.  Pages are returned
     * locked and with an incremented refcount.  Pages which are locked by
     * somebody else or under writeback are skipped.  Only the head page of
     * a THP is returned.  Pages which are partially outside the range are
     * not returned.
     *
     * The entries have ascending indexes.  The indices may not be consecutive
     * due to not-present entries, THP pages, pages which could not be locked
     * or pages under writeback.
     *
     * Return: The number of entries which were found.
     */
    unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
    		pgoff_t end, struct pagevec *pvec, pgoff_t *indices)
    {
    	XA_STATE(xas, &mapping->i_pages, start);
    	struct page *page;
    
    	rcu_read_lock();
    	while ((page = find_get_entry(&xas, end, XA_PRESENT))) {
    		if (!xa_is_value(page)) {
    			if (page->index < start)
    				goto put;
    			VM_BUG_ON_PAGE(page->index != xas.xa_index, page);
    			if (page->index + thp_nr_pages(page) - 1 > end)
    				goto put;
    			if (!trylock_page(page))
    				goto put;
    			if (page->mapping != mapping || PageWriteback(page))
    				goto unlock;
    			VM_BUG_ON_PAGE(!thp_contains(page, xas.xa_index),
    					page);
    		}
    		indices[pvec->nr] = xas.xa_index;
    		if (!pagevec_add(pvec, page))
    			break;
    		goto next;
    unlock:
    		unlock_page(page);
    put:
    		put_page(page);
    next:
    		if (!xa_is_value(page) && PageTransHuge(page)) {
    			unsigned int nr_pages = thp_nr_pages(page);
    
    			/* Final THP may cross MAX_LFS_FILESIZE on 32-bit */
    			xas_set(&xas, page->index + nr_pages);
    			if (xas.xa_index < nr_pages)
    				break;
    		}
    	}
    	rcu_read_unlock();
    
    	return pagevec_count(pvec);
    }
    
    /**
     * find_get_pages_range - gang pagecache lookup
     * @mapping:	The address_space to search
     * @start:	The starting page index
     * @end:	The final page index (inclusive)
     * @nr_pages:	The maximum number of pages
     * @pages:	Where the resulting pages are placed
     *
     * find_get_pages_range() will search for and return a group of up to @nr_pages
     * pages in the mapping starting at index @start and up to index @end
     * (inclusive).  The pages are placed at @pages.  find_get_pages_range() takes
     * a reference against the returned pages.
     *
     * The search returns a group of mapping-contiguous pages with ascending
     * indexes.  There may be holes in the indices due to not-present pages.
     * We also update @start to index the next page for the traversal.
     *
     * Return: the number of pages which were found. If this number is
     * smaller than @nr_pages, the end of specified range has been
     * reached.
     */
    unsigned find_get_pages_range(struct address_space *mapping, pgoff_t *start,
    			      pgoff_t end, unsigned int nr_pages,
    			      struct page **pages)
    {
    	XA_STATE(xas, &mapping->i_pages, *start);
    	struct page *page;
    	unsigned ret = 0;
    
    	if (unlikely(!nr_pages))
    		return 0;
    
    	rcu_read_lock();
    	while ((page = find_get_entry(&xas, end, XA_PRESENT))) {
    		/* Skip over shadow, swap and DAX entries */
    		if (xa_is_value(page))
    			continue;
    
    		pages[ret] = find_subpage(page, xas.xa_index);
    		if (++ret == nr_pages) {
    			*start = xas.xa_index + 1;
    			goto out;
    		}
    	}
    
    	/*
    	 * We come here when there is no page beyond @end. We take care to not
    	 * overflow the index @start as it confuses some of the callers. This
    	 * breaks the iteration when there is a page at index -1 but that is
    	 * already broken anyway.
    	 */
    	if (end == (pgoff_t)-1)
    		*start = (pgoff_t)-1;
    	else
    		*start = end + 1;
    out:
    	rcu_read_unlock();
    
    	return ret;
    }
    
    /**
     * find_get_pages_contig - gang contiguous pagecache lookup
     * @mapping:	The address_space to search
     * @index:	The starting page index
     * @nr_pages:	The maximum number of pages
     * @pages:	Where the resulting pages are placed
     *
     * find_get_pages_contig() works exactly like find_get_pages(), except
     * that the returned number of pages are guaranteed to be contiguous.
     *
     * Return: the number of pages which were found.
     */
    unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
    			       unsigned int nr_pages, struct page **pages)
    {
    	XA_STATE(xas, &mapping->i_pages, index);
    	struct page *page;
    	unsigned int ret = 0;
    
    	if (unlikely(!nr_pages))
    		return 0;
    
    	rcu_read_lock();
    	for (page = xas_load(&xas); page; page = xas_next(&xas)) {
    		if (xas_retry(&xas, page))
    			continue;
    		/*
    		 * If the entry has been swapped out, we can stop looking.
    		 * No current caller is looking for DAX entries.
    		 */
    		if (xa_is_value(page))
    			break;
    
    		if (!page_cache_get_speculative(page))
    			goto retry;
    
    		/* Has the page moved or been split? */
    		if (unlikely(page != xas_reload(&xas)))
    			goto put_page;
    
    		pages[ret] = find_subpage(page, xas.xa_index);
    		if (++ret == nr_pages)
    			break;
    		continue;
    put_page:
    		put_page(page);
    retry:
    		xas_reset(&xas);
    	}
    	rcu_read_unlock();
    	return ret;
    }
    EXPORT_SYMBOL(find_get_pages_contig);
    
    /**
     * find_get_pages_range_tag - Find and return head pages matching @tag.
     * @mapping:	the address_space to search
     * @index:	the starting page index
     * @end:	The final page index (inclusive)
     * @tag:	the tag index
     * @nr_pages:	the maximum number of pages
     * @pages:	where the resulting pages are placed
     *
     * Like find_get_pages(), except we only return head pages which are tagged
     * with @tag.  @index is updated to the index immediately after the last
     * page we return, ready for the next iteration.
     *
     * Return: the number of pages which were found.
     */
    unsigned find_get_pages_range_tag(struct address_space *mapping, pgoff_t *index,
    			pgoff_t end, xa_mark_t tag, unsigned int nr_pages,
    			struct page **pages)
    {
    	XA_STATE(xas, &mapping->i_pages, *index);
    	struct page *page;
    	unsigned ret = 0;
    
    	if (unlikely(!nr_pages))
    		return 0;
    
    	rcu_read_lock();
    	while ((page = find_get_entry(&xas, end, tag))) {
    		/*
    		 * Shadow entries should never be tagged, but this iteration
    		 * is lockless so there is a window for page reclaim to evict
    		 * a page we saw tagged.  Skip over it.
    		 */
    		if (xa_is_value(page))
    			continue;
    
    		pages[ret] = page;
    		if (++ret == nr_pages) {
    			*index = page->index + thp_nr_pages(page);
    			goto out;
    		}
    	}
    
    	/*
    	 * We come here when we got to @end. We take care to not overflow the
    	 * index @index as it confuses some of the callers. This breaks the
    	 * iteration when there is a page at index -1 but that is already
    	 * broken anyway.
    	 */
    	if (end == (pgoff_t)-1)
    		*index = (pgoff_t)-1;
    	else
    		*index = end + 1;
    out:
    	rcu_read_unlock();
    
    	return ret;
    }
    EXPORT_SYMBOL(find_get_pages_range_tag);
    
    /*
     * CD/DVDs are error prone. When a medium error occurs, the driver may fail
     * a _large_ part of the i/o request. Imagine the worst scenario:
     *
     *      ---R__________________________________________B__________
     *         ^ reading here                             ^ bad block(assume 4k)
     *
     * read(R) => miss => readahead(R...B) => media error => frustrating retries
     * => failing the whole request => read(R) => read(R+1) =>
     * readahead(R+1...B+1) => bang => read(R+2) => read(R+3) =>
     * readahead(R+3...B+2) => bang => read(R+3) => read(R+4) =>
     * readahead(R+4...B+3) => bang => read(R+4) => read(R+5) => ......
     *
     * It is going insane. Fix it by quickly scaling down the readahead size.
     */
    static void shrink_readahead_size_eio(struct file_ra_state *ra)
    {
    	ra->ra_pages /= 4;
    }
    
