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32 results

vmscan.c

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  • vmscan.c 43.89 KiB
    /*
     *  linux/mm/vmscan.c
     *
     *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
     *
     *  Swap reorganised 29.12.95, Stephen Tweedie.
     *  kswapd added: 7.1.96  sct
     *  Removed kswapd_ctl limits, and swap out as many pages as needed
     *  to bring the system back to freepages.high: 2.4.97, Rik van Riel.
     *  Zone aware kswapd started 02/00, Kanoj Sarcar (kanoj@sgi.com).
     *  Multiqueue VM started 5.8.00, Rik van Riel.
     */
    
    #include <linux/mm.h>
    #include <linux/module.h>
    #include <linux/slab.h>
    #include <linux/kernel_stat.h>
    #include <linux/swap.h>
    #include <linux/pagemap.h>
    #include <linux/init.h>
    #include <linux/highmem.h>
    #include <linux/file.h>
    #include <linux/writeback.h>
    #include <linux/blkdev.h>
    #include <linux/buffer_head.h>	/* for try_to_release_page(),
    					buffer_heads_over_limit */
    #include <linux/mm_inline.h>
    #include <linux/pagevec.h>
    #include <linux/backing-dev.h>
    #include <linux/rmap.h>
    #include <linux/topology.h>
    #include <linux/cpu.h>
    #include <linux/cpuset.h>
    #include <linux/notifier.h>
    #include <linux/rwsem.h>
    #include <linux/delay.h>
    #include <linux/kthread.h>
    
    #include <asm/tlbflush.h>
    #include <asm/div64.h>
    
    #include <linux/swapops.h>
    
    #include "internal.h"
    
    struct scan_control {
    	/* Incremented by the number of inactive pages that were scanned */
    	unsigned long nr_scanned;
    
    	/* This context's GFP mask */
    	gfp_t gfp_mask;
    
    	int may_writepage;
    
    	/* Can pages be swapped as part of reclaim? */
    	int may_swap;
    
    	/* This context's SWAP_CLUSTER_MAX. If freeing memory for
    	 * suspend, we effectively ignore SWAP_CLUSTER_MAX.
    	 * In this context, it doesn't matter that we scan the
    	 * whole list at once. */
    	int swap_cluster_max;
    
    	int swappiness;
    };
    
    /*
     * The list of shrinker callbacks used by to apply pressure to
     * ageable caches.
     */
    struct shrinker {
    	shrinker_t		shrinker;
    	struct list_head	list;
    	int			seeks;	/* seeks to recreate an obj */
    	long			nr;	/* objs pending delete */
    };
    
    #define lru_to_page(_head) (list_entry((_head)->prev, struct page, lru))
    
    #ifdef ARCH_HAS_PREFETCH
    #define prefetch_prev_lru_page(_page, _base, _field)			\
    	do {								\
    		if ((_page)->lru.prev != _base) {			\
    			struct page *prev;				\
    									\
    			prev = lru_to_page(&(_page->lru));		\
    			prefetch(&prev->_field);			\
    		}							\
    	} while (0)
    #else
    #define prefetch_prev_lru_page(_page, _base, _field) do { } while (0)
    #endif
    
    #ifdef ARCH_HAS_PREFETCHW
    #define prefetchw_prev_lru_page(_page, _base, _field)			\
    	do {								\
    		if ((_page)->lru.prev != _base) {			\
    			struct page *prev;				\
    									\
    			prev = lru_to_page(&(_page->lru));		\
    			prefetchw(&prev->_field);			\
    		}							\
    	} while (0)
    #else
    #define prefetchw_prev_lru_page(_page, _base, _field) do { } while (0)
    #endif
    
    /*
     * From 0 .. 100.  Higher means more swappy.
     */
    int vm_swappiness = 60;
    long vm_total_pages;	/* The total number of pages which the VM controls */
    
    static LIST_HEAD(shrinker_list);
    static DECLARE_RWSEM(shrinker_rwsem);
    
    /*
     * Add a shrinker callback to be called from the vm
     */
    struct shrinker *set_shrinker(int seeks, shrinker_t theshrinker)
    {
            struct shrinker *shrinker;
    
            shrinker = kmalloc(sizeof(*shrinker), GFP_KERNEL);
            if (shrinker) {
    	        shrinker->shrinker = theshrinker;
    	        shrinker->seeks = seeks;
    	        shrinker->nr = 0;
    	        down_write(&shrinker_rwsem);
    	        list_add_tail(&shrinker->list, &shrinker_list);
    	        up_write(&shrinker_rwsem);
    	}
    	return shrinker;
    }
    EXPORT_SYMBOL(set_shrinker);
    
    /*
     * Remove one
     */
    void remove_shrinker(struct shrinker *shrinker)
    {
    	down_write(&shrinker_rwsem);
    	list_del(&shrinker->list);
    	up_write(&shrinker_rwsem);
    	kfree(shrinker);
    }
    EXPORT_SYMBOL(remove_shrinker);
    
    #define SHRINK_BATCH 128
    /*
     * Call the shrink functions to age shrinkable caches
     *
     * Here we assume it costs one seek to replace a lru page and that it also
     * takes a seek to recreate a cache object.  With this in mind we age equal
     * percentages of the lru and ageable caches.  This should balance the seeks
     * generated by these structures.
     *
     * If the vm encounted mapped pages on the LRU it increase the pressure on
     * slab to avoid swapping.
     *
     * We do weird things to avoid (scanned*seeks*entries) overflowing 32 bits.
     *
     * `lru_pages' represents the number of on-LRU pages in all the zones which
     * are eligible for the caller's allocation attempt.  It is used for balancing
     * slab reclaim versus page reclaim.
     *
     * Returns the number of slab objects which we shrunk.
     */
    unsigned long shrink_slab(unsigned long scanned, gfp_t gfp_mask,
    			unsigned long lru_pages)
    {
    	struct shrinker *shrinker;
    	unsigned long ret = 0;
    
    	if (scanned == 0)
    		scanned = SWAP_CLUSTER_MAX;
    
    	if (!down_read_trylock(&shrinker_rwsem))
    		return 1;	/* Assume we'll be able to shrink next time */
    
    	list_for_each_entry(shrinker, &shrinker_list, list) {
    		unsigned long long delta;
    		unsigned long total_scan;
    		unsigned long max_pass = (*shrinker->shrinker)(0, gfp_mask);
    
    		delta = (4 * scanned) / shrinker->seeks;
    		delta *= max_pass;
    		do_div(delta, lru_pages + 1);
    		shrinker->nr += delta;
    		if (shrinker->nr < 0) {
    			printk(KERN_ERR "%s: nr=%ld\n",
    					__FUNCTION__, shrinker->nr);
    			shrinker->nr = max_pass;
    		}
    
