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32 results

segment.h

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  • compaction.c 71.75 KiB
    // SPDX-License-Identifier: GPL-2.0
    /*
     * linux/mm/compaction.c
     *
     * Memory compaction for the reduction of external fragmentation. Note that
     * this heavily depends upon page migration to do all the real heavy
     * lifting
     *
     * Copyright IBM Corp. 2007-2010 Mel Gorman <mel@csn.ul.ie>
     */
    #include <linux/cpu.h>
    #include <linux/swap.h>
    #include <linux/migrate.h>
    #include <linux/compaction.h>
    #include <linux/mm_inline.h>
    #include <linux/sched/signal.h>
    #include <linux/backing-dev.h>
    #include <linux/sysctl.h>
    #include <linux/sysfs.h>
    #include <linux/page-isolation.h>
    #include <linux/kasan.h>
    #include <linux/kthread.h>
    #include <linux/freezer.h>
    #include <linux/page_owner.h>
    #include <linux/psi.h>
    #include "internal.h"
    
    #ifdef CONFIG_COMPACTION
    static inline void count_compact_event(enum vm_event_item item)
    {
    	count_vm_event(item);
    }
    
    static inline void count_compact_events(enum vm_event_item item, long delta)
    {
    	count_vm_events(item, delta);
    }
    #else
    #define count_compact_event(item) do { } while (0)
    #define count_compact_events(item, delta) do { } while (0)
    #endif
    
    #if defined CONFIG_COMPACTION || defined CONFIG_CMA
    
    #define CREATE_TRACE_POINTS
    #include <trace/events/compaction.h>
    
    #define block_start_pfn(pfn, order)	round_down(pfn, 1UL << (order))
    #define block_end_pfn(pfn, order)	ALIGN((pfn) + 1, 1UL << (order))
    #define pageblock_start_pfn(pfn)	block_start_pfn(pfn, pageblock_order)
    #define pageblock_end_pfn(pfn)		block_end_pfn(pfn, pageblock_order)
    
    static unsigned long release_freepages(struct list_head *freelist)
    {
    	struct page *page, *next;
    	unsigned long high_pfn = 0;
    
    	list_for_each_entry_safe(page, next, freelist, lru) {
    		unsigned long pfn = page_to_pfn(page);
    		list_del(&page->lru);
    		__free_page(page);
    		if (pfn > high_pfn)
    			high_pfn = pfn;
    	}
    
    	return high_pfn;
    }
    
    static void split_map_pages(struct list_head *list)
    {
    	unsigned int i, order, nr_pages;
    	struct page *page, *next;
    	LIST_HEAD(tmp_list);
    
    	list_for_each_entry_safe(page, next, list, lru) {
    		list_del(&page->lru);
    
    		order = page_private(page);
    		nr_pages = 1 << order;
    
    		post_alloc_hook(page, order, __GFP_MOVABLE);
    		if (order)
    			split_page(page, order);
    
    		for (i = 0; i < nr_pages; i++) {
    			list_add(&page->lru, &tmp_list);
    			page++;
    		}
    	}
    
    	list_splice(&tmp_list, list);
    }
    
    #ifdef CONFIG_COMPACTION
    
    int PageMovable(struct page *page)
    {
    	struct address_space *mapping;
    
    	VM_BUG_ON_PAGE(!PageLocked(page), page);
    	if (!__PageMovable(page))
    		return 0;
    
    	mapping = page_mapping(page);
    	if (mapping && mapping->a_ops && mapping->a_ops->isolate_page)
    		return 1;
    
    	return 0;
    }
    EXPORT_SYMBOL(PageMovable);
    
    void __SetPageMovable(struct page *page, struct address_space *mapping)
    {
    	VM_BUG_ON_PAGE(!PageLocked(page), page);
    	VM_BUG_ON_PAGE((unsigned long)mapping & PAGE_MAPPING_MOVABLE, page);
    	page->mapping = (void *)((unsigned long)mapping | PAGE_MAPPING_MOVABLE);
    }
    EXPORT_SYMBOL(__SetPageMovable);
    
    void __ClearPageMovable(struct page *page)
    {
    	VM_BUG_ON_PAGE(!PageLocked(page), page);
    	VM_BUG_ON_PAGE(!PageMovable(page), page);
    	/*
    	 * Clear registered address_space val with keeping PAGE_MAPPING_MOVABLE
    	 * flag so that VM can catch up released page by driver after isolation.
    	 * With it, VM migration doesn't try to put it back.
    	 */
    	page->mapping = (void *)((unsigned long)page->mapping &
    				PAGE_MAPPING_MOVABLE);
    }
    EXPORT_SYMBOL(__ClearPageMovable);
    
    /* Do not skip compaction more than 64 times */
    #define COMPACT_MAX_DEFER_SHIFT 6
    
    /*
     * Compaction is deferred when compaction fails to result in a page
     * allocation success. 1 << compact_defer_limit compactions are skipped up
     * to a limit of 1 << COMPACT_MAX_DEFER_SHIFT
     */
    void defer_compaction(struct zone *zone, int order)
    {
    	zone->compact_considered = 0;
    	zone->compact_defer_shift++;
    
    	if (order < zone->compact_order_failed)
    		zone->compact_order_failed = order;
    
    	if (zone->compact_defer_shift > COMPACT_MAX_DEFER_SHIFT)
    		zone->compact_defer_shift = COMPACT_MAX_DEFER_SHIFT;
    
    	trace_mm_compaction_defer_compaction(zone, order);
    }
    
    /* Returns true if compaction should be skipped this time */
    bool compaction_deferred(struct zone *zone, int order)
    {
    	unsigned long defer_limit = 1UL << zone->compact_defer_shift;
    
    	if (order < zone->compact_order_failed)
    		return false;
    
    	/* Avoid possible overflow */
    	if (++zone->compact_considered > defer_limit)
    		zone->compact_considered = defer_limit;
    
    	if (zone->compact_considered >= defer_limit)
    		return false;
    
    	trace_mm_compaction_deferred(zone, order);
    
    	return true;
    }
    
    /*
     * Update defer tracking counters after successful compaction of given order,
     * which means an allocation either succeeded (alloc_success == true) or is
     * expected to succeed.
     */
    void compaction_defer_reset(struct zone *zone, int order,
    		bool alloc_success)
    {
    	if (alloc_success) {
    		zone->compact_considered = 0;
    		zone->compact_defer_shift = 0;
    	}
    	if (order >= zone->compact_order_failed)
    		zone->compact_order_failed = order + 1;
    
    	trace_mm_compaction_defer_reset(zone, order);
    }
    
    /* Returns true if restarting compaction after many failures */
    bool compaction_restarting(struct zone *zone, int order)
    {
    	if (order < zone->compact_order_failed)
    		return false;
    
    	return zone->compact_defer_shift == COMPACT_MAX_DEFER_SHIFT &&
    		zone->compact_considered >= 1UL << zone->compact_defer_shift;
    }
    
    /* Returns true if the pageblock should be scanned for pages to isolate. */
    static inline bool isolation_suitable(struct compact_control *cc,
    					struct page *page)
    {
    	if (cc->ignore_skip_hint)
    		return true;
    
    	return !get_pageblock_skip(page);
    }
    
    static void reset_cached_positions(struct zone *zone)
    {
    	zone->compact_cached_migrate_pfn[0] = zone->zone_start_pfn;
    	zone->compact_cached_migrate_pfn[1] = zone->zone_start_pfn;
    	zone->compact_cached_free_pfn =
    				pageblock_start_pfn(zone_end_pfn(zone) - 1);
    }
    
    /*
     * Compound pages of >= pageblock_order should consistenly be skipped until
     * released. It is always pointless to compact pages of such order (if they are
     * migratable), and the pageblocks they occupy cannot contain any free pages.
     */
    static bool pageblock_skip_persistent(struct page *page)
    {
    	if (!PageCompound(page))
    		return false;
    
    	page = compound_head(page);
    
    	if (compound_order(page) >= pageblock_order)
    		return true;
    
    	return false;
    }
    
    /*
     * This function is called to clear all cached information on pageblocks that
     * should be skipped for page isolation when the migrate and free page scanner
     * meet.
     */
    static void __reset_isolation_suitable(struct zone *zone)
    {
    	unsigned long start_pfn = zone->zone_start_pfn;
    	unsigned long end_pfn = zone_end_pfn(zone);
    	unsigned long pfn;
    
    	zone->compact_blockskip_flush = false;
    
    	/* Walk the zone and mark every pageblock as suitable for isolation */
    	for (pfn = start_pfn; pfn < end_pfn; pfn += pageblock_nr_pages) {
    		struct page *page;
    
    		cond_resched();
    
    		page = pfn_to_online_page(pfn);
    		if (!page)
    			continue;
    		if (zone != page_zone(page))
    			continue;
    		if (pageblock_skip_persistent(page))
    			continue;
    
    		clear_pageblock_skip(page);
    	}
    
    	reset_cached_positions(zone);
    }
    
    void reset_isolation_suitable(pg_data_t *pgdat)
    {
    	int zoneid;
    
    	for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
    		struct zone *zone = &pgdat->node_zones[zoneid];
    		if (!populated_zone(zone))
    			continue;
    
    		/* Only flush if a full compaction finished recently */
    		if (zone->compact_blockskip_flush)
    			__reset_isolation_suitable(zone);
    	}
    }
    
    /*
     * Sets the pageblock skip bit if it was clear. Note that this is a hint as
     * locks are not required for read/writers. Returns true if it was already set.
     */
    static bool test_and_set_skip(struct compact_control *cc, struct page *page,
    							unsigned long pfn)
    {
    	bool skip;
    