    /*
     * filemap_get_read_batch - Get a batch of pages for read
     *
     * Get a batch of pages which represent a contiguous range of bytes
     * in the file.  No tail pages will be returned.  If @index is in the
     * middle of a THP, the entire THP will be returned.  The last page in
     * the batch may have Readahead set or be not Uptodate so that the
     * caller can take the appropriate action.
     */
    static void filemap_get_read_batch(struct address_space *mapping,
    		pgoff_t index, pgoff_t max, struct pagevec *pvec)
    {
    	XA_STATE(xas, &mapping->i_pages, index);
    	struct page *head;
    
    	rcu_read_lock();
    	for (head = xas_load(&xas); head; head = xas_next(&xas)) {
    		if (xas_retry(&xas, head))
    			continue;
    		if (xas.xa_index > max || xa_is_value(head))
    			break;
    		if (!page_cache_get_speculative(head))
    			goto retry;
    
    		/* Has the page moved or been split? */
    		if (unlikely(head != xas_reload(&xas)))
    			goto put_page;
    
    		if (!pagevec_add(pvec, head))
    			break;
    		if (!PageUptodate(head))
    			break;
    		if (PageReadahead(head))
    			break;
    		xas.xa_index = head->index + thp_nr_pages(head) - 1;
    		xas.xa_offset = (xas.xa_index >> xas.xa_shift) & XA_CHUNK_MASK;
    		continue;
    put_page:
    		put_page(head);
    retry:
    		xas_reset(&xas);
    	}
    	rcu_read_unlock();
    }
    
    static int filemap_read_page(struct file *file, struct address_space *mapping,
    		struct page *page)
    {
    	int error;
    
    	/*
    	 * A previous I/O error may have been due to temporary failures,
    	 * eg. multipath errors.  PG_error will be set again if readpage
    	 * fails.
    	 */
    	ClearPageError(page);
    	/* Start the actual read. The read will unlock the page. */
    	error = mapping->a_ops->readpage(file, page);
    	if (error)
    		return error;
    
    	error = wait_on_page_locked_killable(page);
    	if (error)
    		return error;
    	if (PageUptodate(page))
    		return 0;
    	if (!page->mapping)	/* page truncated */
    		return AOP_TRUNCATED_PAGE;
    	shrink_readahead_size_eio(&file->f_ra);
    	return -EIO;
    }
    
    static bool filemap_range_uptodate(struct address_space *mapping,
    		loff_t pos, struct iov_iter *iter, struct page *page)
    {
    	int count;
    
    	if (PageUptodate(page))
    		return true;
    	/* pipes can't handle partially uptodate pages */
    	if (iov_iter_is_pipe(iter))
    		return false;
    	if (!mapping->a_ops->is_partially_uptodate)
    		return false;
    	if (mapping->host->i_blkbits >= (PAGE_SHIFT + thp_order(page)))
    		return false;
    
    	count = iter->count;
    	if (page_offset(page) > pos) {
    		count -= page_offset(page) - pos;
    		pos = 0;
    	} else {
    		pos -= page_offset(page);
    	}
    
    	return mapping->a_ops->is_partially_uptodate(page, pos, count);
    }
    
    static int filemap_update_page(struct kiocb *iocb,
    		struct address_space *mapping, struct iov_iter *iter,
    		struct page *page)
    {
    	int error;
    
    	if (!trylock_page(page)) {
    		if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_NOIO))
    			return -EAGAIN;
    		if (!(iocb->ki_flags & IOCB_WAITQ)) {
    			put_and_wait_on_page_locked(page, TASK_KILLABLE);
    			return AOP_TRUNCATED_PAGE;
    		}
    		error = __lock_page_async(page, iocb->ki_waitq);
    		if (error)
    			return error;
    	}
    
    	if (!page->mapping)
    		goto truncated;
    
    	error = 0;
    	if (filemap_range_uptodate(mapping, iocb->ki_pos, iter, page))
    		goto unlock;
    
    	error = -EAGAIN;
    	if (iocb->ki_flags & (IOCB_NOIO | IOCB_NOWAIT | IOCB_WAITQ))
    		goto unlock;
    
    	error = filemap_read_page(iocb->ki_filp, mapping, page);
    	if (error == AOP_TRUNCATED_PAGE)
    		put_page(page);
    	return error;
    truncated:
    	unlock_page(page);
    	put_page(page);
    	return AOP_TRUNCATED_PAGE;
    unlock:
    	unlock_page(page);
    	return error;
    }
    
    static int filemap_create_page(struct file *file,
    		struct address_space *mapping, pgoff_t index,
    		struct pagevec *pvec)
    {
    	struct page *page;
    	int error;
    
    	page = page_cache_alloc(mapping);
    	if (!page)
    		return -ENOMEM;
    
    	error = add_to_page_cache_lru(page, mapping, index,
    			mapping_gfp_constraint(mapping, GFP_KERNEL));
    	if (error == -EEXIST)
    		error = AOP_TRUNCATED_PAGE;
    	if (error)
    		goto error;
    
    	error = filemap_read_page(file, mapping, page);
    	if (error)
    		goto error;
    
    	pagevec_add(pvec, page);
    	return 0;
    error:
    	put_page(page);
    	return error;
    }
    
    static int filemap_readahead(struct kiocb *iocb, struct file *file,
    		struct address_space *mapping, struct page *page,
    		pgoff_t last_index)
    {
    	if (iocb->ki_flags & IOCB_NOIO)
    		return -EAGAIN;
    	page_cache_async_readahead(mapping, &file->f_ra, file, page,
    			page->index, last_index - page->index);
    	return 0;
    }
    
    static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
    		struct pagevec *pvec)
    {
    	struct file *filp = iocb->ki_filp;
    	struct address_space *mapping = filp->f_mapping;
    	struct file_ra_state *ra = &filp->f_ra;
    	pgoff_t index = iocb->ki_pos >> PAGE_SHIFT;
    	pgoff_t last_index;
    	struct page *page;
    	int err = 0;
    
    	last_index = DIV_ROUND_UP(iocb->ki_pos + iter->count, PAGE_SIZE);
    retry:
    	if (fatal_signal_pending(current))
    		return -EINTR;
    
    	filemap_get_read_batch(mapping, index, last_index, pvec);
    	if (!pagevec_count(pvec)) {
    		if (iocb->ki_flags & IOCB_NOIO)
    			return -EAGAIN;
    		page_cache_sync_readahead(mapping, ra, filp, index,
    				last_index - index);
    		filemap_get_read_batch(mapping, index, last_index, pvec);
    	}
    	if (!pagevec_count(pvec)) {
    		if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_WAITQ))
    			return -EAGAIN;
    		err = filemap_create_page(filp, mapping,
    				iocb->ki_pos >> PAGE_SHIFT, pvec);
    		if (err == AOP_TRUNCATED_PAGE)
    			goto retry;
    		return err;
    	}
    
    	page = pvec->pages[pagevec_count(pvec) - 1];
    	if (PageReadahead(page)) {
    		err = filemap_readahead(iocb, filp, mapping, page, last_index);
    		if (err)
    			goto err;
    	}
    	if (!PageUptodate(page)) {
    		if ((iocb->ki_flags & IOCB_WAITQ) && pagevec_count(pvec) > 1)
    			iocb->ki_flags |= IOCB_NOWAIT;
    		err = filemap_update_page(iocb, mapping, iter, page);
    		if (err)
    			goto err;
    	}
    
    	return 0;
    err:
    	if (err < 0)
    		put_page(page);
    	if (likely(--pvec->nr))
    		return 0;
    	if (err == AOP_TRUNCATED_PAGE)
    		goto retry;
    	return err;
    }
    