    		/*
    		 * Avoid risking looping forever due to too large nr value:
    		 * never try to free more than twice the estimate number of
    		 * freeable entries.
    		 */
    		if (shrinker->nr > max_pass * 2)
    			shrinker->nr = max_pass * 2;
    
    		total_scan = shrinker->nr;
    		shrinker->nr = 0;
    
    		while (total_scan >= SHRINK_BATCH) {
    			long this_scan = SHRINK_BATCH;
    			int shrink_ret;
    			int nr_before;
    
    			nr_before = (*shrinker->shrinker)(0, gfp_mask);
    			shrink_ret = (*shrinker->shrinker)(this_scan, gfp_mask);
    			if (shrink_ret == -1)
    				break;
    			if (shrink_ret < nr_before)
    				ret += nr_before - shrink_ret;
    			mod_page_state(slabs_scanned, this_scan);
    			total_scan -= this_scan;
    
    			cond_resched();
    		}
    
    		shrinker->nr += total_scan;
    	}
    	up_read(&shrinker_rwsem);
    	return ret;
    }
    
    /* Called without lock on whether page is mapped, so answer is unstable */
    static inline int page_mapping_inuse(struct page *page)
    {
    	struct address_space *mapping;
    
    	/* Page is in somebody's page tables. */
    	if (page_mapped(page))
    		return 1;
    
    	/* Be more reluctant to reclaim swapcache than pagecache */
    	if (PageSwapCache(page))
    		return 1;
    
    	mapping = page_mapping(page);
    	if (!mapping)
    		return 0;
    
    	/* File is mmap'd by somebody? */
    	return mapping_mapped(mapping);
    }
    
    static inline int is_page_cache_freeable(struct page *page)
    {
    	return page_count(page) - !!PagePrivate(page) == 2;
    }
    
    static int may_write_to_queue(struct backing_dev_info *bdi)
    {
    	if (current->flags & PF_SWAPWRITE)
    		return 1;
    	if (!bdi_write_congested(bdi))
    		return 1;
    	if (bdi == current->backing_dev_info)
    		return 1;
    	return 0;
    }
    
    /*
     * We detected a synchronous write error writing a page out.  Probably
     * -ENOSPC.  We need to propagate that into the address_space for a subsequent
     * fsync(), msync() or close().
     *
     * The tricky part is that after writepage we cannot touch the mapping: nothing
     * prevents it from being freed up.  But we have a ref on the page and once
     * that page is locked, the mapping is pinned.
     *
     * We're allowed to run sleeping lock_page() here because we know the caller has
     * __GFP_FS.
     */
    static void handle_write_error(struct address_space *mapping,
    				struct page *page, int error)
    {
    	lock_page(page);
    	if (page_mapping(page) == mapping) {
    		if (error == -ENOSPC)
    			set_bit(AS_ENOSPC, &mapping->flags);
    		else
    			set_bit(AS_EIO, &mapping->flags);
    	}
    	unlock_page(page);
    }
    
    /* possible outcome of pageout() */
    typedef enum {
    	/* failed to write page out, page is locked */
    	PAGE_KEEP,
    	/* move page to the active list, page is locked */
    	PAGE_ACTIVATE,
    	/* page has been sent to the disk successfully, page is unlocked */
    	PAGE_SUCCESS,
    	/* page is clean and locked */
    	PAGE_CLEAN,
    } pageout_t;
    
    /*
     * pageout is called by shrink_page_list() for each dirty page.
     * Calls ->writepage().
     */
    static pageout_t pageout(struct page *page, struct address_space *mapping)
    {
    	/*
    	 * If the page is dirty, only perform writeback if that write
    	 * will be non-blocking.  To prevent this allocation from being
    	 * stalled by pagecache activity.  But note that there may be
    	 * stalls if we need to run get_block().  We could test
    	 * PagePrivate for that.
    	 *
    	 * If this process is currently in generic_file_write() against
    	 * this page's queue, we can perform writeback even if that
    	 * will block.
    	 *
    	 * If the page is swapcache, write it back even if that would
    	 * block, for some throttling. This happens by accident, because
    	 * swap_backing_dev_info is bust: it doesn't reflect the
    	 * congestion state of the swapdevs.  Easy to fix, if needed.
    	 * See swapfile.c:page_queue_congested().
    	 */
    	if (!is_page_cache_freeable(page))
    		return PAGE_KEEP;
    	if (!mapping) {
    		/*
    		 * Some data journaling orphaned pages can have
    		 * page->mapping == NULL while being dirty with clean buffers.
    		 */
    		if (PagePrivate(page)) {
    			if (try_to_free_buffers(page)) {
    				ClearPageDirty(page);
    				printk("%s: orphaned page\n", __FUNCTION__);
    				return PAGE_CLEAN;
    			}
    		}
    		return PAGE_KEEP;
    	}
    	if (mapping->a_ops->writepage == NULL)
    		return PAGE_ACTIVATE;
    	if (!may_write_to_queue(mapping->backing_dev_info))
    		return PAGE_KEEP;
    
    	if (clear_page_dirty_for_io(page)) {
    		int res;
    		struct writeback_control wbc = {
    			.sync_mode = WB_SYNC_NONE,
    			.nr_to_write = SWAP_CLUSTER_MAX,
    			.range_start = 0,
    			.range_end = LLONG_MAX,
    			.nonblocking = 1,
    			.for_reclaim = 1,
    		};
    
    		SetPageReclaim(page);
    		res = mapping->a_ops->writepage(page, &wbc);
    		if (res < 0)
    			handle_write_error(mapping, page, res);
    		if (res == AOP_WRITEPAGE_ACTIVATE) {
    			ClearPageReclaim(page);
    			return PAGE_ACTIVATE;
    		}
    		if (!PageWriteback(page)) {
    			/* synchronous write or broken a_ops? */
    			ClearPageReclaim(page);
    		}
    
    		return PAGE_SUCCESS;
    	}
    
    	return PAGE_CLEAN;
    }
    
    int remove_mapping(struct address_space *mapping, struct page *page)
    {
    	if (!mapping)
    		return 0;		/* truncate got there first */
    
    	write_lock_irq(&mapping->tree_lock);
    
    	/*
    	 * The non-racy check for busy page.  It is critical to check
    	 * PageDirty _after_ making sure that the page is freeable and
    	 * not in use by anybody. 	(pagecache + us == 2)
    	 */
    	if (unlikely(page_count(page) != 2))
    		goto cannot_free;
    	smp_rmb();
    	if (unlikely(PageDirty(page)))
    		goto cannot_free;
    
    	if (PageSwapCache(page)) {
    		swp_entry_t swap = { .val = page_private(page) };
    		__delete_from_swap_cache(page);
    		write_unlock_irq(&mapping->tree_lock);
    		swap_free(swap);
    		__put_page(page);	/* The pagecache ref */
    		return 1;
    	}
    
    	__remove_from_page_cache(page);
    	write_unlock_irq(&mapping->tree_lock);
    	__put_page(page);
    	return 1;
    
    cannot_free:
    	write_unlock_irq(&mapping->tree_lock);
    	return 0;
    }
    
    /*
     * shrink_page_list() returns the number of reclaimed pages
     */
    static unsigned long shrink_page_list(struct list_head *page_list,
    					struct scan_control *sc)
    {
    	LIST_HEAD(ret_pages);
    	struct pagevec freed_pvec;
    	int pgactivate = 0;
    	unsigned long nr_reclaimed = 0;
    
    	cond_resched();
    
    	pagevec_init(&freed_pvec, 1);
    	while (!list_empty(page_list)) {
    		struct address_space *mapping;
    		struct page *page;
    		int may_enter_fs;
    		int referenced;
    
    		cond_resched();
    
    		page = lru_to_page(page_list);
    		list_del(&page->lru);
    
    		if (TestSetPageLocked(page))
    			goto keep;
    