    	/* Do no update if skip hint is being ignored */
    	if (cc->ignore_skip_hint)
    		return false;
    
    	if (!IS_ALIGNED(pfn, pageblock_nr_pages))
    		return false;
    
    	skip = get_pageblock_skip(page);
    	if (!skip && !cc->no_set_skip_hint)
    		set_pageblock_skip(page);
    
    	return skip;
    }
    
    static void update_cached_migrate(struct compact_control *cc, unsigned long pfn)
    {
    	struct zone *zone = cc->zone;
    
    	pfn = pageblock_end_pfn(pfn);
    
    	/* Set for isolation rather than compaction */
    	if (cc->no_set_skip_hint)
    		return;
    
    	if (pfn > zone->compact_cached_migrate_pfn[0])
    		zone->compact_cached_migrate_pfn[0] = pfn;
    	if (cc->mode != MIGRATE_ASYNC &&
    	    pfn > zone->compact_cached_migrate_pfn[1])
    		zone->compact_cached_migrate_pfn[1] = pfn;
    }
    
    /*
     * If no pages were isolated then mark this pageblock to be skipped in the
     * future. The information is later cleared by __reset_isolation_suitable().
     */
    static void update_pageblock_skip(struct compact_control *cc,
    			struct page *page, unsigned long nr_isolated)
    {
    	struct zone *zone = cc->zone;
    	unsigned long pfn;
    
    	if (cc->no_set_skip_hint)
    		return;
    
    	if (!page)
    		return;
    
    	if (nr_isolated)
    		return;
    
    	set_pageblock_skip(page);
    
    	pfn = page_to_pfn(page);
    
    	/* Update where async and sync compaction should restart */
    	if (pfn < zone->compact_cached_free_pfn)
    		zone->compact_cached_free_pfn = pfn;
    }
    #else
    static inline bool isolation_suitable(struct compact_control *cc,
    					struct page *page)
    {
    	return true;
    }
    
    static inline bool pageblock_skip_persistent(struct page *page)
    {
    	return false;
    }
    
    static inline void update_pageblock_skip(struct compact_control *cc,
    			struct page *page, unsigned long nr_isolated)
    {
    }
    
    static void update_cached_migrate(struct compact_control *cc, unsigned long pfn)
    {
    }
    
    static bool test_and_set_skip(struct compact_control *cc, struct page *page,
    							unsigned long pfn)
    {
    	return false;
    }
    #endif /* CONFIG_COMPACTION */
    
    /*
     * Compaction requires the taking of some coarse locks that are potentially
     * very heavily contended. For async compaction, trylock and record if the
     * lock is contended. The lock will still be acquired but compaction will
     * abort when the current block is finished regardless of success rate.
     * Sync compaction acquires the lock.
     *
     * Always returns true which makes it easier to track lock state in callers.
     */
    static bool compact_lock_irqsave(spinlock_t *lock, unsigned long *flags,
    						struct compact_control *cc)
    {
    	/* Track if the lock is contended in async mode */
    	if (cc->mode == MIGRATE_ASYNC && !cc->contended) {
    		if (spin_trylock_irqsave(lock, *flags))
    			return true;
    
    		cc->contended = true;
    	}
    
    	spin_lock_irqsave(lock, *flags);
    	return true;
    }
    
    /*
     * Compaction requires the taking of some coarse locks that are potentially
     * very heavily contended. The lock should be periodically unlocked to avoid
     * having disabled IRQs for a long time, even when there is nobody waiting on
     * the lock. It might also be that allowing the IRQs will result in
     * need_resched() becoming true. If scheduling is needed, async compaction
     * aborts. Sync compaction schedules.
     * Either compaction type will also abort if a fatal signal is pending.
     * In either case if the lock was locked, it is dropped and not regained.
     *
     * Returns true if compaction should abort due to fatal signal pending, or
     *		async compaction due to need_resched()
     * Returns false when compaction can continue (sync compaction might have
     *		scheduled)
     */
    static bool compact_unlock_should_abort(spinlock_t *lock,
    		unsigned long flags, bool *locked, struct compact_control *cc)
    {
    	if (*locked) {
    		spin_unlock_irqrestore(lock, flags);
    		*locked = false;
    	}
    
    	if (fatal_signal_pending(current)) {
    		cc->contended = true;
    		return true;
    	}
    
    	if (need_resched()) {
    		if (cc->mode == MIGRATE_ASYNC)
    			cc->contended = true;
    		cond_resched();
    	}
    
    	return false;
    }
    
    /*
     * Aside from avoiding lock contention, compaction also periodically checks
     * need_resched() and either schedules in sync compaction or aborts async
     * compaction. This is similar to what compact_unlock_should_abort() does, but
     * is used where no lock is concerned.
     *
     * Returns false when no scheduling was needed, or sync compaction scheduled.
     * Returns true when async compaction should abort.
     */
    static inline bool compact_should_abort(struct compact_control *cc)
    {
    	/* async compaction aborts if contended */
    	if (need_resched()) {
    		if (cc->mode == MIGRATE_ASYNC)
    			cc->contended = true;
    
    		cond_resched();
    	}
    
    	return false;
    }
    
    /*
     * Isolate free pages onto a private freelist. If @strict is true, will abort
     * returning 0 on any invalid PFNs or non-free pages inside of the pageblock
     * (even though it may still end up isolating some pages).
     */
    static unsigned long isolate_freepages_block(struct compact_control *cc,
    				unsigned long *start_pfn,
    				unsigned long end_pfn,
    				struct list_head *freelist,
    				bool strict)
    {
    	int nr_scanned = 0, total_isolated = 0;
    	struct page *cursor, *valid_page = NULL;
    	unsigned long flags = 0;
    	bool locked = false;
    	unsigned long blockpfn = *start_pfn;
    	unsigned int order;
    
    	cursor = pfn_to_page(blockpfn);
    
    	/* Isolate free pages. */
    	for (; blockpfn < end_pfn; blockpfn++, cursor++) {
    		int isolated;
    		struct page *page = cursor;
    
    		/*
    		 * Periodically drop the lock (if held) regardless of its
    		 * contention, to give chance to IRQs. Abort if fatal signal
    		 * pending or async compaction detects need_resched()
    		 */
    		if (!(blockpfn % SWAP_CLUSTER_MAX)
    		    && compact_unlock_should_abort(&cc->zone->lock, flags,
    								&locked, cc))
    			break;
    
    		nr_scanned++;
    		if (!pfn_valid_within(blockpfn))
    			goto isolate_fail;
    
    		if (!valid_page)
    			valid_page = page;
    
    		/*
    		 * For compound pages such as THP and hugetlbfs, we can save
    		 * potentially a lot of iterations if we skip them at once.
    		 * The check is racy, but we can consider only valid values
    		 * and the only danger is skipping too much.
    		 */
    		if (PageCompound(page)) {
    			const unsigned int order = compound_order(page);
    
    			if (likely(order < MAX_ORDER)) {
    				blockpfn += (1UL << order) - 1;
    				cursor += (1UL << order) - 1;
    			}
    			goto isolate_fail;
    		}
    
    		if (!PageBuddy(page))
    			goto isolate_fail;
    
    		/*
    		 * If we already hold the lock, we can skip some rechecking.
    		 * Note that if we hold the lock now, checked_pageblock was
    		 * already set in some previous iteration (or strict is true),
    		 * so it is correct to skip the suitable migration target
    		 * recheck as well.
    		 */
    		if (!locked) {
    			locked = compact_lock_irqsave(&cc->zone->lock,
    								&flags, cc);
    
    			/* Recheck this is a buddy page under lock */
    			if (!PageBuddy(page))
    				goto isolate_fail;
    		}
    
    		/* Found a free page, will break it into order-0 pages */
    		order = page_order(page);
    		isolated = __isolate_free_page(page, order);
    		if (!isolated)
    			break;
    		set_page_private(page, order);
    
    		total_isolated += isolated;
    		cc->nr_freepages += isolated;
    		list_add_tail(&page->lru, freelist);
    
    		if (!strict && cc->nr_migratepages <= cc->nr_freepages) {
    			blockpfn += isolated;
    			break;
    		}
    		/* Advance to the end of split page */
    		blockpfn += isolated - 1;
    		cursor += isolated - 1;
    		continue;
    
    isolate_fail:
    		if (strict)
    			break;
    		else
    			continue;
    
    	}
    
    	if (locked)
    		spin_unlock_irqrestore(&cc->zone->lock, flags);
    
    	/*
    	 * There is a tiny chance that we have read bogus compound_order(),
    	 * so be careful to not go outside of the pageblock.
    	 */
    	if (unlikely(blockpfn > end_pfn))
    		blockpfn = end_pfn;
    
    	trace_mm_compaction_isolate_freepages(*start_pfn, blockpfn,
    					nr_scanned, total_isolated);
    
    	/* Record how far we have got within the block */
    	*start_pfn = blockpfn;
    
    	/*
    	 * If strict isolation is requested by CMA then check that all the
    	 * pages requested were isolated. If there were any failures, 0 is
    	 * returned and CMA will fail.
    	 */
    	if (strict && blockpfn < end_pfn)
    		total_isolated = 0;
    
    	/* Update the pageblock-skip if the whole pageblock was scanned */
    	if (blockpfn == end_pfn)
    		update_pageblock_skip(cc, valid_page, total_isolated);
    
    	cc->total_free_scanned += nr_scanned;
    	if (total_isolated)
    		count_compact_events(COMPACTISOLATED, total_isolated);
    	return total_isolated;
    }
    