    /**
     * filemap_read - Read data from the page cache.
     * @iocb: The iocb to read.
     * @iter: Destination for the data.
     * @already_read: Number of bytes already read by the caller.
     *
     * Copies data from the page cache.  If the data is not currently present,
     * uses the readahead and readpage address_space operations to fetch it.
     *
     * Return: Total number of bytes copied, including those already read by
     * the caller.  If an error happens before any bytes are copied, returns
     * a negative error number.
     */
    ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
    		ssize_t already_read)
    {
    	struct file *filp = iocb->ki_filp;
    	struct file_ra_state *ra = &filp->f_ra;
    	struct address_space *mapping = filp->f_mapping;
    	struct inode *inode = mapping->host;
    	struct pagevec pvec;
    	int i, error = 0;
    	bool writably_mapped;
    	loff_t isize, end_offset;
    
    	if (unlikely(iocb->ki_pos >= inode->i_sb->s_maxbytes))
    		return 0;
    	if (unlikely(!iov_iter_count(iter)))
    		return 0;
    
    	iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
    	pagevec_init(&pvec);
    
    	do {
    		cond_resched();
    
    		/*
    		 * If we've already successfully copied some data, then we
    		 * can no longer safely return -EIOCBQUEUED. Hence mark
    		 * an async read NOWAIT at that point.
    		 */
    		if ((iocb->ki_flags & IOCB_WAITQ) && already_read)
    			iocb->ki_flags |= IOCB_NOWAIT;
    
    		error = filemap_get_pages(iocb, iter, &pvec);
    		if (error < 0)
    			break;
    
    		/*
    		 * i_size must be checked after we know the pages are Uptodate.
    		 *
    		 * Checking i_size after the check allows us to calculate
    		 * the correct value for "nr", which means the zero-filled
    		 * part of the page is not copied back to userspace (unless
    		 * another truncate extends the file - this is desired though).
    		 */
    		isize = i_size_read(inode);
    		if (unlikely(iocb->ki_pos >= isize))
    			goto put_pages;
    		end_offset = min_t(loff_t, isize, iocb->ki_pos + iter->count);
    
    		/*
    		 * Once we start copying data, we don't want to be touching any
    		 * cachelines that might be contended:
    		 */
    		writably_mapped = mapping_writably_mapped(mapping);
    
    		/*
    		 * When a sequential read accesses a page several times, only
    		 * mark it as accessed the first time.
    		 */
    		if (iocb->ki_pos >> PAGE_SHIFT !=
    		    ra->prev_pos >> PAGE_SHIFT)
    			mark_page_accessed(pvec.pages[0]);
    
    		for (i = 0; i < pagevec_count(&pvec); i++) {
    			struct page *page = pvec.pages[i];
    			size_t page_size = thp_size(page);
    			size_t offset = iocb->ki_pos & (page_size - 1);
    			size_t bytes = min_t(loff_t, end_offset - iocb->ki_pos,
    					     page_size - offset);
    			size_t copied;
    
    			if (end_offset < page_offset(page))
    				break;
    			if (i > 0)
    				mark_page_accessed(page);
    			/*
    			 * If users can be writing to this page using arbitrary
    			 * virtual addresses, take care about potential aliasing
    			 * before reading the page on the kernel side.
    			 */
    			if (writably_mapped) {
    				int j;
    
    				for (j = 0; j < thp_nr_pages(page); j++)
    					flush_dcache_page(page + j);
    			}
    
    			copied = copy_page_to_iter(page, offset, bytes, iter);
    
    			already_read += copied;
    			iocb->ki_pos += copied;
    			ra->prev_pos = iocb->ki_pos;
    
    			if (copied < bytes) {
    				error = -EFAULT;
    				break;
    			}
    		}
    put_pages:
    		for (i = 0; i < pagevec_count(&pvec); i++)
    			put_page(pvec.pages[i]);
    		pagevec_reinit(&pvec);
    	} while (iov_iter_count(iter) && iocb->ki_pos < isize && !error);
    
    	file_accessed(filp);
    
    	return already_read ? already_read : error;
    }
    EXPORT_SYMBOL_GPL(filemap_read);
    
    /**
     * generic_file_read_iter - generic filesystem read routine
     * @iocb:	kernel I/O control block
     * @iter:	destination for the data read
     *
     * This is the "read_iter()" routine for all filesystems
     * that can use the page cache directly.
     *
     * The IOCB_NOWAIT flag in iocb->ki_flags indicates that -EAGAIN shall
     * be returned when no data can be read without waiting for I/O requests
     * to complete; it doesn't prevent readahead.
     *
     * The IOCB_NOIO flag in iocb->ki_flags indicates that no new I/O
     * requests shall be made for the read or for readahead.  When no data
     * can be read, -EAGAIN shall be returned.  When readahead would be
     * triggered, a partial, possibly empty read shall be returned.
     *
     * Return:
     * * number of bytes copied, even for partial reads
     * * negative error code (or 0 if IOCB_NOIO) if nothing was read
     */
    ssize_t
    generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
    {
    	size_t count = iov_iter_count(iter);
    	ssize_t retval = 0;
    
    	if (!count)
    		return 0; /* skip atime */
    
    	if (iocb->ki_flags & IOCB_DIRECT) {
    		struct file *file = iocb->ki_filp;
    		struct address_space *mapping = file->f_mapping;
    		struct inode *inode = mapping->host;
    		loff_t size;
    
    		size = i_size_read(inode);
    		if (iocb->ki_flags & IOCB_NOWAIT) {
    			if (filemap_range_has_page(mapping, iocb->ki_pos,
    						   iocb->ki_pos + count - 1))
    				return -EAGAIN;
    		} else {
    			retval = filemap_write_and_wait_range(mapping,
    						iocb->ki_pos,
    					        iocb->ki_pos + count - 1);
    			if (retval < 0)
    				return retval;
    		}
    
    		file_accessed(file);
    
    		retval = mapping->a_ops->direct_IO(iocb, iter);
    		if (retval >= 0) {
    			iocb->ki_pos += retval;
    			count -= retval;
    		}
    		if (retval != -EIOCBQUEUED)
    			iov_iter_revert(iter, count - iov_iter_count(iter));
    
    		/*
    		 * Btrfs can have a short DIO read if we encounter
    		 * compressed extents, so if there was an error, or if
    		 * we've already read everything we wanted to, or if
    		 * there was a short read because we hit EOF, go ahead
    		 * and return.  Otherwise fallthrough to buffered io for
    		 * the rest of the read.  Buffered reads will not work for
    		 * DAX files, so don't bother trying.
    		 */
    		if (retval < 0 || !count || iocb->ki_pos >= size ||
    		    IS_DAX(inode))
    			return retval;
    	}
    
    	return filemap_read(iocb, iter, retval);
    }
    EXPORT_SYMBOL(generic_file_read_iter);
    
    static inline loff_t page_seek_hole_data(struct xa_state *xas,
    		struct address_space *mapping, struct page *page,
    		loff_t start, loff_t end, bool seek_data)
    {
    	const struct address_space_operations *ops = mapping->a_ops;
    	size_t offset, bsz = i_blocksize(mapping->host);
    
    	if (xa_is_value(page) || PageUptodate(page))
    		return seek_data ? start : end;
    	if (!ops->is_partially_uptodate)
    		return seek_data ? end : start;
    
    	xas_pause(xas);
    	rcu_read_unlock();
    	lock_page(page);
    	if (unlikely(page->mapping != mapping))
    		goto unlock;
    
    	offset = offset_in_thp(page, start) & ~(bsz - 1);
    
    	do {
    		if (ops->is_partially_uptodate(page, offset, bsz) == seek_data)
    			break;
    		start = (start + bsz) & ~(bsz - 1);
    		offset += bsz;
    	} while (offset < thp_size(page));
    unlock:
    	unlock_page(page);
    	rcu_read_lock();
    	return start;
    }
    
    static inline
    unsigned int seek_page_size(struct xa_state *xas, struct page *page)
    {
    	if (xa_is_value(page))
    		return PAGE_SIZE << xa_get_order(xas->xa, xas->xa_index);
    	return thp_size(page);
    }
    