    		BUG_ON(PageActive(page));
    
    		sc->nr_scanned++;
    
    		if (!sc->may_swap && page_mapped(page))
    			goto keep_locked;
    
    		/* Double the slab pressure for mapped and swapcache pages */
    		if (page_mapped(page) || PageSwapCache(page))
    			sc->nr_scanned++;
    
    		if (PageWriteback(page))
    			goto keep_locked;
    
    		referenced = page_referenced(page, 1);
    		/* In active use or really unfreeable?  Activate it. */
    		if (referenced && page_mapping_inuse(page))
    			goto activate_locked;
    
    #ifdef CONFIG_SWAP
    		/*
    		 * Anonymous process memory has backing store?
    		 * Try to allocate it some swap space here.
    		 */
    		if (PageAnon(page) && !PageSwapCache(page))
    			if (!add_to_swap(page, GFP_ATOMIC))
    				goto activate_locked;
    #endif /* CONFIG_SWAP */
    
    		mapping = page_mapping(page);
    		may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
    			(PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
    
    		/*
    		 * The page is mapped into the page tables of one or more
    		 * processes. Try to unmap it here.
    		 */
    		if (page_mapped(page) && mapping) {
    			switch (try_to_unmap(page, 0)) {
    			case SWAP_FAIL:
    				goto activate_locked;
    			case SWAP_AGAIN:
    				goto keep_locked;
    			case SWAP_SUCCESS:
    				; /* try to free the page below */
    			}
    		}
    
    		if (PageDirty(page)) {
    			if (referenced)
    				goto keep_locked;
    			if (!may_enter_fs)
    				goto keep_locked;
    			if (!sc->may_writepage)
    				goto keep_locked;
    
    			/* Page is dirty, try to write it out here */
    			switch(pageout(page, mapping)) {
    			case PAGE_KEEP:
    				goto keep_locked;
    			case PAGE_ACTIVATE:
    				goto activate_locked;
    			case PAGE_SUCCESS:
    				if (PageWriteback(page) || PageDirty(page))
    					goto keep;
    				/*
    				 * A synchronous write - probably a ramdisk.  Go
    				 * ahead and try to reclaim the page.
    				 */
    				if (TestSetPageLocked(page))
    					goto keep;
    				if (PageDirty(page) || PageWriteback(page))
    					goto keep_locked;
    				mapping = page_mapping(page);
    			case PAGE_CLEAN:
    				; /* try to free the page below */
    			}
    		}
    
    		/*
    		 * If the page has buffers, try to free the buffer mappings
    		 * associated with this page. If we succeed we try to free
    		 * the page as well.
    		 *
    		 * We do this even if the page is PageDirty().
    		 * try_to_release_page() does not perform I/O, but it is
    		 * possible for a page to have PageDirty set, but it is actually
    		 * clean (all its buffers are clean).  This happens if the
    		 * buffers were written out directly, with submit_bh(). ext3
    		 * will do this, as well as the blockdev mapping. 
    		 * try_to_release_page() will discover that cleanness and will
    		 * drop the buffers and mark the page clean - it can be freed.
    		 *
    		 * Rarely, pages can have buffers and no ->mapping.  These are
    		 * the pages which were not successfully invalidated in
    		 * truncate_complete_page().  We try to drop those buffers here
    		 * and if that worked, and the page is no longer mapped into
    		 * process address space (page_count == 1) it can be freed.
    		 * Otherwise, leave the page on the LRU so it is swappable.
    		 */
    		if (PagePrivate(page)) {
    			if (!try_to_release_page(page, sc->gfp_mask))
    				goto activate_locked;
    			if (!mapping && page_count(page) == 1)
    				goto free_it;
    		}
    
    		if (!remove_mapping(mapping, page))
    			goto keep_locked;
    
    free_it:
    		unlock_page(page);
    		nr_reclaimed++;
    		if (!pagevec_add(&freed_pvec, page))
    			__pagevec_release_nonlru(&freed_pvec);
    		continue;
    
    activate_locked:
    		SetPageActive(page);
    		pgactivate++;
    keep_locked:
    		unlock_page(page);
    keep:
    		list_add(&page->lru, &ret_pages);
    		BUG_ON(PageLRU(page));
    	}
    	list_splice(&ret_pages, page_list);
    	if (pagevec_count(&freed_pvec))
    		__pagevec_release_nonlru(&freed_pvec);
    	mod_page_state(pgactivate, pgactivate);
    	return nr_reclaimed;
    }
    
    /*
     * zone->lru_lock is heavily contended.  Some of the functions that
     * shrink the lists perform better by taking out a batch of pages
     * and working on them outside the LRU lock.
     *
     * For pagecache intensive workloads, this function is the hottest
     * spot in the kernel (apart from copy_*_user functions).
     *
     * Appropriate locks must be held before calling this function.
     *
     * @nr_to_scan:	The number of pages to look through on the list.
     * @src:	The LRU list to pull pages off.
     * @dst:	The temp list to put pages on to.
     * @scanned:	The number of pages that were scanned.
     *
     * returns how many pages were moved onto *@dst.
     */
    static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
    		struct list_head *src, struct list_head *dst,
    		unsigned long *scanned)
    {
    	unsigned long nr_taken = 0;
    	struct page *page;
    	unsigned long scan;
    
    	for (scan = 0; scan < nr_to_scan && !list_empty(src); scan++) {
    		struct list_head *target;
    		page = lru_to_page(src);
    		prefetchw_prev_lru_page(page, src, flags);
    
    		BUG_ON(!PageLRU(page));
    
    		list_del(&page->lru);
    		target = src;
    		if (likely(get_page_unless_zero(page))) {
    			/*
    			 * Be careful not to clear PageLRU until after we're
    			 * sure the page is not being freed elsewhere -- the
    			 * page release code relies on it.
    			 */
    			ClearPageLRU(page);
    			target = dst;
    			nr_taken++;
    		} /* else it is being freed elsewhere */
    
    		list_add(&page->lru, target);
    	}
    
    	*scanned = scan;
    	return nr_taken;
    }
    
    /*
     * shrink_inactive_list() is a helper for shrink_zone().  It returns the number
     * of reclaimed pages
     */
    static unsigned long shrink_inactive_list(unsigned long max_scan,
    				struct zone *zone, struct scan_control *sc)
    {
    	LIST_HEAD(page_list);
    	struct pagevec pvec;
    	unsigned long nr_scanned = 0;
    	unsigned long nr_reclaimed = 0;
    
    	pagevec_init(&pvec, 1);
    