    /**
     * isolate_freepages_range() - isolate free pages.
     * @cc:        Compaction control structure.
     * @start_pfn: The first PFN to start isolating.
     * @end_pfn:   The one-past-last PFN.
     *
     * Non-free pages, invalid PFNs, or zone boundaries within the
     * [start_pfn, end_pfn) range are considered errors, cause function to
     * undo its actions and return zero.
     *
     * Otherwise, function returns one-past-the-last PFN of isolated page
     * (which may be greater then end_pfn if end fell in a middle of
     * a free page).
     */
    unsigned long
    isolate_freepages_range(struct compact_control *cc,
    			unsigned long start_pfn, unsigned long end_pfn)
    {
    	unsigned long isolated, pfn, block_start_pfn, block_end_pfn;
    	LIST_HEAD(freelist);
    
    	pfn = start_pfn;
    	block_start_pfn = pageblock_start_pfn(pfn);
    	if (block_start_pfn < cc->zone->zone_start_pfn)
    		block_start_pfn = cc->zone->zone_start_pfn;
    	block_end_pfn = pageblock_end_pfn(pfn);
    
    	for (; pfn < end_pfn; pfn += isolated,
    				block_start_pfn = block_end_pfn,
    				block_end_pfn += pageblock_nr_pages) {
    		/* Protect pfn from changing by isolate_freepages_block */
    		unsigned long isolate_start_pfn = pfn;
    
    		block_end_pfn = min(block_end_pfn, end_pfn);
    
    		/*
    		 * pfn could pass the block_end_pfn if isolated freepage
    		 * is more than pageblock order. In this case, we adjust
    		 * scanning range to right one.
    		 */
    		if (pfn >= block_end_pfn) {
    			block_start_pfn = pageblock_start_pfn(pfn);
    			block_end_pfn = pageblock_end_pfn(pfn);
    			block_end_pfn = min(block_end_pfn, end_pfn);
    		}
    
    		if (!pageblock_pfn_to_page(block_start_pfn,
    					block_end_pfn, cc->zone))
    			break;
    
    		isolated = isolate_freepages_block(cc, &isolate_start_pfn,
    						block_end_pfn, &freelist, true);
    
    		/*
    		 * In strict mode, isolate_freepages_block() returns 0 if
    		 * there are any holes in the block (ie. invalid PFNs or
    		 * non-free pages).
    		 */
    		if (!isolated)
    			break;
    
    		/*
    		 * If we managed to isolate pages, it is always (1 << n) *
    		 * pageblock_nr_pages for some non-negative n.  (Max order
    		 * page may span two pageblocks).
    		 */
    	}
    
    	/* __isolate_free_page() does not map the pages */
    	split_map_pages(&freelist);
    
    	if (pfn < end_pfn) {
    		/* Loop terminated early, cleanup. */
    		release_freepages(&freelist);
    		return 0;
    	}
    
    	/* We don't use freelists for anything. */
    	return pfn;
    }
    
    /* Similar to reclaim, but different enough that they don't share logic */
    static bool too_many_isolated(struct zone *zone)
    {
    	unsigned long active, inactive, isolated;
    
    	inactive = node_page_state(zone->zone_pgdat, NR_INACTIVE_FILE) +
    			node_page_state(zone->zone_pgdat, NR_INACTIVE_ANON);
    	active = node_page_state(zone->zone_pgdat, NR_ACTIVE_FILE) +
    			node_page_state(zone->zone_pgdat, NR_ACTIVE_ANON);
    	isolated = node_page_state(zone->zone_pgdat, NR_ISOLATED_FILE) +
    			node_page_state(zone->zone_pgdat, NR_ISOLATED_ANON);
    
    	return isolated > (inactive + active) / 2;
    }
    
    /**
     * isolate_migratepages_block() - isolate all migrate-able pages within
     *				  a single pageblock
     * @cc:		Compaction control structure.
     * @low_pfn:	The first PFN to isolate
     * @end_pfn:	The one-past-the-last PFN to isolate, within same pageblock
     * @isolate_mode: Isolation mode to be used.
     *
     * Isolate all pages that can be migrated from the range specified by
     * [low_pfn, end_pfn). The range is expected to be within same pageblock.
     * Returns zero if there is a fatal signal pending, otherwise PFN of the
     * first page that was not scanned (which may be both less, equal to or more
     * than end_pfn).
     *
     * The pages are isolated on cc->migratepages list (not required to be empty),
     * and cc->nr_migratepages is updated accordingly. The cc->migrate_pfn field
     * is neither read nor updated.
     */
    static unsigned long
    isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
    			unsigned long end_pfn, isolate_mode_t isolate_mode)
    {
    	struct zone *zone = cc->zone;
    	unsigned long nr_scanned = 0, nr_isolated = 0;
    	struct lruvec *lruvec;
    	unsigned long flags = 0;
    	bool locked = false;
    	struct page *page = NULL, *valid_page = NULL;
    	unsigned long start_pfn = low_pfn;
    	bool skip_on_failure = false;
    	unsigned long next_skip_pfn = 0;
    	bool skip_updated = false;
    
    	/*
    	 * Ensure that there are not too many pages isolated from the LRU
    	 * list by either parallel reclaimers or compaction. If there are,
    	 * delay for some time until fewer pages are isolated
    	 */
    	while (unlikely(too_many_isolated(zone))) {
    		/* async migration should just abort */
    		if (cc->mode == MIGRATE_ASYNC)
    			return 0;
    
    		congestion_wait(BLK_RW_ASYNC, HZ/10);
    
    		if (fatal_signal_pending(current))
    			return 0;
    	}
    
    	if (compact_should_abort(cc))
    		return 0;
    
    	if (cc->direct_compaction && (cc->mode == MIGRATE_ASYNC)) {
    		skip_on_failure = true;
    		next_skip_pfn = block_end_pfn(low_pfn, cc->order);
    	}
    
    	/* Time to isolate some pages for migration */
    	for (; low_pfn < end_pfn; low_pfn++) {
    
    		if (skip_on_failure && low_pfn >= next_skip_pfn) {
    			/*
    			 * We have isolated all migration candidates in the
    			 * previous order-aligned block, and did not skip it due
    			 * to failure. We should migrate the pages now and
    			 * hopefully succeed compaction.
    			 */
    			if (nr_isolated)
    				break;
    
    			/*
    			 * We failed to isolate in the previous order-aligned
    			 * block. Set the new boundary to the end of the
    			 * current block. Note we can't simply increase
    			 * next_skip_pfn by 1 << order, as low_pfn might have
    			 * been incremented by a higher number due to skipping
    			 * a compound or a high-order buddy page in the
    			 * previous loop iteration.
    			 */
    			next_skip_pfn = block_end_pfn(low_pfn, cc->order);
    		}
    
    		/*
    		 * Periodically drop the lock (if held) regardless of its
    		 * contention, to give chance to IRQs. Abort async compaction
    		 * if contended.
    		 */
    		if (!(low_pfn % SWAP_CLUSTER_MAX)
    		    && compact_unlock_should_abort(zone_lru_lock(zone), flags,
    								&locked, cc))
    			break;
    
    		if (!pfn_valid_within(low_pfn))
    			goto isolate_fail;
    		nr_scanned++;
    
    		page = pfn_to_page(low_pfn);
    
    		/*
    		 * Check if the pageblock has already been marked skipped.
    		 * Only the aligned PFN is checked as the caller isolates
    		 * COMPACT_CLUSTER_MAX at a time so the second call must
    		 * not falsely conclude that the block should be skipped.
    		 */
    		if (!valid_page && IS_ALIGNED(low_pfn, pageblock_nr_pages)) {
    			if (!cc->ignore_skip_hint && get_pageblock_skip(page)) {
    				low_pfn = end_pfn;
    				goto isolate_abort;
    			}
    			valid_page = page;
    		}
    
    		/*
    		 * Skip if free. We read page order here without zone lock
    		 * which is generally unsafe, but the race window is small and
    		 * the worst thing that can happen is that we skip some
    		 * potential isolation targets.
    		 */
    		if (PageBuddy(page)) {
    			unsigned long freepage_order = page_order_unsafe(page);
    
    			/*
    			 * Without lock, we cannot be sure that what we got is
    			 * a valid page order. Consider only values in the
    			 * valid order range to prevent low_pfn overflow.
    			 */
    			if (freepage_order > 0 && freepage_order < MAX_ORDER)
    				low_pfn += (1UL << freepage_order) - 1;
    			continue;
    		}
    
    		/*
    		 * Regardless of being on LRU, compound pages such as THP and
    		 * hugetlbfs are not to be compacted. We can potentially save
    		 * a lot of iterations if we skip them at once. The check is
    		 * racy, but we can consider only valid values and the only
    		 * danger is skipping too much.
    		 */
    		if (PageCompound(page)) {
    			const unsigned int order = compound_order(page);
    
    			if (likely(order < MAX_ORDER))
    				low_pfn += (1UL << order) - 1;
    			goto isolate_fail;
    		}
    
    		/*
    		 * Check may be lockless but that's ok as we recheck later.
    		 * It's possible to migrate LRU and non-lru movable pages.
    		 * Skip any other type of page
    		 */
    		if (!PageLRU(page)) {
    			/*
    			 * __PageMovable can return false positive so we need
    			 * to verify it under page_lock.
    			 */
    			if (unlikely(__PageMovable(page)) &&
    					!PageIsolated(page)) {
    				if (locked) {
    					spin_unlock_irqrestore(zone_lru_lock(zone),
    									flags);
    					locked = false;
    				}
    
    				if (!isolate_movable_page(page, isolate_mode))
    					goto isolate_success;
    			}
    
    			goto isolate_fail;
    		}
    
    		/*
    		 * Migration will fail if an anonymous page is pinned in memory,
    		 * so avoid taking lru_lock and isolating it unnecessarily in an
    		 * admittedly racy check.
    		 */
    		if (!page_mapping(page) &&
    		    page_count(page) > page_mapcount(page))
    			goto isolate_fail;
    
    		/*
    		 * Only allow to migrate anonymous pages in GFP_NOFS context
    		 * because those do not depend on fs locks.
    		 */
    		if (!(cc->gfp_mask & __GFP_FS) && page_mapping(page))
    			goto isolate_fail;
    
    		/* If we already hold the lock, we can skip some rechecking */
    		if (!locked) {
    			locked = compact_lock_irqsave(zone_lru_lock(zone),
    								&flags, cc);
    