    /**
     * mapping_seek_hole_data - Seek for SEEK_DATA / SEEK_HOLE in the page cache.
     * @mapping: Address space to search.
     * @start: First byte to consider.
     * @end: Limit of search (exclusive).
     * @whence: Either SEEK_HOLE or SEEK_DATA.
     *
     * If the page cache knows which blocks contain holes and which blocks
     * contain data, your filesystem can use this function to implement
     * SEEK_HOLE and SEEK_DATA.  This is useful for filesystems which are
     * entirely memory-based such as tmpfs, and filesystems which support
     * unwritten extents.
     *
     * Return: The requested offset on successs, or -ENXIO if @whence specifies
     * SEEK_DATA and there is no data after @start.  There is an implicit hole
     * after @end - 1, so SEEK_HOLE returns @end if all the bytes between @start
     * and @end contain data.
     */
    loff_t mapping_seek_hole_data(struct address_space *mapping, loff_t start,
    		loff_t end, int whence)
    {
    	XA_STATE(xas, &mapping->i_pages, start >> PAGE_SHIFT);
    	pgoff_t max = (end - 1) >> PAGE_SHIFT;
    	bool seek_data = (whence == SEEK_DATA);
    	struct page *page;
    
    	if (end <= start)
    		return -ENXIO;
    
    	rcu_read_lock();
    	while ((page = find_get_entry(&xas, max, XA_PRESENT))) {
    		loff_t pos = (u64)xas.xa_index << PAGE_SHIFT;
    		unsigned int seek_size;
    
    		if (start < pos) {
    			if (!seek_data)
    				goto unlock;
    			start = pos;
    		}
    
    		seek_size = seek_page_size(&xas, page);
    		pos = round_up(pos + 1, seek_size);
    		start = page_seek_hole_data(&xas, mapping, page, start, pos,
    				seek_data);
    		if (start < pos)
    			goto unlock;
    		if (start >= end)
    			break;
    		if (seek_size > PAGE_SIZE)
    			xas_set(&xas, pos >> PAGE_SHIFT);
    		if (!xa_is_value(page))
    			put_page(page);
    	}
    	if (seek_data)
    		start = -ENXIO;
    unlock:
    	rcu_read_unlock();
    	if (page && !xa_is_value(page))
    		put_page(page);
    	if (start > end)
    		return end;
    	return start;
    }
    
    #ifdef CONFIG_MMU
    #define MMAP_LOTSAMISS  (100)
    /*
     * lock_page_maybe_drop_mmap - lock the page, possibly dropping the mmap_lock
     * @vmf - the vm_fault for this fault.
     * @page - the page to lock.
     * @fpin - the pointer to the file we may pin (or is already pinned).
     *
     * This works similar to lock_page_or_retry in that it can drop the mmap_lock.
     * It differs in that it actually returns the page locked if it returns 1 and 0
     * if it couldn't lock the page.  If we did have to drop the mmap_lock then fpin
     * will point to the pinned file and needs to be fput()'ed at a later point.
     */
    static int lock_page_maybe_drop_mmap(struct vm_fault *vmf, struct page *page,
    				     struct file **fpin)
    {
    	if (trylock_page(page))
    		return 1;
    
    	/*
    	 * NOTE! This will make us return with VM_FAULT_RETRY, but with
    	 * the mmap_lock still held. That's how FAULT_FLAG_RETRY_NOWAIT
    	 * is supposed to work. We have way too many special cases..
    	 */
    	if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
    		return 0;
    
    	*fpin = maybe_unlock_mmap_for_io(vmf, *fpin);
    	if (vmf->flags & FAULT_FLAG_KILLABLE) {
    		if (__lock_page_killable(page)) {
    			/*
    			 * We didn't have the right flags to drop the mmap_lock,
    			 * but all fault_handlers only check for fatal signals
    			 * if we return VM_FAULT_RETRY, so we need to drop the
    			 * mmap_lock here and return 0 if we don't have a fpin.
    			 */
    			if (*fpin == NULL)
    				mmap_read_unlock(vmf->vma->vm_mm);
    			return 0;
    		}
    	} else
    		__lock_page(page);
    	return 1;
    }
    
    
    /*
     * Synchronous readahead happens when we don't even find a page in the page
     * cache at all.  We don't want to perform IO under the mmap sem, so if we have
     * to drop the mmap sem we return the file that was pinned in order for us to do
     * that.  If we didn't pin a file then we return NULL.  The file that is
     * returned needs to be fput()'ed when we're done with it.
     */
    static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
    {
    	struct file *file = vmf->vma->vm_file;
    	struct file_ra_state *ra = &file->f_ra;
    	struct address_space *mapping = file->f_mapping;
    	DEFINE_READAHEAD(ractl, file, mapping, vmf->pgoff);
    	struct file *fpin = NULL;
    	unsigned int mmap_miss;
    
    	/* If we don't want any read-ahead, don't bother */
    	if (vmf->vma->vm_flags & VM_RAND_READ)
    		return fpin;
    	if (!ra->ra_pages)
    		return fpin;
    
    	if (vmf->vma->vm_flags & VM_SEQ_READ) {
    		fpin = maybe_unlock_mmap_for_io(vmf, fpin);
    		page_cache_sync_ra(&ractl, ra, ra->ra_pages);
    		return fpin;
    	}
    
    	/* Avoid banging the cache line if not needed */
    	mmap_miss = READ_ONCE(ra->mmap_miss);
    	if (mmap_miss < MMAP_LOTSAMISS * 10)
    		WRITE_ONCE(ra->mmap_miss, ++mmap_miss);
    
    	/*
    	 * Do we miss much more than hit in this file? If so,
    	 * stop bothering with read-ahead. It will only hurt.
    	 */
    	if (mmap_miss > MMAP_LOTSAMISS)
    		return fpin;
    
    	/*
    	 * mmap read-around
    	 */
    	fpin = maybe_unlock_mmap_for_io(vmf, fpin);
    	ra->start = max_t(long, 0, vmf->pgoff - ra->ra_pages / 2);
    	ra->size = ra->ra_pages;
    	ra->async_size = ra->ra_pages / 4;
    	ractl._index = ra->start;
    	do_page_cache_ra(&ractl, ra->size, ra->async_size);
    	return fpin;
    }
    
    /*
     * Asynchronous readahead happens when we find the page and PG_readahead,
     * so we want to possibly extend the readahead further.  We return the file that
     * was pinned if we have to drop the mmap_lock in order to do IO.
     */
    static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
    					    struct page *page)
    {
    	struct file *file = vmf->vma->vm_file;
    	struct file_ra_state *ra = &file->f_ra;
    	struct address_space *mapping = file->f_mapping;
    	struct file *fpin = NULL;
    	unsigned int mmap_miss;
    	pgoff_t offset = vmf->pgoff;
    
    	/* If we don't want any read-ahead, don't bother */
    	if (vmf->vma->vm_flags & VM_RAND_READ || !ra->ra_pages)
    		return fpin;
    	mmap_miss = READ_ONCE(ra->mmap_miss);
    	if (mmap_miss)
    		WRITE_ONCE(ra->mmap_miss, --mmap_miss);
    	if (PageReadahead(page)) {
    		fpin = maybe_unlock_mmap_for_io(vmf, fpin);
    		page_cache_async_readahead(mapping, ra, file,
    					   page, offset, ra->ra_pages);
    	}
    	return fpin;
    }
    