    	lru_add_drain();
    	spin_lock_irq(&zone->lru_lock);
    	do {
    		struct page *page;
    		unsigned long nr_taken;
    		unsigned long nr_scan;
    		unsigned long nr_freed;
    
    		nr_taken = isolate_lru_pages(sc->swap_cluster_max,
    					     &zone->inactive_list,
    					     &page_list, &nr_scan);
    		zone->nr_inactive -= nr_taken;
    		zone->pages_scanned += nr_scan;
    		spin_unlock_irq(&zone->lru_lock);
    
    		nr_scanned += nr_scan;
    		nr_freed = shrink_page_list(&page_list, sc);
    		nr_reclaimed += nr_freed;
    		local_irq_disable();
    		if (current_is_kswapd()) {
    			__mod_page_state_zone(zone, pgscan_kswapd, nr_scan);
    			__mod_page_state(kswapd_steal, nr_freed);
    		} else
    			__mod_page_state_zone(zone, pgscan_direct, nr_scan);
    		__mod_page_state_zone(zone, pgsteal, nr_freed);
    
    		if (nr_taken == 0)
    			goto done;
    
    		spin_lock(&zone->lru_lock);
    		/*
    		 * Put back any unfreeable pages.
    		 */
    		while (!list_empty(&page_list)) {
    			page = lru_to_page(&page_list);
    			BUG_ON(PageLRU(page));
    			SetPageLRU(page);
    			list_del(&page->lru);
    			if (PageActive(page))
    				add_page_to_active_list(zone, page);
    			else
    				add_page_to_inactive_list(zone, page);
    			if (!pagevec_add(&pvec, page)) {
    				spin_unlock_irq(&zone->lru_lock);
    				__pagevec_release(&pvec);
    				spin_lock_irq(&zone->lru_lock);
    			}
    		}
      	} while (nr_scanned < max_scan);
    	spin_unlock(&zone->lru_lock);
    done:
    	local_irq_enable();
    	pagevec_release(&pvec);
    	return nr_reclaimed;
    }
    
    /*
     * This moves pages from the active list to the inactive list.
     *
     * We move them the other way if the page is referenced by one or more
     * processes, from rmap.
     *
     * If the pages are mostly unmapped, the processing is fast and it is
     * appropriate to hold zone->lru_lock across the whole operation.  But if
     * the pages are mapped, the processing is slow (page_referenced()) so we
     * should drop zone->lru_lock around each page.  It's impossible to balance
     * this, so instead we remove the pages from the LRU while processing them.
     * It is safe to rely on PG_active against the non-LRU pages in here because
     * nobody will play with that bit on a non-LRU page.
     *
     * The downside is that we have to touch page->_count against each page.
     * But we had to alter page->flags anyway.
     */
    static void shrink_active_list(unsigned long nr_pages, struct zone *zone,
    				struct scan_control *sc)
    {
    	unsigned long pgmoved;
    	int pgdeactivate = 0;
    	unsigned long pgscanned;
    	LIST_HEAD(l_hold);	/* The pages which were snipped off */
    	LIST_HEAD(l_inactive);	/* Pages to go onto the inactive_list */
    	LIST_HEAD(l_active);	/* Pages to go onto the active_list */
    	struct page *page;
    	struct pagevec pvec;
    	int reclaim_mapped = 0;
    
    	if (sc->may_swap) {
    		long mapped_ratio;
    		long distress;
    		long swap_tendency;
    
    		/*
    		 * `distress' is a measure of how much trouble we're having
    		 * reclaiming pages.  0 -> no problems.  100 -> great trouble.
    		 */
    		distress = 100 >> zone->prev_priority;
    
    		/*
    		 * The point of this algorithm is to decide when to start
    		 * reclaiming mapped memory instead of just pagecache.  Work out
    		 * how much memory
    		 * is mapped.
    		 */
    		mapped_ratio = (global_page_state(NR_FILE_MAPPED) * 100) /
    					vm_total_pages;
    
    		/*
    		 * Now decide how much we really want to unmap some pages.  The
    		 * mapped ratio is downgraded - just because there's a lot of
    		 * mapped memory doesn't necessarily mean that page reclaim
    		 * isn't succeeding.
    		 *
    		 * The distress ratio is important - we don't want to start
    		 * going oom.
    		 *
    		 * A 100% value of vm_swappiness overrides this algorithm
    		 * altogether.
    		 */
    		swap_tendency = mapped_ratio / 2 + distress + sc->swappiness;
    
    		/*
    		 * Now use this metric to decide whether to start moving mapped
    		 * memory onto the inactive list.
    		 */
    		if (swap_tendency >= 100)
    			reclaim_mapped = 1;
    	}
    
    	lru_add_drain();
    	spin_lock_irq(&zone->lru_lock);
    	pgmoved = isolate_lru_pages(nr_pages, &zone->active_list,
    				    &l_hold, &pgscanned);
    	zone->pages_scanned += pgscanned;
    	zone->nr_active -= pgmoved;
    	spin_unlock_irq(&zone->lru_lock);
    
    	while (!list_empty(&l_hold)) {
    		cond_resched();
    		page = lru_to_page(&l_hold);
    		list_del(&page->lru);
    		if (page_mapped(page)) {
    			if (!reclaim_mapped ||
    			    (total_swap_pages == 0 && PageAnon(page)) ||
    			    page_referenced(page, 0)) {
    				list_add(&page->lru, &l_active);
    				continue;
    			}
    		}
    		list_add(&page->lru, &l_inactive);
    	}
    
    	pagevec_init(&pvec, 1);
    	pgmoved = 0;
    	spin_lock_irq(&zone->lru_lock);
    	while (!list_empty(&l_inactive)) {
    		page = lru_to_page(&l_inactive);
    		prefetchw_prev_lru_page(page, &l_inactive, flags);
    		BUG_ON(PageLRU(page));
    		SetPageLRU(page);
    		BUG_ON(!PageActive(page));
    		ClearPageActive(page);
    
    		list_move(&page->lru, &zone->inactive_list);
    		pgmoved++;
    		if (!pagevec_add(&pvec, page)) {
    			zone->nr_inactive += pgmoved;
    			spin_unlock_irq(&zone->lru_lock);
    			pgdeactivate += pgmoved;
    			pgmoved = 0;
    			if (buffer_heads_over_limit)
    				pagevec_strip(&pvec);
    			__pagevec_release(&pvec);
    			spin_lock_irq(&zone->lru_lock);
    		}
    	}
    	zone->nr_inactive += pgmoved;
    	pgdeactivate += pgmoved;
    	if (buffer_heads_over_limit) {
    		spin_unlock_irq(&zone->lru_lock);
    		pagevec_strip(&pvec);
    		spin_lock_irq(&zone->lru_lock);
    	}
    
    	pgmoved = 0;
    	while (!list_empty(&l_active)) {
    		page = lru_to_page(&l_active);
    		prefetchw_prev_lru_page(page, &l_active, flags);
    		BUG_ON(PageLRU(page));
    		SetPageLRU(page);
    		BUG_ON(!PageActive(page));
    		list_move(&page->lru, &zone->active_list);
    		pgmoved++;
    		if (!pagevec_add(&pvec, page)) {
    			zone->nr_active += pgmoved;
    			pgmoved = 0;
    			spin_unlock_irq(&zone->lru_lock);
    			__pagevec_release(&pvec);
    			spin_lock_irq(&zone->lru_lock);
    		}
    	}
    	zone->nr_active += pgmoved;
    	spin_unlock(&zone->lru_lock);
    