    			/* Try get exclusive access under lock */
    			if (!skip_updated) {
    				skip_updated = true;
    				if (test_and_set_skip(cc, page, low_pfn))
    					goto isolate_abort;
    			}
    
    			/* Recheck PageLRU and PageCompound under lock */
    			if (!PageLRU(page))
    				goto isolate_fail;
    
    			/*
    			 * Page become compound since the non-locked check,
    			 * and it's on LRU. It can only be a THP so the order
    			 * is safe to read and it's 0 for tail pages.
    			 */
    			if (unlikely(PageCompound(page))) {
    				low_pfn += (1UL << compound_order(page)) - 1;
    				goto isolate_fail;
    			}
    		}
    
    		lruvec = mem_cgroup_page_lruvec(page, zone->zone_pgdat);
    
    		/* Try isolate the page */
    		if (__isolate_lru_page(page, isolate_mode) != 0)
    			goto isolate_fail;
    
    		VM_BUG_ON_PAGE(PageCompound(page), page);
    
    		/* Successfully isolated */
    		del_page_from_lru_list(page, lruvec, page_lru(page));
    		inc_node_page_state(page,
    				NR_ISOLATED_ANON + page_is_file_cache(page));
    
    isolate_success:
    		list_add(&page->lru, &cc->migratepages);
    		cc->nr_migratepages++;
    		nr_isolated++;
    
    		/*
    		 * Avoid isolating too much unless this block is being
    		 * rescanned (e.g. dirty/writeback pages, parallel allocation)
    		 * or a lock is contended. For contention, isolate quickly to
    		 * potentially remove one source of contention.
    		 */
    		if (cc->nr_migratepages == COMPACT_CLUSTER_MAX &&
    		    !cc->rescan && !cc->contended) {
    			++low_pfn;
    			break;
    		}
    
    		continue;
    isolate_fail:
    		if (!skip_on_failure)
    			continue;
    
    		/*
    		 * We have isolated some pages, but then failed. Release them
    		 * instead of migrating, as we cannot form the cc->order buddy
    		 * page anyway.
    		 */
    		if (nr_isolated) {
    			if (locked) {
    				spin_unlock_irqrestore(zone_lru_lock(zone), flags);
    				locked = false;
    			}
    			putback_movable_pages(&cc->migratepages);
    			cc->nr_migratepages = 0;
    			nr_isolated = 0;
    		}
    
    		if (low_pfn < next_skip_pfn) {
    			low_pfn = next_skip_pfn - 1;
    			/*
    			 * The check near the loop beginning would have updated
    			 * next_skip_pfn too, but this is a bit simpler.
    			 */
    			next_skip_pfn += 1UL << cc->order;
    		}
    	}
    
    	/*
    	 * The PageBuddy() check could have potentially brought us outside
    	 * the range to be scanned.
    	 */
    	if (unlikely(low_pfn > end_pfn))
    		low_pfn = end_pfn;
    
    isolate_abort:
    	if (locked)
    		spin_unlock_irqrestore(zone_lru_lock(zone), flags);
    
    	/*
    	 * Updated the cached scanner pfn once the pageblock has been scanned
    	 * Pages will either be migrated in which case there is no point
    	 * scanning in the near future or migration failed in which case the
    	 * failure reason may persist. The block is marked for skipping if
    	 * there were no pages isolated in the block or if the block is
    	 * rescanned twice in a row.
    	 */
    	if (low_pfn == end_pfn && (!nr_isolated || cc->rescan)) {
    		if (valid_page && !skip_updated)
    			set_pageblock_skip(valid_page);
    		update_cached_migrate(cc, low_pfn);
    	}
    
    	trace_mm_compaction_isolate_migratepages(start_pfn, low_pfn,
    						nr_scanned, nr_isolated);
    
    	cc->total_migrate_scanned += nr_scanned;
    	if (nr_isolated)
    		count_compact_events(COMPACTISOLATED, nr_isolated);
    
    	return low_pfn;
    }
    
    /**
     * isolate_migratepages_range() - isolate migrate-able pages in a PFN range
     * @cc:        Compaction control structure.
     * @start_pfn: The first PFN to start isolating.
     * @end_pfn:   The one-past-last PFN.
     *
     * Returns zero if isolation fails fatally due to e.g. pending signal.
     * Otherwise, function returns one-past-the-last PFN of isolated page
     * (which may be greater than end_pfn if end fell in a middle of a THP page).
     */
    unsigned long
    isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
    							unsigned long end_pfn)
    {
    	unsigned long pfn, block_start_pfn, block_end_pfn;
    
    	/* Scan block by block. First and last block may be incomplete */
    	pfn = start_pfn;
    	block_start_pfn = pageblock_start_pfn(pfn);
    	if (block_start_pfn < cc->zone->zone_start_pfn)
    		block_start_pfn = cc->zone->zone_start_pfn;
    	block_end_pfn = pageblock_end_pfn(pfn);
    
    	for (; pfn < end_pfn; pfn = block_end_pfn,
    				block_start_pfn = block_end_pfn,
    				block_end_pfn += pageblock_nr_pages) {
    
    		block_end_pfn = min(block_end_pfn, end_pfn);
    
    		if (!pageblock_pfn_to_page(block_start_pfn,
    					block_end_pfn, cc->zone))
    			continue;
    
    		pfn = isolate_migratepages_block(cc, pfn, block_end_pfn,
    							ISOLATE_UNEVICTABLE);
    
    		if (!pfn)
    			break;
    
    		if (cc->nr_migratepages == COMPACT_CLUSTER_MAX)
    			break;
    	}
    
    	return pfn;
    }
    
    #endif /* CONFIG_COMPACTION || CONFIG_CMA */
    #ifdef CONFIG_COMPACTION
    
    static bool suitable_migration_source(struct compact_control *cc,
    							struct page *page)
    {
    	int block_mt;
    
    	if (pageblock_skip_persistent(page))
    		return false;
    
    	if ((cc->mode != MIGRATE_ASYNC) || !cc->direct_compaction)
    		return true;
    
    	block_mt = get_pageblock_migratetype(page);
    
    	if (cc->migratetype == MIGRATE_MOVABLE)
    		return is_migrate_movable(block_mt);
    	else
    		return block_mt == cc->migratetype;
    }
    
    /* Returns true if the page is within a block suitable for migration to */
    static bool suitable_migration_target(struct compact_control *cc,
    							struct page *page)
    {
    	/* If the page is a large free page, then disallow migration */
    	if (PageBuddy(page)) {
    		/*
    		 * We are checking page_order without zone->lock taken. But
    		 * the only small danger is that we skip a potentially suitable
    		 * pageblock, so it's not worth to check order for valid range.
    		 */
    		if (page_order_unsafe(page) >= pageblock_order)
    			return false;
    	}
    
    	if (cc->ignore_block_suitable)
    		return true;
    
    	/* If the block is MIGRATE_MOVABLE or MIGRATE_CMA, allow migration */
    	if (is_migrate_movable(get_pageblock_migratetype(page)))
    		return true;
    
    	/* Otherwise skip the block */
    	return false;
    }
    
    static inline unsigned int
    freelist_scan_limit(struct compact_control *cc)
    {
    	return (COMPACT_CLUSTER_MAX >> cc->fast_search_fail) + 1;
    }
    
    /*
     * Test whether the free scanner has reached the same or lower pageblock than
     * the migration scanner, and compaction should thus terminate.
     */
    static inline bool compact_scanners_met(struct compact_control *cc)
    {
    	return (cc->free_pfn >> pageblock_order)
    		<= (cc->migrate_pfn >> pageblock_order);
    }
    
    /*
     * Used when scanning for a suitable migration target which scans freelists
     * in reverse. Reorders the list such as the unscanned pages are scanned
     * first on the next iteration of the free scanner
     */
    static void
    move_freelist_head(struct list_head *freelist, struct page *freepage)
    {
    	LIST_HEAD(sublist);
    
    	if (!list_is_last(freelist, &freepage->lru)) {
    		list_cut_before(&sublist, freelist, &freepage->lru);
    		if (!list_empty(&sublist))
    			list_splice_tail(&sublist, freelist);
    	}
    }
    
    /*
     * Similar to move_freelist_head except used by the migration scanner
     * when scanning forward. It's possible for these list operations to
     * move against each other if they search the free list exactly in
     * lockstep.
     */
    static void
    move_freelist_tail(struct list_head *freelist, struct page *freepage)
    {
    	LIST_HEAD(sublist);
    
    	if (!list_is_first(freelist, &freepage->lru)) {
    		list_cut_position(&sublist, freelist, &freepage->lru);
    		if (!list_empty(&sublist))
    			list_splice_tail(&sublist, freelist);
    	}
    }
    
    static void
    fast_isolate_around(struct compact_control *cc, unsigned long pfn, unsigned long nr_isolated)
    {
    	unsigned long start_pfn, end_pfn;
    	struct page *page = pfn_to_page(pfn);
    
    	/* Do not search around if there are enough pages already */
    	if (cc->nr_freepages >= cc->nr_migratepages)
    		return;
    
    	/* Minimise scanning during async compaction */
    	if (cc->direct_compaction && cc->mode == MIGRATE_ASYNC)
    		return;
    
    	/* Pageblock boundaries */
    	start_pfn = pageblock_start_pfn(pfn);
    	end_pfn = min(start_pfn + pageblock_nr_pages, zone_end_pfn(cc->zone));
    
    	/* Scan before */
    	if (start_pfn != pfn) {
    		isolate_freepages_block(cc, &start_pfn, pfn, &cc->freepages, false);
    		if (cc->nr_freepages >= cc->nr_migratepages)
    			return;
    	}
    
    	/* Scan after */
    	start_pfn = pfn + nr_isolated;
    	if (start_pfn != end_pfn)
    		isolate_freepages_block(cc, &start_pfn, end_pfn, &cc->freepages, false);
    