    /**
     * filemap_fault - read in file data for page fault handling
     * @vmf:	struct vm_fault containing details of the fault
     *
     * filemap_fault() is invoked via the vma operations vector for a
     * mapped memory region to read in file data during a page fault.
     *
     * The goto's are kind of ugly, but this streamlines the normal case of having
     * it in the page cache, and handles the special cases reasonably without
     * having a lot of duplicated code.
     *
     * vma->vm_mm->mmap_lock must be held on entry.
     *
     * If our return value has VM_FAULT_RETRY set, it's because the mmap_lock
     * may be dropped before doing I/O or by lock_page_maybe_drop_mmap().
     *
     * If our return value does not have VM_FAULT_RETRY set, the mmap_lock
     * has not been released.
     *
     * We never return with VM_FAULT_RETRY and a bit from VM_FAULT_ERROR set.
     *
     * Return: bitwise-OR of %VM_FAULT_ codes.
     */
    vm_fault_t filemap_fault(struct vm_fault *vmf)
    {
    	int error;
    	struct file *file = vmf->vma->vm_file;
    	struct file *fpin = NULL;
    	struct address_space *mapping = file->f_mapping;
    	struct file_ra_state *ra = &file->f_ra;
    	struct inode *inode = mapping->host;
    	pgoff_t offset = vmf->pgoff;
    	pgoff_t max_off;
    	struct page *page;
    	vm_fault_t ret = 0;
    
    	max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
    	if (unlikely(offset >= max_off))
    		return VM_FAULT_SIGBUS;
    
    	/*
    	 * Do we have something in the page cache already?
    	 */
    	page = find_get_page(mapping, offset);
    	if (likely(page) && !(vmf->flags & FAULT_FLAG_TRIED)) {
    		/*
    		 * We found the page, so try async readahead before
    		 * waiting for the lock.
    		 */
    		fpin = do_async_mmap_readahead(vmf, page);
    	} else if (!page) {
    		/* No page in the page cache at all */
    		count_vm_event(PGMAJFAULT);
    		count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
    		ret = VM_FAULT_MAJOR;
    		fpin = do_sync_mmap_readahead(vmf);
    retry_find:
    		page = pagecache_get_page(mapping, offset,
    					  FGP_CREAT|FGP_FOR_MMAP,
    					  vmf->gfp_mask);
    		if (!page) {
    			if (fpin)
    				goto out_retry;
    			return VM_FAULT_OOM;
    		}
    	}
    
    	if (!lock_page_maybe_drop_mmap(vmf, page, &fpin))
    		goto out_retry;
    
    	/* Did it get truncated? */
    	if (unlikely(compound_head(page)->mapping != mapping)) {
    		unlock_page(page);
    		put_page(page);
    		goto retry_find;
    	}
    	VM_BUG_ON_PAGE(page_to_pgoff(page) != offset, page);
    
    	/*
    	 * We have a locked page in the page cache, now we need to check
    	 * that it's up-to-date. If not, it is going to be due to an error.
    	 */
    	if (unlikely(!PageUptodate(page)))
    		goto page_not_uptodate;
    
    	/*
    	 * We've made it this far and we had to drop our mmap_lock, now is the
    	 * time to return to the upper layer and have it re-find the vma and
    	 * redo the fault.
    	 */
    	if (fpin) {
    		unlock_page(page);
    		goto out_retry;
    	}
    
    	/*
    	 * Found the page and have a reference on it.
    	 * We must recheck i_size under page lock.
    	 */
    	max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
    	if (unlikely(offset >= max_off)) {
    		unlock_page(page);
    		put_page(page);
    		return VM_FAULT_SIGBUS;
    	}
    
    	vmf->page = page;
    	return ret | VM_FAULT_LOCKED;
    
    page_not_uptodate:
    	/*
    	 * Umm, take care of errors if the page isn't up-to-date.
    	 * Try to re-read it _once_. We do this synchronously,
    	 * because there really aren't any performance issues here
    	 * and we need to check for errors.
    	 */
    	ClearPageError(page);
    	fpin = maybe_unlock_mmap_for_io(vmf, fpin);
    	error = mapping->a_ops->readpage(file, page);
    	if (!error) {
    		wait_on_page_locked(page);
    		if (!PageUptodate(page))
    			error = -EIO;
    	}
    	if (fpin)
    		goto out_retry;
    	put_page(page);
    
    	if (!error || error == AOP_TRUNCATED_PAGE)
    		goto retry_find;
    
    	shrink_readahead_size_eio(ra);
    	return VM_FAULT_SIGBUS;
    
    out_retry:
    	/*
    	 * We dropped the mmap_lock, we need to return to the fault handler to
    	 * re-find the vma and come back and find our hopefully still populated
    	 * page.
    	 */
    	if (page)
    		put_page(page);
    	if (fpin)
    		fput(fpin);
    	return ret | VM_FAULT_RETRY;
    }
    EXPORT_SYMBOL(filemap_fault);
    
    static bool filemap_map_pmd(struct vm_fault *vmf, struct page *page)
    {
    	struct mm_struct *mm = vmf->vma->vm_mm;
    
    	/* Huge page is mapped? No need to proceed. */
    	if (pmd_trans_huge(*vmf->pmd)) {
    		unlock_page(page);
    		put_page(page);
    		return true;
    	}
    
    	if (pmd_none(*vmf->pmd) && PageTransHuge(page)) {
    	    vm_fault_t ret = do_set_pmd(vmf, page);
    	    if (!ret) {
    		    /* The page is mapped successfully, reference consumed. */
    		    unlock_page(page);
    		    return true;
    	    }
    	}
    
    	if (pmd_none(*vmf->pmd)) {
    		vmf->ptl = pmd_lock(mm, vmf->pmd);
    		if (likely(pmd_none(*vmf->pmd))) {
    			mm_inc_nr_ptes(mm);
    			pmd_populate(mm, vmf->pmd, vmf->prealloc_pte);
    			vmf->prealloc_pte = NULL;
    		}
    		spin_unlock(vmf->ptl);
    	}
    
    	/* See comment in handle_pte_fault() */
    	if (pmd_devmap_trans_unstable(vmf->pmd)) {
    		unlock_page(page);
    		put_page(page);
    		return true;
    	}
    
    	return false;
    }
    
    static struct page *next_uptodate_page(struct page *page,
    				       struct address_space *mapping,
    				       struct xa_state *xas, pgoff_t end_pgoff)
    {
    	unsigned long max_idx;
    
    	do {
    		if (!page)
    			return NULL;
    		if (xas_retry(xas, page))
    			continue;
    		if (xa_is_value(page))
    			continue;
    		if (PageLocked(page))
    			continue;
    		if (!page_cache_get_speculative(page))
    			continue;
    		/* Has the page moved or been split? */
    		if (unlikely(page != xas_reload(xas)))
    			goto skip;
    		if (!PageUptodate(page) || PageReadahead(page))
    			goto skip;
    		if (PageHWPoison(page))
    			goto skip;
    		if (!trylock_page(page))
    			goto skip;
    		if (page->mapping != mapping)
    			goto unlock;
    		if (!PageUptodate(page))
    			goto unlock;
    		max_idx = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE);
    		if (xas->xa_index >= max_idx)
    			goto unlock;
    		return page;
    unlock:
    		unlock_page(page);
    skip:
    		put_page(page);
    	} while ((page = xas_next_entry(xas, end_pgoff)) != NULL);
    
    	return NULL;
    }
    
    static inline struct page *first_map_page(struct address_space *mapping,
    					  struct xa_state *xas,
    					  pgoff_t end_pgoff)
    {
    	return next_uptodate_page(xas_find(xas, end_pgoff),
    				  mapping, xas, end_pgoff);
    }
    