    	__mod_page_state_zone(zone, pgrefill, pgscanned);
    	__mod_page_state(pgdeactivate, pgdeactivate);
    	local_irq_enable();
    
    	pagevec_release(&pvec);
    }
    
    /*
     * This is a basic per-zone page freer.  Used by both kswapd and direct reclaim.
     */
    static unsigned long shrink_zone(int priority, struct zone *zone,
    				struct scan_control *sc)
    {
    	unsigned long nr_active;
    	unsigned long nr_inactive;
    	unsigned long nr_to_scan;
    	unsigned long nr_reclaimed = 0;
    
    	atomic_inc(&zone->reclaim_in_progress);
    
    	/*
    	 * Add one to `nr_to_scan' just to make sure that the kernel will
    	 * slowly sift through the active list.
    	 */
    	zone->nr_scan_active += (zone->nr_active >> priority) + 1;
    	nr_active = zone->nr_scan_active;
    	if (nr_active >= sc->swap_cluster_max)
    		zone->nr_scan_active = 0;
    	else
    		nr_active = 0;
    
    	zone->nr_scan_inactive += (zone->nr_inactive >> priority) + 1;
    	nr_inactive = zone->nr_scan_inactive;
    	if (nr_inactive >= sc->swap_cluster_max)
    		zone->nr_scan_inactive = 0;
    	else
    		nr_inactive = 0;
    
    	while (nr_active || nr_inactive) {
    		if (nr_active) {
    			nr_to_scan = min(nr_active,
    					(unsigned long)sc->swap_cluster_max);
    			nr_active -= nr_to_scan;
    			shrink_active_list(nr_to_scan, zone, sc);
    		}
    
    		if (nr_inactive) {
    			nr_to_scan = min(nr_inactive,
    					(unsigned long)sc->swap_cluster_max);
    			nr_inactive -= nr_to_scan;
    			nr_reclaimed += shrink_inactive_list(nr_to_scan, zone,
    								sc);
    		}
    	}
    
    	throttle_vm_writeout();
    
    	atomic_dec(&zone->reclaim_in_progress);
    	return nr_reclaimed;
    }
    
    /*
     * This is the direct reclaim path, for page-allocating processes.  We only
     * try to reclaim pages from zones which will satisfy the caller's allocation
     * request.
     *
     * We reclaim from a zone even if that zone is over pages_high.  Because:
     * a) The caller may be trying to free *extra* pages to satisfy a higher-order
     *    allocation or
     * b) The zones may be over pages_high but they must go *over* pages_high to
     *    satisfy the `incremental min' zone defense algorithm.
     *
     * Returns the number of reclaimed pages.
     *
     * If a zone is deemed to be full of pinned pages then just give it a light
     * scan then give up on it.
     */
    static unsigned long shrink_zones(int priority, struct zone **zones,
    					struct scan_control *sc)
    {
    	unsigned long nr_reclaimed = 0;
    	int i;
    
    	for (i = 0; zones[i] != NULL; i++) {
    		struct zone *zone = zones[i];
    
    		if (!populated_zone(zone))
    			continue;
    
    		if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
    			continue;
    
    		zone->temp_priority = priority;
    		if (zone->prev_priority > priority)
    			zone->prev_priority = priority;
    
    		if (zone->all_unreclaimable && priority != DEF_PRIORITY)
    			continue;	/* Let kswapd poll it */
    
    		nr_reclaimed += shrink_zone(priority, zone, sc);
    	}
    	return nr_reclaimed;
    }
     
    /*
     * This is the main entry point to direct page reclaim.
     *
     * If a full scan of the inactive list fails to free enough memory then we
     * are "out of memory" and something needs to be killed.
     *
     * If the caller is !__GFP_FS then the probability of a failure is reasonably
     * high - the zone may be full of dirty or under-writeback pages, which this
     * caller can't do much about.  We kick pdflush and take explicit naps in the
     * hope that some of these pages can be written.  But if the allocating task
     * holds filesystem locks which prevent writeout this might not work, and the
     * allocation attempt will fail.
     */
    unsigned long try_to_free_pages(struct zone **zones, gfp_t gfp_mask)
    {
    	int priority;
    	int ret = 0;
    	unsigned long total_scanned = 0;
    	unsigned long nr_reclaimed = 0;
    	struct reclaim_state *reclaim_state = current->reclaim_state;
    	unsigned long lru_pages = 0;
    	int i;
    	struct scan_control sc = {
    		.gfp_mask = gfp_mask,
    		.may_writepage = !laptop_mode,
    		.swap_cluster_max = SWAP_CLUSTER_MAX,
    		.may_swap = 1,
    		.swappiness = vm_swappiness,
    	};
    
    	inc_page_state(allocstall);
    
    	for (i = 0; zones[i] != NULL; i++) {
    		struct zone *zone = zones[i];
    
    		if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
    			continue;
    
    		zone->temp_priority = DEF_PRIORITY;
    		lru_pages += zone->nr_active + zone->nr_inactive;
    	}
    
    	for (priority = DEF_PRIORITY; priority >= 0; priority--) {
    		sc.nr_scanned = 0;
    		if (!priority)
    			disable_swap_token();
    		nr_reclaimed += shrink_zones(priority, zones, &sc);
    		shrink_slab(sc.nr_scanned, gfp_mask, lru_pages);
    		if (reclaim_state) {
    			nr_reclaimed += reclaim_state->reclaimed_slab;
    			reclaim_state->reclaimed_slab = 0;
    		}
    		total_scanned += sc.nr_scanned;
    		if (nr_reclaimed >= sc.swap_cluster_max) {
    			ret = 1;
    			goto out;
    		}
    
    		/*
    		 * Try to write back as many pages as we just scanned.  This
    		 * tends to cause slow streaming writers to write data to the
    		 * disk smoothly, at the dirtying rate, which is nice.   But
    		 * that's undesirable in laptop mode, where we *want* lumpy
    		 * writeout.  So in laptop mode, write out the whole world.
    		 */
    		if (total_scanned > sc.swap_cluster_max +
    					sc.swap_cluster_max / 2) {
    			wakeup_pdflush(laptop_mode ? 0 : total_scanned);
    			sc.may_writepage = 1;
    		}
    