    	/* Skip this pageblock in the future as it's full or nearly full */
    	if (cc->nr_freepages < cc->nr_migratepages)
    		set_pageblock_skip(page);
    }
    
    static unsigned long
    fast_isolate_freepages(struct compact_control *cc)
    {
    	unsigned int limit = min(1U, freelist_scan_limit(cc) >> 1);
    	unsigned int nr_scanned = 0;
    	unsigned long low_pfn, min_pfn, high_pfn = 0, highest = 0;
    	unsigned long nr_isolated = 0;
    	unsigned long distance;
    	struct page *page = NULL;
    	bool scan_start = false;
    	int order;
    
    	/* Full compaction passes in a negative order */
    	if (cc->order <= 0)
    		return cc->free_pfn;
    
    	/*
    	 * If starting the scan, use a deeper search and use the highest
    	 * PFN found if a suitable one is not found.
    	 */
    	if (cc->free_pfn == pageblock_start_pfn(zone_end_pfn(cc->zone) - 1)) {
    		limit = pageblock_nr_pages >> 1;
    		scan_start = true;
    	}
    
    	/*
    	 * Preferred point is in the top quarter of the scan space but take
    	 * a pfn from the top half if the search is problematic.
    	 */
    	distance = (cc->free_pfn - cc->migrate_pfn);
    	low_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 2));
    	min_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 1));
    
    	if (WARN_ON_ONCE(min_pfn > low_pfn))
    		low_pfn = min_pfn;
    
    	for (order = cc->order - 1;
    	     order >= 0 && !page;
    	     order--) {
    		struct free_area *area = &cc->zone->free_area[order];
    		struct list_head *freelist;
    		struct page *freepage;
    		unsigned long flags;
    		unsigned int order_scanned = 0;
    
    		if (!area->nr_free)
    			continue;
    
    		spin_lock_irqsave(&cc->zone->lock, flags);
    		freelist = &area->free_list[MIGRATE_MOVABLE];
    		list_for_each_entry_reverse(freepage, freelist, lru) {
    			unsigned long pfn;
    
    			order_scanned++;
    			nr_scanned++;
    			pfn = page_to_pfn(freepage);
    
    			if (pfn >= highest)
    				highest = pageblock_start_pfn(pfn);
    
    			if (pfn >= low_pfn) {
    				cc->fast_search_fail = 0;
    				page = freepage;
    				break;
    			}
    
    			if (pfn >= min_pfn && pfn > high_pfn) {
    				high_pfn = pfn;
    
    				/* Shorten the scan if a candidate is found */
    				limit >>= 1;
    			}
    
    			if (order_scanned >= limit)
    				break;
    		}
    
    		/* Use a minimum pfn if a preferred one was not found */
    		if (!page && high_pfn) {
    			page = pfn_to_page(high_pfn);
    
    			/* Update freepage for the list reorder below */
    			freepage = page;
    		}
    
    		/* Reorder to so a future search skips recent pages */
    		move_freelist_head(freelist, freepage);
    
    		/* Isolate the page if available */
    		if (page) {
    			if (__isolate_free_page(page, order)) {
    				set_page_private(page, order);
    				nr_isolated = 1 << order;
    				cc->nr_freepages += nr_isolated;
    				list_add_tail(&page->lru, &cc->freepages);
    				count_compact_events(COMPACTISOLATED, nr_isolated);
    			} else {
    				/* If isolation fails, abort the search */
    				order = -1;
    				page = NULL;
    			}
    		}
    
    		spin_unlock_irqrestore(&cc->zone->lock, flags);
    
    		/*
    		 * Smaller scan on next order so the total scan ig related
    		 * to freelist_scan_limit.
    		 */
    		if (order_scanned >= limit)
    			limit = min(1U, limit >> 1);
    	}
    
    	if (!page) {
    		cc->fast_search_fail++;
    		if (scan_start) {
    			/*
    			 * Use the highest PFN found above min. If one was
    			 * not found, be pessemistic for direct compaction
    			 * and use the min mark.
    			 */
    			if (highest) {
    				page = pfn_to_page(highest);
    				cc->free_pfn = highest;
    			} else {
    				if (cc->direct_compaction) {
    					page = pfn_to_page(min_pfn);
    					cc->free_pfn = min_pfn;
    				}
    			}
    		}
    	}
    
    	if (highest && highest > cc->zone->compact_cached_free_pfn)
    		cc->zone->compact_cached_free_pfn = highest;
    
    	cc->total_free_scanned += nr_scanned;
    	if (!page)
    		return cc->free_pfn;
    
    	low_pfn = page_to_pfn(page);
    	fast_isolate_around(cc, low_pfn, nr_isolated);
    	return low_pfn;
    }
    
    /*
     * Based on information in the current compact_control, find blocks
     * suitable for isolating free pages from and then isolate them.
     */
    static void isolate_freepages(struct compact_control *cc)
    {
    	struct zone *zone = cc->zone;
    	struct page *page;
    	unsigned long block_start_pfn;	/* start of current pageblock */
    	unsigned long isolate_start_pfn; /* exact pfn we start at */
    	unsigned long block_end_pfn;	/* end of current pageblock */
    	unsigned long low_pfn;	     /* lowest pfn scanner is able to scan */
    	struct list_head *freelist = &cc->freepages;
    
    	/* Try a small search of the free lists for a candidate */
    	isolate_start_pfn = fast_isolate_freepages(cc);
    	if (cc->nr_freepages)
    		goto splitmap;
    
    	/*
    	 * Initialise the free scanner. The starting point is where we last
    	 * successfully isolated from, zone-cached value, or the end of the
    	 * zone when isolating for the first time. For looping we also need
    	 * this pfn aligned down to the pageblock boundary, because we do
    	 * block_start_pfn -= pageblock_nr_pages in the for loop.
    	 * For ending point, take care when isolating in last pageblock of a
    	 * a zone which ends in the middle of a pageblock.
    	 * The low boundary is the end of the pageblock the migration scanner
    	 * is using.
    	 */
    	isolate_start_pfn = cc->free_pfn;
    	block_start_pfn = pageblock_start_pfn(isolate_start_pfn);
    	block_end_pfn = min(block_start_pfn + pageblock_nr_pages,
    						zone_end_pfn(zone));
    	low_pfn = pageblock_end_pfn(cc->migrate_pfn);
    
    	/*
    	 * Isolate free pages until enough are available to migrate the
    	 * pages on cc->migratepages. We stop searching if the migrate
    	 * and free page scanners meet or enough free pages are isolated.
    	 */
    	for (; block_start_pfn >= low_pfn;
    				block_end_pfn = block_start_pfn,
    				block_start_pfn -= pageblock_nr_pages,
    				isolate_start_pfn = block_start_pfn) {
    		/*
    		 * This can iterate a massively long zone without finding any
    		 * suitable migration targets, so periodically check if we need
    		 * to schedule, or even abort async compaction.
    		 */
    		if (!(block_start_pfn % (SWAP_CLUSTER_MAX * pageblock_nr_pages))
    						&& compact_should_abort(cc))
    			break;
    
    		page = pageblock_pfn_to_page(block_start_pfn, block_end_pfn,
    									zone);
    		if (!page)
    			continue;
    
    		/* Check the block is suitable for migration */
    		if (!suitable_migration_target(cc, page))
    			continue;
    
    		/* If isolation recently failed, do not retry */
    		if (!isolation_suitable(cc, page))
    			continue;
    
    		/* Found a block suitable for isolating free pages from. */
    		isolate_freepages_block(cc, &isolate_start_pfn, block_end_pfn,
    					freelist, false);
    
    		/* Are enough freepages isolated? */
    		if (cc->nr_freepages >= cc->nr_migratepages) {
    			if (isolate_start_pfn >= block_end_pfn) {
    				/*
    				 * Restart at previous pageblock if more
    				 * freepages can be isolated next time.
    				 */
    				isolate_start_pfn =
    					block_start_pfn - pageblock_nr_pages;
    			}
    			break;
    		} else if (isolate_start_pfn < block_end_pfn) {
    			/*
    			 * If isolation failed early, do not continue
    			 * needlessly.
    			 */
    			break;
    		}
    	}
    
    	/*
    	 * Record where the free scanner will restart next time. Either we
    	 * broke from the loop and set isolate_start_pfn based on the last
    	 * call to isolate_freepages_block(), or we met the migration scanner
    	 * and the loop terminated due to isolate_start_pfn < low_pfn
    	 */
    	cc->free_pfn = isolate_start_pfn;
    
    splitmap:
    	/* __isolate_free_page() does not map the pages */
    	split_map_pages(freelist);
    }
    
    /*
     * This is a migrate-callback that "allocates" freepages by taking pages
     * from the isolated freelists in the block we are migrating to.
     */
    static struct page *compaction_alloc(struct page *migratepage,
    					unsigned long data)
    {
    	struct compact_control *cc = (struct compact_control *)data;
    	struct page *freepage;
    
    	if (list_empty(&cc->freepages)) {
    		isolate_freepages(cc);
    
    		if (list_empty(&cc->freepages))
    			return NULL;
    	}
    
    	freepage = list_entry(cc->freepages.next, struct page, lru);
    	list_del(&freepage->lru);
    	cc->nr_freepages--;
    
    	return freepage;
    }
    
    /*
     * This is a migrate-callback that "frees" freepages back to the isolated
     * freelist.  All pages on the freelist are from the same zone, so there is no
     * special handling needed for NUMA.
     */
    static void compaction_free(struct page *page, unsigned long data)
    {
    	struct compact_control *cc = (struct compact_control *)data;
    
    	list_add(&page->lru, &cc->freepages);
    	cc->nr_freepages++;
    }
    
    /* possible outcome of isolate_migratepages */
    typedef enum {
    	ISOLATE_ABORT,		/* Abort compaction now */
    	ISOLATE_NONE,		/* No pages isolated, continue scanning */
    	ISOLATE_SUCCESS,	/* Pages isolated, migrate */
    } isolate_migrate_t;
    