    static inline struct page *next_map_page(struct address_space *mapping,
    					 struct xa_state *xas,
    					 pgoff_t end_pgoff)
    {
    	return next_uptodate_page(xas_next_entry(xas, end_pgoff),
    				  mapping, xas, end_pgoff);
    }
    
    vm_fault_t filemap_map_pages(struct vm_fault *vmf,
    			     pgoff_t start_pgoff, pgoff_t end_pgoff)
    {
    	struct vm_area_struct *vma = vmf->vma;
    	struct file *file = vma->vm_file;
    	struct address_space *mapping = file->f_mapping;
    	pgoff_t last_pgoff = start_pgoff;
    	unsigned long addr;
    	XA_STATE(xas, &mapping->i_pages, start_pgoff);
    	struct page *head, *page;
    	unsigned int mmap_miss = READ_ONCE(file->f_ra.mmap_miss);
    	vm_fault_t ret = 0;
    
    	rcu_read_lock();
    	head = first_map_page(mapping, &xas, end_pgoff);
    	if (!head)
    		goto out;
    
    	if (filemap_map_pmd(vmf, head)) {
    		ret = VM_FAULT_NOPAGE;
    		goto out;
    	}
    
    	addr = vma->vm_start + ((start_pgoff - vma->vm_pgoff) << PAGE_SHIFT);
    	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, addr, &vmf->ptl);
    	do {
    		page = find_subpage(head, xas.xa_index);
    		if (PageHWPoison(page))
    			goto unlock;
    
    		if (mmap_miss > 0)
    			mmap_miss--;
    
    		addr += (xas.xa_index - last_pgoff) << PAGE_SHIFT;
    		vmf->pte += xas.xa_index - last_pgoff;
    		last_pgoff = xas.xa_index;
    
    		if (!pte_none(*vmf->pte))
    			goto unlock;
    
    		/* We're about to handle the fault */
    		if (vmf->address == addr)
    			ret = VM_FAULT_NOPAGE;
    
    		do_set_pte(vmf, page, addr);
    		/* no need to invalidate: a not-present page won't be cached */
    		update_mmu_cache(vma, addr, vmf->pte);
    		unlock_page(head);
    		continue;
    unlock:
    		unlock_page(head);
    		put_page(head);
    	} while ((head = next_map_page(mapping, &xas, end_pgoff)) != NULL);
    	pte_unmap_unlock(vmf->pte, vmf->ptl);
    out:
    	rcu_read_unlock();
    	WRITE_ONCE(file->f_ra.mmap_miss, mmap_miss);
    	return ret;
    }
    EXPORT_SYMBOL(filemap_map_pages);
    
    vm_fault_t filemap_page_mkwrite(struct vm_fault *vmf)
    {
    	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
    	struct page *page = vmf->page;
    	vm_fault_t ret = VM_FAULT_LOCKED;
    
    	sb_start_pagefault(mapping->host->i_sb);
    	file_update_time(vmf->vma->vm_file);
    	lock_page(page);
    	if (page->mapping != mapping) {
    		unlock_page(page);
    		ret = VM_FAULT_NOPAGE;
    		goto out;
    	}
    	/*
    	 * We mark the page dirty already here so that when freeze is in
    	 * progress, we are guaranteed that writeback during freezing will
    	 * see the dirty page and writeprotect it again.
    	 */
    	set_page_dirty(page);
    	wait_for_stable_page(page);
    out:
    	sb_end_pagefault(mapping->host->i_sb);
    	return ret;
    }
    
    const struct vm_operations_struct generic_file_vm_ops = {
    	.fault		= filemap_fault,
    	.map_pages	= filemap_map_pages,
    	.page_mkwrite	= filemap_page_mkwrite,
    };
    
    /* This is used for a general mmap of a disk file */
    
    int generic_file_mmap(struct file * file, struct vm_area_struct * vma)
    {
    	struct address_space *mapping = file->f_mapping;
    
    	if (!mapping->a_ops->readpage)
    		return -ENOEXEC;
    	file_accessed(file);
    	vma->vm_ops = &generic_file_vm_ops;
    	return 0;
    }
    
    /*
     * This is for filesystems which do not implement ->writepage.
     */
    int generic_file_readonly_mmap(struct file *file, struct vm_area_struct *vma)
    {
    	if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_MAYWRITE))
    		return -EINVAL;
    	return generic_file_mmap(file, vma);
    }
    #else
    vm_fault_t filemap_page_mkwrite(struct vm_fault *vmf)
    {
    	return VM_FAULT_SIGBUS;
    }
    int generic_file_mmap(struct file * file, struct vm_area_struct * vma)
    {
    	return -ENOSYS;
    }
    int generic_file_readonly_mmap(struct file * file, struct vm_area_struct * vma)
    {
    	return -ENOSYS;
    }
    #endif /* CONFIG_MMU */
    
    EXPORT_SYMBOL(filemap_page_mkwrite);
    EXPORT_SYMBOL(generic_file_mmap);
    EXPORT_SYMBOL(generic_file_readonly_mmap);
    
    static struct page *wait_on_page_read(struct page *page)
    {
    	if (!IS_ERR(page)) {
    		wait_on_page_locked(page);
    		if (!PageUptodate(page)) {
    			put_page(page);
    			page = ERR_PTR(-EIO);
    		}
    	}
    	return page;
    }
    
    static struct page *do_read_cache_page(struct address_space *mapping,
    				pgoff_t index,
    				int (*filler)(void *, struct page *),
    				void *data,
    				gfp_t gfp)
    {
    	struct page *page;
    	int err;
    repeat:
    	page = find_get_page(mapping, index);
    	if (!page) {
    		page = __page_cache_alloc(gfp);
    		if (!page)
    			return ERR_PTR(-ENOMEM);
    		err = add_to_page_cache_lru(page, mapping, index, gfp);
    		if (unlikely(err)) {
    			put_page(page);
    			if (err == -EEXIST)
    				goto repeat;
    			/* Presumably ENOMEM for xarray node */
    			return ERR_PTR(err);
    		}
    
    filler:
    		if (filler)
    			err = filler(data, page);
    		else
    			err = mapping->a_ops->readpage(data, page);
    
    		if (err < 0) {
    			put_page(page);
    			return ERR_PTR(err);
    		}
    
    		page = wait_on_page_read(page);
    		if (IS_ERR(page))
    			return page;
    		goto out;
    	}
    	if (PageUptodate(page))
    		goto out;
    
    	/*
    	 * Page is not up to date and may be locked due to one of the following
    	 * case a: Page is being filled and the page lock is held
    	 * case b: Read/write error clearing the page uptodate status
    	 * case c: Truncation in progress (page locked)
    	 * case d: Reclaim in progress
    	 *
    	 * Case a, the page will be up to date when the page is unlocked.
    	 *    There is no need to serialise on the page lock here as the page
    	 *    is pinned so the lock gives no additional protection. Even if the
    	 *    page is truncated, the data is still valid if PageUptodate as
    	 *    it's a race vs truncate race.
    	 * Case b, the page will not be up to date
    	 * Case c, the page may be truncated but in itself, the data may still
    	 *    be valid after IO completes as it's a read vs truncate race. The
    	 *    operation must restart if the page is not uptodate on unlock but
    	 *    otherwise serialising on page lock to stabilise the mapping gives
    	 *    no additional guarantees to the caller as the page lock is
    	 *    released before return.
    	 * Case d, similar to truncation. If reclaim holds the page lock, it
    	 *    will be a race with remove_mapping that determines if the mapping
    	 *    is valid on unlock but otherwise the data is valid and there is
    	 *    no need to serialise with page lock.
    	 *
    	 * As the page lock gives no additional guarantee, we optimistically
    	 * wait on the page to be unlocked and check if it's up to date and
    	 * use the page if it is. Otherwise, the page lock is required to
    	 * distinguish between the different cases. The motivation is that we
    	 * avoid spurious serialisations and wakeups when multiple processes
    	 * wait on the same page for IO to complete.
    	 */
    	wait_on_page_locked(page);
    	if (PageUptodate(page))
    		goto out;
    