    		/* Take a nap, wait for some writeback to complete */
    		if (sc.nr_scanned && priority < DEF_PRIORITY - 2)
    			blk_congestion_wait(WRITE, HZ/10);
    	}
    out:
    	for (i = 0; zones[i] != 0; i++) {
    		struct zone *zone = zones[i];
    
    		if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
    			continue;
    
    		zone->prev_priority = zone->temp_priority;
    	}
    	return ret;
    }
    
    /*
     * For kswapd, balance_pgdat() will work across all this node's zones until
     * they are all at pages_high.
     *
     * Returns the number of pages which were actually freed.
     *
     * There is special handling here for zones which are full of pinned pages.
     * This can happen if the pages are all mlocked, or if they are all used by
     * device drivers (say, ZONE_DMA).  Or if they are all in use by hugetlb.
     * What we do is to detect the case where all pages in the zone have been
     * scanned twice and there has been zero successful reclaim.  Mark the zone as
     * dead and from now on, only perform a short scan.  Basically we're polling
     * the zone for when the problem goes away.
     *
     * kswapd scans the zones in the highmem->normal->dma direction.  It skips
     * zones which have free_pages > pages_high, but once a zone is found to have
     * free_pages <= pages_high, we scan that zone and the lower zones regardless
     * of the number of free pages in the lower zones.  This interoperates with
     * the page allocator fallback scheme to ensure that aging of pages is balanced
     * across the zones.
     */
    static unsigned long balance_pgdat(pg_data_t *pgdat, int order)
    {
    	int all_zones_ok;
    	int priority;
    	int i;
    	unsigned long total_scanned;
    	unsigned long nr_reclaimed;
    	struct reclaim_state *reclaim_state = current->reclaim_state;
    	struct scan_control sc = {
    		.gfp_mask = GFP_KERNEL,
    		.may_swap = 1,
    		.swap_cluster_max = SWAP_CLUSTER_MAX,
    		.swappiness = vm_swappiness,
    	};
    
    loop_again:
    	total_scanned = 0;
    	nr_reclaimed = 0;
    	sc.may_writepage = !laptop_mode;
    	inc_page_state(pageoutrun);
    
    	for (i = 0; i < pgdat->nr_zones; i++) {
    		struct zone *zone = pgdat->node_zones + i;
    
    		zone->temp_priority = DEF_PRIORITY;
    	}
    
    	for (priority = DEF_PRIORITY; priority >= 0; priority--) {
    		int end_zone = 0;	/* Inclusive.  0 = ZONE_DMA */
    		unsigned long lru_pages = 0;
    
    		/* The swap token gets in the way of swapout... */
    		if (!priority)
    			disable_swap_token();
    
    		all_zones_ok = 1;
    
    		/*
    		 * Scan in the highmem->dma direction for the highest
    		 * zone which needs scanning
    		 */
    		for (i = pgdat->nr_zones - 1; i >= 0; i--) {
    			struct zone *zone = pgdat->node_zones + i;
    
    			if (!populated_zone(zone))
    				continue;
    
    			if (zone->all_unreclaimable && priority != DEF_PRIORITY)
    				continue;
    
    			if (!zone_watermark_ok(zone, order, zone->pages_high,
    					       0, 0)) {
    				end_zone = i;
    				goto scan;
    			}
    		}
    		goto out;
    scan:
    		for (i = 0; i <= end_zone; i++) {
    			struct zone *zone = pgdat->node_zones + i;
    
    			lru_pages += zone->nr_active + zone->nr_inactive;
    		}
    
    		/*
    		 * Now scan the zone in the dma->highmem direction, stopping
    		 * at the last zone which needs scanning.
    		 *
    		 * We do this because the page allocator works in the opposite
    		 * direction.  This prevents the page allocator from allocating
    		 * pages behind kswapd's direction of progress, which would
    		 * cause too much scanning of the lower zones.
    		 */
    		for (i = 0; i <= end_zone; i++) {
    			struct zone *zone = pgdat->node_zones + i;
    			int nr_slab;
    
    			if (!populated_zone(zone))
    				continue;
    
    			if (zone->all_unreclaimable && priority != DEF_PRIORITY)
    				continue;
    
    			if (!zone_watermark_ok(zone, order, zone->pages_high,
    					       end_zone, 0))
    				all_zones_ok = 0;
    			zone->temp_priority = priority;
    			if (zone->prev_priority > priority)
    				zone->prev_priority = priority;
    			sc.nr_scanned = 0;
    			nr_reclaimed += shrink_zone(priority, zone, &sc);
    			reclaim_state->reclaimed_slab = 0;
    			nr_slab = shrink_slab(sc.nr_scanned, GFP_KERNEL,
    						lru_pages);
    			nr_reclaimed += reclaim_state->reclaimed_slab;
    			total_scanned += sc.nr_scanned;
    			if (zone->all_unreclaimable)
    				continue;
    			if (nr_slab == 0 && zone->pages_scanned >=
    				    (zone->nr_active + zone->nr_inactive) * 4)
    				zone->all_unreclaimable = 1;
    			/*
    			 * If we've done a decent amount of scanning and
    			 * the reclaim ratio is low, start doing writepage
    			 * even in laptop mode
    			 */
    			if (total_scanned > SWAP_CLUSTER_MAX * 2 &&
    			    total_scanned > nr_reclaimed + nr_reclaimed / 2)
    				sc.may_writepage = 1;
    		}
    		if (all_zones_ok)
    			break;		/* kswapd: all done */
    		/*
    		 * OK, kswapd is getting into trouble.  Take a nap, then take
    		 * another pass across the zones.
    		 */
    		if (total_scanned && priority < DEF_PRIORITY - 2)
    			blk_congestion_wait(WRITE, HZ/10);
    
    		/*
    		 * We do this so kswapd doesn't build up large priorities for
    		 * example when it is freeing in parallel with allocators. It
    		 * matches the direct reclaim path behaviour in terms of impact
    		 * on zone->*_priority.
    		 */
    		if (nr_reclaimed >= SWAP_CLUSTER_MAX)
    			break;
    	}
    out:
    	for (i = 0; i < pgdat->nr_zones; i++) {
    		struct zone *zone = pgdat->node_zones + i;
    
    		zone->prev_priority = zone->temp_priority;
    	}
    	if (!all_zones_ok) {
    		cond_resched();
    		goto loop_again;
    	}
    
    	return nr_reclaimed;
    }
    
    /*
     * The background pageout daemon, started as a kernel thread
     * from the init process. 
     *
     * This basically trickles out pages so that we have _some_
     * free memory available even if there is no other activity
     * that frees anything up. This is needed for things like routing
     * etc, where we otherwise might have all activity going on in
     * asynchronous contexts that cannot page things out.
     *
     * If there are applications that are active memory-allocators
     * (most normal use), this basically shouldn't matter.
     */
    static int kswapd(void *p)
    {
    	unsigned long order;
    	pg_data_t *pgdat = (pg_data_t*)p;
    	struct task_struct *tsk = current;
    	DEFINE_WAIT(wait);
    	struct reclaim_state reclaim_state = {
    		.reclaimed_slab = 0,
    	};
    	cpumask_t cpumask;
    
    	cpumask = node_to_cpumask(pgdat->node_id);
    	if (!cpus_empty(cpumask))
    		set_cpus_allowed(tsk, cpumask);
    	current->reclaim_state = &reclaim_state;
    