    /*
     * Allow userspace to control policy on scanning the unevictable LRU for
     * compactable pages.
     */
    int sysctl_compact_unevictable_allowed __read_mostly = 1;
    
    static inline void
    update_fast_start_pfn(struct compact_control *cc, unsigned long pfn)
    {
    	if (cc->fast_start_pfn == ULONG_MAX)
    		return;
    
    	if (!cc->fast_start_pfn)
    		cc->fast_start_pfn = pfn;
    
    	cc->fast_start_pfn = min(cc->fast_start_pfn, pfn);
    }
    
    static inline unsigned long
    reinit_migrate_pfn(struct compact_control *cc)
    {
    	if (!cc->fast_start_pfn || cc->fast_start_pfn == ULONG_MAX)
    		return cc->migrate_pfn;
    
    	cc->migrate_pfn = cc->fast_start_pfn;
    	cc->fast_start_pfn = ULONG_MAX;
    
    	return cc->migrate_pfn;
    }
    
    /*
     * Briefly search the free lists for a migration source that already has
     * some free pages to reduce the number of pages that need migration
     * before a pageblock is free.
     */
    static unsigned long fast_find_migrateblock(struct compact_control *cc)
    {
    	unsigned int limit = freelist_scan_limit(cc);
    	unsigned int nr_scanned = 0;
    	unsigned long distance;
    	unsigned long pfn = cc->migrate_pfn;
    	unsigned long high_pfn;
    	int order;
    
    	/* Skip hints are relied on to avoid repeats on the fast search */
    	if (cc->ignore_skip_hint)
    		return pfn;
    
    	/*
    	 * If the migrate_pfn is not at the start of a zone or the start
    	 * of a pageblock then assume this is a continuation of a previous
    	 * scan restarted due to COMPACT_CLUSTER_MAX.
    	 */
    	if (pfn != cc->zone->zone_start_pfn && pfn != pageblock_start_pfn(pfn))
    		return pfn;
    
    	/*
    	 * For smaller orders, just linearly scan as the number of pages
    	 * to migrate should be relatively small and does not necessarily
    	 * justify freeing up a large block for a small allocation.
    	 */
    	if (cc->order <= PAGE_ALLOC_COSTLY_ORDER)
    		return pfn;
    
    	/*
    	 * Only allow kcompactd and direct requests for movable pages to
    	 * quickly clear out a MOVABLE pageblock for allocation. This
    	 * reduces the risk that a large movable pageblock is freed for
    	 * an unmovable/reclaimable small allocation.
    	 */
    	if (cc->direct_compaction && cc->migratetype != MIGRATE_MOVABLE)
    		return pfn;
    
    	/*
    	 * When starting the migration scanner, pick any pageblock within the
    	 * first half of the search space. Otherwise try and pick a pageblock
    	 * within the first eighth to reduce the chances that a migration
    	 * target later becomes a source.
    	 */
    	distance = (cc->free_pfn - cc->migrate_pfn) >> 1;
    	if (cc->migrate_pfn != cc->zone->zone_start_pfn)
    		distance >>= 2;
    	high_pfn = pageblock_start_pfn(cc->migrate_pfn + distance);
    
    	for (order = cc->order - 1;
    	     order >= PAGE_ALLOC_COSTLY_ORDER && pfn == cc->migrate_pfn && nr_scanned < limit;
    	     order--) {
    		struct free_area *area = &cc->zone->free_area[order];
    		struct list_head *freelist;
    		unsigned long flags;
    		struct page *freepage;
    
    		if (!area->nr_free)
    			continue;
    
    		spin_lock_irqsave(&cc->zone->lock, flags);
    		freelist = &area->free_list[MIGRATE_MOVABLE];
    		list_for_each_entry(freepage, freelist, lru) {
    			unsigned long free_pfn;
    
    			nr_scanned++;
    			free_pfn = page_to_pfn(freepage);
    			if (free_pfn < high_pfn) {
    				/*
    				 * Avoid if skipped recently. Ideally it would
    				 * move to the tail but even safe iteration of
    				 * the list assumes an entry is deleted, not
    				 * reordered.
    				 */
    				if (get_pageblock_skip(freepage)) {
    					if (list_is_last(freelist, &freepage->lru))
    						break;
    
    					continue;
    				}
    
    				/* Reorder to so a future search skips recent pages */
    				move_freelist_tail(freelist, freepage);
    
    				update_fast_start_pfn(cc, free_pfn);
    				pfn = pageblock_start_pfn(free_pfn);
    				cc->fast_search_fail = 0;
    				set_pageblock_skip(freepage);
    				break;
    			}
    
    			if (nr_scanned >= limit) {
    				cc->fast_search_fail++;
    				move_freelist_tail(freelist, freepage);
    				break;
    			}
    		}
    		spin_unlock_irqrestore(&cc->zone->lock, flags);
    	}
    
    	cc->total_migrate_scanned += nr_scanned;
    
    	/*
    	 * If fast scanning failed then use a cached entry for a page block
    	 * that had free pages as the basis for starting a linear scan.
    	 */
    	if (pfn == cc->migrate_pfn)
    		pfn = reinit_migrate_pfn(cc);
    
    	return pfn;
    }
    
    /*
     * Isolate all pages that can be migrated from the first suitable block,
     * starting at the block pointed to by the migrate scanner pfn within
     * compact_control.
     */
    static isolate_migrate_t isolate_migratepages(struct zone *zone,
    					struct compact_control *cc)
    {
    	unsigned long block_start_pfn;
    	unsigned long block_end_pfn;
    	unsigned long low_pfn;
    	struct page *page;
    	const isolate_mode_t isolate_mode =
    		(sysctl_compact_unevictable_allowed ? ISOLATE_UNEVICTABLE : 0) |
    		(cc->mode != MIGRATE_SYNC ? ISOLATE_ASYNC_MIGRATE : 0);
    	bool fast_find_block;
    
    	/*
    	 * Start at where we last stopped, or beginning of the zone as
    	 * initialized by compact_zone(). The first failure will use
    	 * the lowest PFN as the starting point for linear scanning.
    	 */
    	low_pfn = fast_find_migrateblock(cc);
    	block_start_pfn = pageblock_start_pfn(low_pfn);
    	if (block_start_pfn < zone->zone_start_pfn)
    		block_start_pfn = zone->zone_start_pfn;
    
    	/*
    	 * fast_find_migrateblock marks a pageblock skipped so to avoid
    	 * the isolation_suitable check below, check whether the fast
    	 * search was successful.
    	 */
    	fast_find_block = low_pfn != cc->migrate_pfn && !cc->fast_search_fail;
    
    	/* Only scan within a pageblock boundary */
    	block_end_pfn = pageblock_end_pfn(low_pfn);
    
    	/*
    	 * Iterate over whole pageblocks until we find the first suitable.
    	 * Do not cross the free scanner.
    	 */
    	for (; block_end_pfn <= cc->free_pfn;
    			fast_find_block = false,
    			low_pfn = block_end_pfn,
    			block_start_pfn = block_end_pfn,
    			block_end_pfn += pageblock_nr_pages) {
    
    		/*
    		 * This can potentially iterate a massively long zone with
    		 * many pageblocks unsuitable, so periodically check if we
    		 * need to schedule, or even abort async compaction.
    		 */
    		if (!(low_pfn % (SWAP_CLUSTER_MAX * pageblock_nr_pages))
    						&& compact_should_abort(cc))
    			break;
    
    		page = pageblock_pfn_to_page(block_start_pfn, block_end_pfn,
    									zone);
    		if (!page)
    			continue;
    
    		/*
    		 * If isolation recently failed, do not retry. Only check the
    		 * pageblock once. COMPACT_CLUSTER_MAX causes a pageblock
    		 * to be visited multiple times. Assume skip was checked
    		 * before making it "skip" so other compaction instances do
    		 * not scan the same block.
    		 */
    		if (IS_ALIGNED(low_pfn, pageblock_nr_pages) &&
    		    !fast_find_block && !isolation_suitable(cc, page))
    			continue;
    
    		/*
    		 * For async compaction, also only scan in MOVABLE blocks
    		 * without huge pages. Async compaction is optimistic to see
    		 * if the minimum amount of work satisfies the allocation.
    		 * The cached PFN is updated as it's possible that all
    		 * remaining blocks between source and target are unsuitable
    		 * and the compaction scanners fail to meet.
    		 */
    		if (!suitable_migration_source(cc, page)) {
    			update_cached_migrate(cc, block_end_pfn);
    			continue;
    		}
    
    		/* Perform the isolation */
    		low_pfn = isolate_migratepages_block(cc, low_pfn,
    						block_end_pfn, isolate_mode);
    
    		if (!low_pfn)
    			return ISOLATE_ABORT;
    
    		/*
    		 * Either we isolated something and proceed with migration. Or
    		 * we failed and compact_zone should decide if we should
    		 * continue or not.
    		 */
    		break;
    	}
    
    	/* Record where migration scanner will be restarted. */
    	cc->migrate_pfn = low_pfn;
    
    	return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
    }
    
    /*
     * order == -1 is expected when compacting via
     * /proc/sys/vm/compact_memory
     */
    static inline bool is_via_compact_memory(int order)
    {
    	return order == -1;
    }
    
    static enum compact_result __compact_finished(struct compact_control *cc)
    {
    	unsigned int order;
    	const int migratetype = cc->migratetype;
    	int ret;
    
    	/* Compaction run completes if the migrate and free scanner meet */
    	if (compact_scanners_met(cc)) {
    		/* Let the next compaction start anew. */
    		reset_cached_positions(cc->zone);
    