    	/* Distinguish between all the cases under the safety of the lock */
    	lock_page(page);
    
    	/* Case c or d, restart the operation */
    	if (!page->mapping) {
    		unlock_page(page);
    		put_page(page);
    		goto repeat;
    	}
    
    	/* Someone else locked and filled the page in a very small window */
    	if (PageUptodate(page)) {
    		unlock_page(page);
    		goto out;
    	}
    
    	/*
    	 * A previous I/O error may have been due to temporary
    	 * failures.
    	 * Clear page error before actual read, PG_error will be
    	 * set again if read page fails.
    	 */
    	ClearPageError(page);
    	goto filler;
    
    out:
    	mark_page_accessed(page);
    	return page;
    }
    
    /**
     * read_cache_page - read into page cache, fill it if needed
     * @mapping:	the page's address_space
     * @index:	the page index
     * @filler:	function to perform the read
     * @data:	first arg to filler(data, page) function, often left as NULL
     *
     * Read into the page cache. If a page already exists, and PageUptodate() is
     * not set, try to fill the page and wait for it to become unlocked.
     *
     * If the page does not get brought uptodate, return -EIO.
     *
     * Return: up to date page on success, ERR_PTR() on failure.
     */
    struct page *read_cache_page(struct address_space *mapping,
    				pgoff_t index,
    				int (*filler)(void *, struct page *),
    				void *data)
    {
    	return do_read_cache_page(mapping, index, filler, data,
    			mapping_gfp_mask(mapping));
    }
    EXPORT_SYMBOL(read_cache_page);
    
    /**
     * read_cache_page_gfp - read into page cache, using specified page allocation flags.
     * @mapping:	the page's address_space
     * @index:	the page index
     * @gfp:	the page allocator flags to use if allocating
     *
     * This is the same as "read_mapping_page(mapping, index, NULL)", but with
     * any new page allocations done using the specified allocation flags.
     *
     * If the page does not get brought uptodate, return -EIO.
     *
     * Return: up to date page on success, ERR_PTR() on failure.
     */
    struct page *read_cache_page_gfp(struct address_space *mapping,
    				pgoff_t index,
    				gfp_t gfp)
    {
    	return do_read_cache_page(mapping, index, NULL, NULL, gfp);
    }
    EXPORT_SYMBOL(read_cache_page_gfp);
    
    int pagecache_write_begin(struct file *file, struct address_space *mapping,
    				loff_t pos, unsigned len, unsigned flags,
    				struct page **pagep, void **fsdata)
    {
    	const struct address_space_operations *aops = mapping->a_ops;
    
    	return aops->write_begin(file, mapping, pos, len, flags,
    							pagep, fsdata);
    }
    EXPORT_SYMBOL(pagecache_write_begin);
    
    int pagecache_write_end(struct file *file, struct address_space *mapping,
    				loff_t pos, unsigned len, unsigned copied,
    				struct page *page, void *fsdata)
    {
    	const struct address_space_operations *aops = mapping->a_ops;
    
    	return aops->write_end(file, mapping, pos, len, copied, page, fsdata);
    }
    EXPORT_SYMBOL(pagecache_write_end);
    
    /*
     * Warn about a page cache invalidation failure during a direct I/O write.
     */
    void dio_warn_stale_pagecache(struct file *filp)
    {
    	static DEFINE_RATELIMIT_STATE(_rs, 86400 * HZ, DEFAULT_RATELIMIT_BURST);
    	char pathname[128];
    	char *path;
    
    	errseq_set(&filp->f_mapping->wb_err, -EIO);
    	if (__ratelimit(&_rs)) {
    		path = file_path(filp, pathname, sizeof(pathname));
    		if (IS_ERR(path))
    			path = "(unknown)";
    		pr_crit("Page cache invalidation failure on direct I/O.  Possible data corruption due to collision with buffered I/O!\n");
    		pr_crit("File: %s PID: %d Comm: %.20s\n", path, current->pid,
    			current->comm);
    	}
    }
    
    ssize_t
    generic_file_direct_write(struct kiocb *iocb, struct iov_iter *from)
    {
    	struct file	*file = iocb->ki_filp;
    	struct address_space *mapping = file->f_mapping;
    	struct inode	*inode = mapping->host;
    	loff_t		pos = iocb->ki_pos;
    	ssize_t		written;
    	size_t		write_len;
    	pgoff_t		end;
    
    	write_len = iov_iter_count(from);
    	end = (pos + write_len - 1) >> PAGE_SHIFT;
    
    	if (iocb->ki_flags & IOCB_NOWAIT) {
    		/* If there are pages to writeback, return */
    		if (filemap_range_has_page(file->f_mapping, pos,
    					   pos + write_len - 1))
    			return -EAGAIN;
    	} else {
    		written = filemap_write_and_wait_range(mapping, pos,
    							pos + write_len - 1);
    		if (written)
    			goto out;
    	}
    
    	/*
    	 * After a write we want buffered reads to be sure to go to disk to get
    	 * the new data.  We invalidate clean cached page from the region we're
    	 * about to write.  We do this *before* the write so that we can return
    	 * without clobbering -EIOCBQUEUED from ->direct_IO().
    	 */
    	written = invalidate_inode_pages2_range(mapping,
    					pos >> PAGE_SHIFT, end);
    	/*
    	 * If a page can not be invalidated, return 0 to fall back
    	 * to buffered write.
    	 */
    	if (written) {
    		if (written == -EBUSY)
    			return 0;
    		goto out;
    	}
    
    	written = mapping->a_ops->direct_IO(iocb, from);
    
    	/*
    	 * Finally, try again to invalidate clean pages which might have been
    	 * cached by non-direct readahead, or faulted in by get_user_pages()
    	 * if the source of the write was an mmap'ed region of the file
    	 * we're writing.  Either one is a pretty crazy thing to do,
    	 * so we don't support it 100%.  If this invalidation
    	 * fails, tough, the write still worked...
    	 *
    	 * Most of the time we do not need this since dio_complete() will do
    	 * the invalidation for us. However there are some file systems that
    	 * do not end up with dio_complete() being called, so let's not break
    	 * them by removing it completely.
    	 *
    	 * Noticeable example is a blkdev_direct_IO().
    	 *
    	 * Skip invalidation for async writes or if mapping has no pages.
    	 */
    	if (written > 0 && mapping->nrpages &&
    	    invalidate_inode_pages2_range(mapping, pos >> PAGE_SHIFT, end))
    		dio_warn_stale_pagecache(file);
    
    	if (written > 0) {
    		pos += written;
    		write_len -= written;
    		if (pos > i_size_read(inode) && !S_ISBLK(inode->i_mode)) {
    			i_size_write(inode, pos);
    			mark_inode_dirty(inode);
    		}
    		iocb->ki_pos = pos;
    	}
    	if (written != -EIOCBQUEUED)
    		iov_iter_revert(from, write_len - iov_iter_count(from));
    out:
    	return written;
    }
    EXPORT_SYMBOL(generic_file_direct_write);
    