    	/*
    	 * Tell the memory management that we're a "memory allocator",
    	 * and that if we need more memory we should get access to it
    	 * regardless (see "__alloc_pages()"). "kswapd" should
    	 * never get caught in the normal page freeing logic.
    	 *
    	 * (Kswapd normally doesn't need memory anyway, but sometimes
    	 * you need a small amount of memory in order to be able to
    	 * page out something else, and this flag essentially protects
    	 * us from recursively trying to free more memory as we're
    	 * trying to free the first piece of memory in the first place).
    	 */
    	tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
    
    	order = 0;
    	for ( ; ; ) {
    		unsigned long new_order;
    
    		try_to_freeze();
    
    		prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
    		new_order = pgdat->kswapd_max_order;
    		pgdat->kswapd_max_order = 0;
    		if (order < new_order) {
    			/*
    			 * Don't sleep if someone wants a larger 'order'
    			 * allocation
    			 */
    			order = new_order;
    		} else {
    			schedule();
    			order = pgdat->kswapd_max_order;
    		}
    		finish_wait(&pgdat->kswapd_wait, &wait);
    
    		balance_pgdat(pgdat, order);
    	}
    	return 0;
    }
    
    /*
     * A zone is low on free memory, so wake its kswapd task to service it.
     */
    void wakeup_kswapd(struct zone *zone, int order)
    {
    	pg_data_t *pgdat;
    
    	if (!populated_zone(zone))
    		return;
    
    	pgdat = zone->zone_pgdat;
    	if (zone_watermark_ok(zone, order, zone->pages_low, 0, 0))
    		return;
    	if (pgdat->kswapd_max_order < order)
    		pgdat->kswapd_max_order = order;
    	if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
    		return;
    	if (!waitqueue_active(&pgdat->kswapd_wait))
    		return;
    	wake_up_interruptible(&pgdat->kswapd_wait);
    }
    
    #ifdef CONFIG_PM
    /*
     * Helper function for shrink_all_memory().  Tries to reclaim 'nr_pages' pages
     * from LRU lists system-wide, for given pass and priority, and returns the
     * number of reclaimed pages
     *
     * For pass > 3 we also try to shrink the LRU lists that contain a few pages
     */
    static unsigned long shrink_all_zones(unsigned long nr_pages, int pass,
    				      int prio, struct scan_control *sc)
    {
    	struct zone *zone;
    	unsigned long nr_to_scan, ret = 0;
    
    	for_each_zone(zone) {
    
    		if (!populated_zone(zone))
    			continue;
    
    		if (zone->all_unreclaimable && prio != DEF_PRIORITY)
    			continue;
    
    		/* For pass = 0 we don't shrink the active list */
    		if (pass > 0) {
    			zone->nr_scan_active += (zone->nr_active >> prio) + 1;
    			if (zone->nr_scan_active >= nr_pages || pass > 3) {
    				zone->nr_scan_active = 0;
    				nr_to_scan = min(nr_pages, zone->nr_active);
    				shrink_active_list(nr_to_scan, zone, sc);
    			}
    		}
    
    		zone->nr_scan_inactive += (zone->nr_inactive >> prio) + 1;
    		if (zone->nr_scan_inactive >= nr_pages || pass > 3) {
    			zone->nr_scan_inactive = 0;
    			nr_to_scan = min(nr_pages, zone->nr_inactive);
    			ret += shrink_inactive_list(nr_to_scan, zone, sc);
    			if (ret >= nr_pages)
    				return ret;
    		}
    	}
    
    	return ret;
    }
    
    /*
     * Try to free `nr_pages' of memory, system-wide, and return the number of
     * freed pages.
     *
     * Rather than trying to age LRUs the aim is to preserve the overall
     * LRU order by reclaiming preferentially
     * inactive > active > active referenced > active mapped
     */
    unsigned long shrink_all_memory(unsigned long nr_pages)
    {
    	unsigned long lru_pages, nr_slab;
    	unsigned long ret = 0;
    	int pass;
    	struct reclaim_state reclaim_state;
    	struct zone *zone;
    	struct scan_control sc = {
    		.gfp_mask = GFP_KERNEL,
    		.may_swap = 0,
    		.swap_cluster_max = nr_pages,
    		.may_writepage = 1,
    		.swappiness = vm_swappiness,
    	};
    
    	current->reclaim_state = &reclaim_state;
    
    	lru_pages = 0;
    	for_each_zone(zone)
    		lru_pages += zone->nr_active + zone->nr_inactive;
    
    	nr_slab = read_page_state(nr_slab);
    	/* If slab caches are huge, it's better to hit them first */
    	while (nr_slab >= lru_pages) {
    		reclaim_state.reclaimed_slab = 0;
    		shrink_slab(nr_pages, sc.gfp_mask, lru_pages);
    		if (!reclaim_state.reclaimed_slab)
    			break;
    
    		ret += reclaim_state.reclaimed_slab;
    		if (ret >= nr_pages)
    			goto out;
    
    		nr_slab -= reclaim_state.reclaimed_slab;
    	}
    
    	/*
    	 * We try to shrink LRUs in 5 passes:
    	 * 0 = Reclaim from inactive_list only
    	 * 1 = Reclaim from active list but don't reclaim mapped
    	 * 2 = 2nd pass of type 1
    	 * 3 = Reclaim mapped (normal reclaim)
    	 * 4 = 2nd pass of type 3
    	 */
    	for (pass = 0; pass < 5; pass++) {
    		int prio;
    
    		/* Needed for shrinking slab caches later on */
    		if (!lru_pages)
    			for_each_zone(zone) {
    				lru_pages += zone->nr_active;
    				lru_pages += zone->nr_inactive;
    			}
    
    		/* Force reclaiming mapped pages in the passes #3 and #4 */
    		if (pass > 2) {
    			sc.may_swap = 1;
    			sc.swappiness = 100;
    		}
    
    		for (prio = DEF_PRIORITY; prio >= 0; prio--) {
    			unsigned long nr_to_scan = nr_pages - ret;
    
    			sc.nr_scanned = 0;
    			ret += shrink_all_zones(nr_to_scan, prio, pass, &sc);
    			if (ret >= nr_pages)
    				goto out;
    
    			reclaim_state.reclaimed_slab = 0;
    			shrink_slab(sc.nr_scanned, sc.gfp_mask, lru_pages);
    			ret += reclaim_state.reclaimed_slab;
    			if (ret >= nr_pages)
    				goto out;
    
    			if (sc.nr_scanned && prio < DEF_PRIORITY - 2)
    				blk_congestion_wait(WRITE, HZ / 10);
    		}
    
    		lru_pages = 0;
    	}
    
    	/*
    	 * If ret = 0, we could not shrink LRUs, but there may be something
    	 * in slab caches
    	 */
    	if (!ret)
    		do {
    			reclaim_state.reclaimed_slab = 0;
    			shrink_slab(nr_pages, sc.gfp_mask, lru_pages);
    			ret += reclaim_state.reclaimed_slab;
    		} while (ret < nr_pages && reclaim_state.reclaimed_slab > 0);
    
    out:
    	current->reclaim_state = NULL;
    
    	return ret;
    }
    #endif
    
    #ifdef CONFIG_HOTPLUG_CPU
    /* It's optimal to keep kswapds on the same CPUs as their memory, but
       not required for correctness.  So if the last cpu in a node goes
       away, we get changed to run anywhere: as the first one comes back,
       restore their cpu bindings. */
    static int __devinit cpu_callback(struct notifier_block *nfb,
    				  unsigned long action, void *hcpu)
    {
    	pg_data_t *pgdat;
    	cpumask_t mask;
    
    	if (action == CPU_ONLINE) {
    		for_each_online_pgdat(pgdat) {
    			mask = node_to_cpumask(pgdat->node_id);
    			if (any_online_cpu(mask) != NR_CPUS)
    				/* One of our CPUs online: restore mask */
    				set_cpus_allowed(pgdat->kswapd, mask);
    		}
    	}
    	return NOTIFY_OK;
    }
    #endif /* CONFIG_HOTPLUG_CPU */
    