    		/*
    		 * Mark that the PG_migrate_skip information should be cleared
    		 * by kswapd when it goes to sleep. kcompactd does not set the
    		 * flag itself as the decision to be clear should be directly
    		 * based on an allocation request.
    		 */
    		if (cc->direct_compaction)
    			cc->zone->compact_blockskip_flush = true;
    
    		if (cc->whole_zone)
    			return COMPACT_COMPLETE;
    		else
    			return COMPACT_PARTIAL_SKIPPED;
    	}
    
    	if (is_via_compact_memory(cc->order))
    		return COMPACT_CONTINUE;
    
    	/*
    	 * Always finish scanning a pageblock to reduce the possibility of
    	 * fallbacks in the future. This is particularly important when
    	 * migration source is unmovable/reclaimable but it's not worth
    	 * special casing.
    	 */
    	if (!IS_ALIGNED(cc->migrate_pfn, pageblock_nr_pages))
    		return COMPACT_CONTINUE;
    
    	/* Direct compactor: Is a suitable page free? */
    	ret = COMPACT_NO_SUITABLE_PAGE;
    	for (order = cc->order; order < MAX_ORDER; order++) {
    		struct free_area *area = &cc->zone->free_area[order];
    		bool can_steal;
    
    		/* Job done if page is free of the right migratetype */
    		if (!list_empty(&area->free_list[migratetype]))
    			return COMPACT_SUCCESS;
    
    #ifdef CONFIG_CMA
    		/* MIGRATE_MOVABLE can fallback on MIGRATE_CMA */
    		if (migratetype == MIGRATE_MOVABLE &&
    			!list_empty(&area->free_list[MIGRATE_CMA]))
    			return COMPACT_SUCCESS;
    #endif
    		/*
    		 * Job done if allocation would steal freepages from
    		 * other migratetype buddy lists.
    		 */
    		if (find_suitable_fallback(area, order, migratetype,
    						true, &can_steal) != -1) {
    
    			/* movable pages are OK in any pageblock */
    			if (migratetype == MIGRATE_MOVABLE)
    				return COMPACT_SUCCESS;
    
    			/*
    			 * We are stealing for a non-movable allocation. Make
    			 * sure we finish compacting the current pageblock
    			 * first so it is as free as possible and we won't
    			 * have to steal another one soon. This only applies
    			 * to sync compaction, as async compaction operates
    			 * on pageblocks of the same migratetype.
    			 */
    			if (cc->mode == MIGRATE_ASYNC ||
    					IS_ALIGNED(cc->migrate_pfn,
    							pageblock_nr_pages)) {
    				return COMPACT_SUCCESS;
    			}
    
    			ret = COMPACT_CONTINUE;
    			break;
    		}
    	}
    
    	if (cc->contended || fatal_signal_pending(current))
    		ret = COMPACT_CONTENDED;
    
    	return ret;
    }
    
    static enum compact_result compact_finished(struct compact_control *cc)
    {
    	int ret;
    
    	ret = __compact_finished(cc);
    	trace_mm_compaction_finished(cc->zone, cc->order, ret);
    	if (ret == COMPACT_NO_SUITABLE_PAGE)
    		ret = COMPACT_CONTINUE;
    
    	return ret;
    }
    
    /*
     * compaction_suitable: Is this suitable to run compaction on this zone now?
     * Returns
     *   COMPACT_SKIPPED  - If there are too few free pages for compaction
     *   COMPACT_SUCCESS  - If the allocation would succeed without compaction
     *   COMPACT_CONTINUE - If compaction should run now
     */
    static enum compact_result __compaction_suitable(struct zone *zone, int order,
    					unsigned int alloc_flags,
    					int classzone_idx,
    					unsigned long wmark_target)
    {
    	unsigned long watermark;
    
    	if (is_via_compact_memory(order))
    		return COMPACT_CONTINUE;
    
    	watermark = wmark_pages(zone, alloc_flags & ALLOC_WMARK_MASK);
    	/*
    	 * If watermarks for high-order allocation are already met, there
    	 * should be no need for compaction at all.
    	 */
    	if (zone_watermark_ok(zone, order, watermark, classzone_idx,
    								alloc_flags))
    		return COMPACT_SUCCESS;
    
    	/*
    	 * Watermarks for order-0 must be met for compaction to be able to
    	 * isolate free pages for migration targets. This means that the
    	 * watermark and alloc_flags have to match, or be more pessimistic than
    	 * the check in __isolate_free_page(). We don't use the direct
    	 * compactor's alloc_flags, as they are not relevant for freepage
    	 * isolation. We however do use the direct compactor's classzone_idx to
    	 * skip over zones where lowmem reserves would prevent allocation even
    	 * if compaction succeeds.
    	 * For costly orders, we require low watermark instead of min for
    	 * compaction to proceed to increase its chances.
    	 * ALLOC_CMA is used, as pages in CMA pageblocks are considered
    	 * suitable migration targets
    	 */
    	watermark = (order > PAGE_ALLOC_COSTLY_ORDER) ?
    				low_wmark_pages(zone) : min_wmark_pages(zone);
    	watermark += compact_gap(order);
    	if (!__zone_watermark_ok(zone, 0, watermark, classzone_idx,
    						ALLOC_CMA, wmark_target))
    		return COMPACT_SKIPPED;
    
    	return COMPACT_CONTINUE;
    }
    
    enum compact_result compaction_suitable(struct zone *zone, int order,
    					unsigned int alloc_flags,
    					int classzone_idx)
    {
    	enum compact_result ret;
    	int fragindex;
    
    	ret = __compaction_suitable(zone, order, alloc_flags, classzone_idx,
    				    zone_page_state(zone, NR_FREE_PAGES));
    	/*
    	 * fragmentation index determines if allocation failures are due to
    	 * low memory or external fragmentation
    	 *
    	 * index of -1000 would imply allocations might succeed depending on
    	 * watermarks, but we already failed the high-order watermark check
    	 * index towards 0 implies failure is due to lack of memory
    	 * index towards 1000 implies failure is due to fragmentation
    	 *
    	 * Only compact if a failure would be due to fragmentation. Also
    	 * ignore fragindex for non-costly orders where the alternative to
    	 * a successful reclaim/compaction is OOM. Fragindex and the
    	 * vm.extfrag_threshold sysctl is meant as a heuristic to prevent
    	 * excessive compaction for costly orders, but it should not be at the
    	 * expense of system stability.
    	 */
    	if (ret == COMPACT_CONTINUE && (order > PAGE_ALLOC_COSTLY_ORDER)) {
    		fragindex = fragmentation_index(zone, order);
    		if (fragindex >= 0 && fragindex <= sysctl_extfrag_threshold)
    			ret = COMPACT_NOT_SUITABLE_ZONE;
    	}
    
    	trace_mm_compaction_suitable(zone, order, ret);
    	if (ret == COMPACT_NOT_SUITABLE_ZONE)
    		ret = COMPACT_SKIPPED;
    
    	return ret;
    }
    
    bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
    		int alloc_flags)
    {
    	struct zone *zone;
    	struct zoneref *z;
    
    	/*
    	 * Make sure at least one zone would pass __compaction_suitable if we continue
    	 * retrying the reclaim.
    	 */
    	for_each_zone_zonelist_nodemask(zone, z, ac->zonelist, ac->high_zoneidx,
    					ac->nodemask) {
    		unsigned long available;
    		enum compact_result compact_result;
    
    		/*
    		 * Do not consider all the reclaimable memory because we do not
    		 * want to trash just for a single high order allocation which
    		 * is even not guaranteed to appear even if __compaction_suitable
    		 * is happy about the watermark check.
    		 */
    		available = zone_reclaimable_pages(zone) / order;
    		available += zone_page_state_snapshot(zone, NR_FREE_PAGES);
    		compact_result = __compaction_suitable(zone, order, alloc_flags,
    				ac_classzone_idx(ac), available);
    		if (compact_result != COMPACT_SKIPPED)
    			return true;
    	}
    
    	return false;
    }
    
    static enum compact_result compact_zone(struct compact_control *cc)
    {
    	enum compact_result ret;
    	unsigned long start_pfn = cc->zone->zone_start_pfn;
    	unsigned long end_pfn = zone_end_pfn(cc->zone);
    	unsigned long last_migrated_pfn;
    	const bool sync = cc->mode != MIGRATE_ASYNC;
    	bool update_cached;
    
    	cc->migratetype = gfpflags_to_migratetype(cc->gfp_mask);
    	ret = compaction_suitable(cc->zone, cc->order, cc->alloc_flags,
    							cc->classzone_idx);
    	/* Compaction is likely to fail */
    	if (ret == COMPACT_SUCCESS || ret == COMPACT_SKIPPED)
    		return ret;
    
    	/* huh, compaction_suitable is returning something unexpected */
    	VM_BUG_ON(ret != COMPACT_CONTINUE);
    
    	/*
    	 * Clear pageblock skip if there were failures recently and compaction
    	 * is about to be retried after being deferred.
    	 */
    	if (compaction_restarting(cc->zone, cc->order))
    		__reset_isolation_suitable(cc->zone);
    
    	/*
    	 * Setup to move all movable pages to the end of the zone. Used cached
    	 * information on where the scanners should start (unless we explicitly
    	 * want to compact the whole zone), but check that it is initialised
    	 * by ensuring the values are within zone boundaries.
    	 */
    	cc->fast_start_pfn = 0;
    	if (cc->whole_zone) {
    		cc->migrate_pfn = start_pfn;
    		cc->free_pfn = pageblock_start_pfn(end_pfn - 1);
    	} else {
    		cc->migrate_pfn = cc->zone->compact_cached_migrate_pfn[sync];
    		cc->free_pfn = cc->zone->compact_cached_free_pfn;
    		if (cc->free_pfn < start_pfn || cc->free_pfn >= end_pfn) {
    			cc->free_pfn = pageblock_start_pfn(end_pfn - 1);
    			cc->zone->compact_cached_free_pfn = cc->free_pfn;
    		}
    		if (cc->migrate_pfn < start_pfn || cc->migrate_pfn >= end_pfn) {
    			cc->migrate_pfn = start_pfn;
    			cc->zone->compact_cached_migrate_pfn[0] = cc->migrate_pfn;
    			cc->zone->compact_cached_migrate_pfn[1] = cc->migrate_pfn;
    		}
    
    		if (cc->migrate_pfn == start_pfn)
    			cc->whole_zone = true;
    	}
    
    	last_migrated_pfn = 0;
    