    /*
     * Find or create a page at the given pagecache position. Return the locked
     * page. This function is specifically for buffered writes.
     */
    struct page *grab_cache_page_write_begin(struct address_space *mapping,
    					pgoff_t index, unsigned flags)
    {
    	struct page *page;
    	int fgp_flags = FGP_LOCK|FGP_WRITE|FGP_CREAT;
    
    	if (flags & AOP_FLAG_NOFS)
    		fgp_flags |= FGP_NOFS;
    
    	page = pagecache_get_page(mapping, index, fgp_flags,
    			mapping_gfp_mask(mapping));
    	if (page)
    		wait_for_stable_page(page);
    
    	return page;
    }
    EXPORT_SYMBOL(grab_cache_page_write_begin);
    
    ssize_t generic_perform_write(struct file *file,
    				struct iov_iter *i, loff_t pos)
    {
    	struct address_space *mapping = file->f_mapping;
    	const struct address_space_operations *a_ops = mapping->a_ops;
    	long status = 0;
    	ssize_t written = 0;
    	unsigned int flags = 0;
    
    	do {
    		struct page *page;
    		unsigned long offset;	/* Offset into pagecache page */
    		unsigned long bytes;	/* Bytes to write to page */
    		size_t copied;		/* Bytes copied from user */
    		void *fsdata;
    
    		offset = (pos & (PAGE_SIZE - 1));
    		bytes = min_t(unsigned long, PAGE_SIZE - offset,
    						iov_iter_count(i));
    
    again:
    		/*
    		 * Bring in the user page that we will copy from _first_.
    		 * Otherwise there's a nasty deadlock on copying from the
    		 * same page as we're writing to, without it being marked
    		 * up-to-date.
    		 *
    		 * Not only is this an optimisation, but it is also required
    		 * to check that the address is actually valid, when atomic
    		 * usercopies are used, below.
    		 */
    		if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
    			status = -EFAULT;
    			break;
    		}
    
    		if (fatal_signal_pending(current)) {
    			status = -EINTR;
    			break;
    		}
    
    		status = a_ops->write_begin(file, mapping, pos, bytes, flags,
    						&page, &fsdata);
    		if (unlikely(status < 0))
    			break;
    
    		if (mapping_writably_mapped(mapping))
    			flush_dcache_page(page);
    
    		copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
    		flush_dcache_page(page);
    
    		status = a_ops->write_end(file, mapping, pos, bytes, copied,
    						page, fsdata);
    		if (unlikely(status < 0))
    			break;
    		copied = status;
    
    		cond_resched();
    
    		iov_iter_advance(i, copied);
    		if (unlikely(copied == 0)) {
    			/*
    			 * If we were unable to copy any data at all, we must
    			 * fall back to a single segment length write.
    			 *
    			 * If we didn't fallback here, we could livelock
    			 * because not all segments in the iov can be copied at
    			 * once without a pagefault.
    			 */
    			bytes = min_t(unsigned long, PAGE_SIZE - offset,
    						iov_iter_single_seg_count(i));
    			goto again;
    		}
    		pos += copied;
    		written += copied;
    
    		balance_dirty_pages_ratelimited(mapping);
    	} while (iov_iter_count(i));
    
    	return written ? written : status;
    }
    EXPORT_SYMBOL(generic_perform_write);
    
    /**
     * __generic_file_write_iter - write data to a file
     * @iocb:	IO state structure (file, offset, etc.)
     * @from:	iov_iter with data to write
     *
     * This function does all the work needed for actually writing data to a
     * file. It does all basic checks, removes SUID from the file, updates
     * modification times and calls proper subroutines depending on whether we
     * do direct IO or a standard buffered write.
     *
     * It expects i_mutex to be grabbed unless we work on a block device or similar
     * object which does not need locking at all.
     *
     * This function does *not* take care of syncing data in case of O_SYNC write.
     * A caller has to handle it. This is mainly due to the fact that we want to
     * avoid syncing under i_mutex.
     *
     * Return:
     * * number of bytes written, even for truncated writes
     * * negative error code if no data has been written at all
     */
    ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
    {
    	struct file *file = iocb->ki_filp;
    	struct address_space * mapping = file->f_mapping;
    	struct inode 	*inode = mapping->host;
    	ssize_t		written = 0;
    	ssize_t		err;
    	ssize_t		status;
    
    	/* We can write back this queue in page reclaim */
    	current->backing_dev_info = inode_to_bdi(inode);
    	err = file_remove_privs(file);
    	if (err)
    		goto out;
    
    	err = file_update_time(file);
    	if (err)
    		goto out;
    
    	if (iocb->ki_flags & IOCB_DIRECT) {
    		loff_t pos, endbyte;
    
    		written = generic_file_direct_write(iocb, from);
    		/*
    		 * If the write stopped short of completing, fall back to
    		 * buffered writes.  Some filesystems do this for writes to
    		 * holes, for example.  For DAX files, a buffered write will
    		 * not succeed (even if it did, DAX does not handle dirty
    		 * page-cache pages correctly).
    		 */
    		if (written < 0 || !iov_iter_count(from) || IS_DAX(inode))
    			goto out;
    
    		status = generic_perform_write(file, from, pos = iocb->ki_pos);
    		/*
    		 * If generic_perform_write() returned a synchronous error
    		 * then we want to return the number of bytes which were
    		 * direct-written, or the error code if that was zero.  Note
    		 * that this differs from normal direct-io semantics, which
    		 * will return -EFOO even if some bytes were written.
    		 */
    		if (unlikely(status < 0)) {
    			err = status;
    			goto out;
    		}
    		/*
    		 * We need to ensure that the page cache pages are written to
    		 * disk and invalidated to preserve the expected O_DIRECT
    		 * semantics.
    		 */
    		endbyte = pos + status - 1;
    		err = filemap_write_and_wait_range(mapping, pos, endbyte);
    		if (err == 0) {
    			iocb->ki_pos = endbyte + 1;
    			written += status;
    			invalidate_mapping_pages(mapping,
    						 pos >> PAGE_SHIFT,
    						 endbyte >> PAGE_SHIFT);
    		} else {
    			/*
    			 * We don't know how much we wrote, so just return
    			 * the number of bytes which were direct-written
    			 */
    		}
    	} else {
    		written = generic_perform_write(file, from, iocb->ki_pos);
    		if (likely(written > 0))
    			iocb->ki_pos += written;
    	}
    out:
    	current->backing_dev_info = NULL;
    	return written ? written : err;
    }
    EXPORT_SYMBOL(__generic_file_write_iter);
    
    /**
     * generic_file_write_iter - write data to a file
     * @iocb:	IO state structure
     * @from:	iov_iter with data to write
     *
     * This is a wrapper around __generic_file_write_iter() to be used by most
     * filesystems. It takes care of syncing the file in case of O_SYNC file
     * and acquires i_mutex as needed.
     * Return:
     * * negative error code if no data has been written at all of
     *   vfs_fsync_range() failed for a synchronous write
     * * number of bytes written, even for truncated writes
     */
    ssize_t generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
    {
    	struct file *file = iocb->ki_filp;
    	struct inode *inode = file->f_mapping->host;
    	ssize_t ret;
    
    	inode_lock(inode);
    	ret = generic_write_checks(iocb, from);
    	if (ret > 0)
    		ret = __generic_file_write_iter(iocb, from);
    	inode_unlock(inode);
    
    	if (ret > 0)
    		ret = generic_write_sync(iocb, ret);
    	return ret;
    }
    EXPORT_SYMBOL(generic_file_write_iter);
    
    /**
     * try_to_release_page() - release old fs-specific metadata on a page
     *
     * @page: the page which the kernel is trying to free
     * @gfp_mask: memory allocation flags (and I/O mode)
     *
     * The address_space is to try to release any data against the page
     * (presumably at page->private).
     *
     * This may also be called if PG_fscache is set on a page, indicating that the
     * page is known to the local caching routines.
     *
     * The @gfp_mask argument specifies whether I/O may be performed to release
     * this page (__GFP_IO), and whether the call may block (__GFP_RECLAIM & __GFP_FS).
     *
     * Return: %1 if the release was successful, otherwise return zero.
     */
    int try_to_release_page(struct page *page, gfp_t gfp_mask)
    {
    	struct address_space * const mapping = page->mapping;
    
    	BUG_ON(!PageLocked(page));
    	if (PageWriteback(page))
    		return 0;
    
    	if (mapping && mapping->a_ops->releasepage)
    		return mapping->a_ops->releasepage(page, gfp_mask);
    	return try_to_free_buffers(page);
    }
    
    EXPORT_SYMBOL(try_to_release_page);