    /*
     * This kswapd start function will be called by init and node-hot-add.
     * On node-hot-add, kswapd will moved to proper cpus if cpus are hot-added.
     */
    int kswapd_run(int nid)
    {
    	pg_data_t *pgdat = NODE_DATA(nid);
    	int ret = 0;
    
    	if (pgdat->kswapd)
    		return 0;
    
    	pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid);
    	if (IS_ERR(pgdat->kswapd)) {
    		/* failure at boot is fatal */
    		BUG_ON(system_state == SYSTEM_BOOTING);
    		printk("Failed to start kswapd on node %d\n",nid);
    		ret = -1;
    	}
    	return ret;
    }
    
    static int __init kswapd_init(void)
    {
    	int nid;
    
    	swap_setup();
    	for_each_online_node(nid)
     		kswapd_run(nid);
    	hotcpu_notifier(cpu_callback, 0);
    	return 0;
    }
    
    module_init(kswapd_init)
    
    #ifdef CONFIG_NUMA
    /*
     * Zone reclaim mode
     *
     * If non-zero call zone_reclaim when the number of free pages falls below
     * the watermarks.
     *
     * In the future we may add flags to the mode. However, the page allocator
     * should only have to check that zone_reclaim_mode != 0 before calling
     * zone_reclaim().
     */
    int zone_reclaim_mode __read_mostly;
    
    #define RECLAIM_OFF 0
    #define RECLAIM_ZONE (1<<0)	/* Run shrink_cache on the zone */
    #define RECLAIM_WRITE (1<<1)	/* Writeout pages during reclaim */
    #define RECLAIM_SWAP (1<<2)	/* Swap pages out during reclaim */
    #define RECLAIM_SLAB (1<<3)	/* Do a global slab shrink if the zone is out of memory */
    
    /*
     * Mininum time between zone reclaim scans
     */
    int zone_reclaim_interval __read_mostly = 30*HZ;
    
    /*
     * Priority for ZONE_RECLAIM. This determines the fraction of pages
     * of a node considered for each zone_reclaim. 4 scans 1/16th of
     * a zone.
     */
    #define ZONE_RECLAIM_PRIORITY 4
    
    /*
     * Try to free up some pages from this zone through reclaim.
     */
    static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
    {
    	/* Minimum pages needed in order to stay on node */
    	const unsigned long nr_pages = 1 << order;
    	struct task_struct *p = current;
    	struct reclaim_state reclaim_state;
    	int priority;
    	unsigned long nr_reclaimed = 0;
    	struct scan_control sc = {
    		.may_writepage = !!(zone_reclaim_mode & RECLAIM_WRITE),
    		.may_swap = !!(zone_reclaim_mode & RECLAIM_SWAP),
    		.swap_cluster_max = max_t(unsigned long, nr_pages,
    					SWAP_CLUSTER_MAX),
    		.gfp_mask = gfp_mask,
    		.swappiness = vm_swappiness,
    	};
    
    	disable_swap_token();
    	cond_resched();
    	/*
    	 * We need to be able to allocate from the reserves for RECLAIM_SWAP
    	 * and we also need to be able to write out pages for RECLAIM_WRITE
    	 * and RECLAIM_SWAP.
    	 */
    	p->flags |= PF_MEMALLOC | PF_SWAPWRITE;
    	reclaim_state.reclaimed_slab = 0;
    	p->reclaim_state = &reclaim_state;
    
    	/*
    	 * Free memory by calling shrink zone with increasing priorities
    	 * until we have enough memory freed.
    	 */
    	priority = ZONE_RECLAIM_PRIORITY;
    	do {
    		nr_reclaimed += shrink_zone(priority, zone, &sc);
    		priority--;
    	} while (priority >= 0 && nr_reclaimed < nr_pages);
    
    	if (nr_reclaimed < nr_pages && (zone_reclaim_mode & RECLAIM_SLAB)) {
    		/*
    		 * shrink_slab() does not currently allow us to determine how
    		 * many pages were freed in this zone. So we just shake the slab
    		 * a bit and then go off node for this particular allocation
    		 * despite possibly having freed enough memory to allocate in
    		 * this zone.  If we freed local memory then the next
    		 * allocations will be local again.
    		 *
    		 * shrink_slab will free memory on all zones and may take
    		 * a long time.
    		 */
    		shrink_slab(sc.nr_scanned, gfp_mask, order);
    	}
    
    	p->reclaim_state = NULL;
    	current->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE);
    
    	if (nr_reclaimed == 0) {
    		/*
    		 * We were unable to reclaim enough pages to stay on node.  We
    		 * now allow off node accesses for a certain time period before
    		 * trying again to reclaim pages from the local zone.
    		 */
    		zone->last_unsuccessful_zone_reclaim = jiffies;
    	}
    
    	return nr_reclaimed >= nr_pages;
    }
    
    int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
    {
    	cpumask_t mask;
    	int node_id;
    
    	/*
    	 * Do not reclaim if there was a recent unsuccessful attempt at zone
    	 * reclaim.  In that case we let allocations go off node for the
    	 * zone_reclaim_interval.  Otherwise we would scan for each off-node
    	 * page allocation.
    	 */
    	if (time_before(jiffies,
    		zone->last_unsuccessful_zone_reclaim + zone_reclaim_interval))
    			return 0;
    
    	/*
    	 * Avoid concurrent zone reclaims, do not reclaim in a zone that does
    	 * not have reclaimable pages and if we should not delay the allocation
    	 * then do not scan.
    	 */
    	if (!(gfp_mask & __GFP_WAIT) ||
    		zone->all_unreclaimable ||
    		atomic_read(&zone->reclaim_in_progress) > 0 ||
    		(current->flags & PF_MEMALLOC))
    			return 0;
    
    	/*
    	 * Only run zone reclaim on the local zone or on zones that do not
    	 * have associated processors. This will favor the local processor
    	 * over remote processors and spread off node memory allocations
    	 * as wide as possible.
    	 */
    	node_id = zone->zone_pgdat->node_id;
    	mask = node_to_cpumask(node_id);
    	if (!cpus_empty(mask) && node_id != numa_node_id())
    		return 0;
    	return __zone_reclaim(zone, gfp_mask, order);
    }
    #endif