    	/*
    	 * Migrate has separate cached PFNs for ASYNC and SYNC* migration on
    	 * the basis that some migrations will fail in ASYNC mode. However,
    	 * if the cached PFNs match and pageblocks are skipped due to having
    	 * no isolation candidates, then the sync state does not matter.
    	 * Until a pageblock with isolation candidates is found, keep the
    	 * cached PFNs in sync to avoid revisiting the same blocks.
    	 */
    	update_cached = !sync &&
    		cc->zone->compact_cached_migrate_pfn[0] == cc->zone->compact_cached_migrate_pfn[1];
    
    	trace_mm_compaction_begin(start_pfn, cc->migrate_pfn,
    				cc->free_pfn, end_pfn, sync);
    
    	migrate_prep_local();
    
    	while ((ret = compact_finished(cc)) == COMPACT_CONTINUE) {
    		int err;
    		unsigned long start_pfn = cc->migrate_pfn;
    
    		/*
    		 * Avoid multiple rescans which can happen if a page cannot be
    		 * isolated (dirty/writeback in async mode) or if the migrated
    		 * pages are being allocated before the pageblock is cleared.
    		 * The first rescan will capture the entire pageblock for
    		 * migration. If it fails, it'll be marked skip and scanning
    		 * will proceed as normal.
    		 */
    		cc->rescan = false;
    		if (pageblock_start_pfn(last_migrated_pfn) ==
    		    pageblock_start_pfn(start_pfn)) {
    			cc->rescan = true;
    		}
    
    		switch (isolate_migratepages(cc->zone, cc)) {
    		case ISOLATE_ABORT:
    			ret = COMPACT_CONTENDED;
    			putback_movable_pages(&cc->migratepages);
    			cc->nr_migratepages = 0;
    			last_migrated_pfn = 0;
    			goto out;
    		case ISOLATE_NONE:
    			if (update_cached) {
    				cc->zone->compact_cached_migrate_pfn[1] =
    					cc->zone->compact_cached_migrate_pfn[0];
    			}
    
    			/*
    			 * We haven't isolated and migrated anything, but
    			 * there might still be unflushed migrations from
    			 * previous cc->order aligned block.
    			 */
    			goto check_drain;
    		case ISOLATE_SUCCESS:
    			update_cached = false;
    			last_migrated_pfn = start_pfn;
    			;
    		}
    
    		err = migrate_pages(&cc->migratepages, compaction_alloc,
    				compaction_free, (unsigned long)cc, cc->mode,
    				MR_COMPACTION);
    
    		trace_mm_compaction_migratepages(cc->nr_migratepages, err,
    							&cc->migratepages);
    
    		/* All pages were either migrated or will be released */
    		cc->nr_migratepages = 0;
    		if (err) {
    			putback_movable_pages(&cc->migratepages);
    			/*
    			 * migrate_pages() may return -ENOMEM when scanners meet
    			 * and we want compact_finished() to detect it
    			 */
    			if (err == -ENOMEM && !compact_scanners_met(cc)) {
    				ret = COMPACT_CONTENDED;
    				goto out;
    			}
    			/*
    			 * We failed to migrate at least one page in the current
    			 * order-aligned block, so skip the rest of it.
    			 */
    			if (cc->direct_compaction &&
    						(cc->mode == MIGRATE_ASYNC)) {
    				cc->migrate_pfn = block_end_pfn(
    						cc->migrate_pfn - 1, cc->order);
    				/* Draining pcplists is useless in this case */
    				last_migrated_pfn = 0;
    			}
    		}
    
    check_drain:
    		/*
    		 * Has the migration scanner moved away from the previous
    		 * cc->order aligned block where we migrated from? If yes,
    		 * flush the pages that were freed, so that they can merge and
    		 * compact_finished() can detect immediately if allocation
    		 * would succeed.
    		 */
    		if (cc->order > 0 && last_migrated_pfn) {
    			int cpu;
    			unsigned long current_block_start =
    				block_start_pfn(cc->migrate_pfn, cc->order);
    
    			if (last_migrated_pfn < current_block_start) {
    				cpu = get_cpu();
    				lru_add_drain_cpu(cpu);
    				drain_local_pages(cc->zone);
    				put_cpu();
    				/* No more flushing until we migrate again */
    				last_migrated_pfn = 0;
    			}
    		}
    
    	}
    
    out:
    	/*
    	 * Release free pages and update where the free scanner should restart,
    	 * so we don't leave any returned pages behind in the next attempt.
    	 */
    	if (cc->nr_freepages > 0) {
    		unsigned long free_pfn = release_freepages(&cc->freepages);
    
    		cc->nr_freepages = 0;
    		VM_BUG_ON(free_pfn == 0);
    		/* The cached pfn is always the first in a pageblock */
    		free_pfn = pageblock_start_pfn(free_pfn);
    		/*
    		 * Only go back, not forward. The cached pfn might have been
    		 * already reset to zone end in compact_finished()
    		 */
    		if (free_pfn > cc->zone->compact_cached_free_pfn)
    			cc->zone->compact_cached_free_pfn = free_pfn;
    	}
    
    	count_compact_events(COMPACTMIGRATE_SCANNED, cc->total_migrate_scanned);
    	count_compact_events(COMPACTFREE_SCANNED, cc->total_free_scanned);
    
    	trace_mm_compaction_end(start_pfn, cc->migrate_pfn,
    				cc->free_pfn, end_pfn, sync, ret);
    
    	return ret;
    }
    
    static enum compact_result compact_zone_order(struct zone *zone, int order,
    		gfp_t gfp_mask, enum compact_priority prio,
    		unsigned int alloc_flags, int classzone_idx)
    {
    	enum compact_result ret;
    	struct compact_control cc = {
    		.nr_freepages = 0,
    		.nr_migratepages = 0,
    		.total_migrate_scanned = 0,
    		.total_free_scanned = 0,
    		.order = order,
    		.gfp_mask = gfp_mask,
    		.zone = zone,
    		.mode = (prio == COMPACT_PRIO_ASYNC) ?
    					MIGRATE_ASYNC :	MIGRATE_SYNC_LIGHT,
    		.alloc_flags = alloc_flags,
    		.classzone_idx = classzone_idx,
    		.direct_compaction = true,
    		.whole_zone = (prio == MIN_COMPACT_PRIORITY),
    		.ignore_skip_hint = (prio == MIN_COMPACT_PRIORITY),
    		.ignore_block_suitable = (prio == MIN_COMPACT_PRIORITY)
    	};
    	INIT_LIST_HEAD(&cc.freepages);
    	INIT_LIST_HEAD(&cc.migratepages);
    
    	ret = compact_zone(&cc);
    
    	VM_BUG_ON(!list_empty(&cc.freepages));
    	VM_BUG_ON(!list_empty(&cc.migratepages));
    
    	return ret;
    }
    
    int sysctl_extfrag_threshold = 500;
    
    /**
     * try_to_compact_pages - Direct compact to satisfy a high-order allocation
     * @gfp_mask: The GFP mask of the current allocation
     * @order: The order of the current allocation
     * @alloc_flags: The allocation flags of the current allocation
     * @ac: The context of current allocation
     * @prio: Determines how hard direct compaction should try to succeed
     *
     * This is the main entry point for direct page compaction.
     */
    enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
    		unsigned int alloc_flags, const struct alloc_context *ac,
    		enum compact_priority prio)
    {
    	int may_perform_io = gfp_mask & __GFP_IO;
    	struct zoneref *z;
    	struct zone *zone;
    	enum compact_result rc = COMPACT_SKIPPED;
    
    	/*
    	 * Check if the GFP flags allow compaction - GFP_NOIO is really
    	 * tricky context because the migration might require IO
    	 */
    	if (!may_perform_io)
    		return COMPACT_SKIPPED;
    
    	trace_mm_compaction_try_to_compact_pages(order, gfp_mask, prio);
    
    	/* Compact each zone in the list */
    	for_each_zone_zonelist_nodemask(zone, z, ac->zonelist, ac->high_zoneidx,
    								ac->nodemask) {
    		enum compact_result status;
    
    		if (prio > MIN_COMPACT_PRIORITY
    					&& compaction_deferred(zone, order)) {
    			rc = max_t(enum compact_result, COMPACT_DEFERRED, rc);
    			continue;
    		}
    
    		status = compact_zone_order(zone, order, gfp_mask, prio,
    					alloc_flags, ac_classzone_idx(ac));
    		rc = max(status, rc);
    
    		/* The allocation should succeed, stop compacting */
    		if (status == COMPACT_SUCCESS) {
    			/*
    			 * We think the allocation will succeed in this zone,
    			 * but it is not certain, hence the false. The caller
    			 * will repeat this with true if allocation indeed
    			 * succeeds in this zone.
    			 */
    			compaction_defer_reset(zone, order, false);
    
    			break;
    		}
    
    		if (prio != COMPACT_PRIO_ASYNC && (status == COMPACT_COMPLETE ||
    					status == COMPACT_PARTIAL_SKIPPED))
    			/*
    			 * We think that allocation won't succeed in this zone
    			 * so we defer compaction there. If it ends up
    			 * succeeding after all, it will be reset.
    			 */
    			defer_compaction(zone, order);
    
    		/*
    		 * We might have stopped compacting due to need_resched() in
    		 * async compaction, or due to a fatal signal detected. In that
    		 * case do not try further zones
    		 */
    		if ((prio == COMPACT_PRIO_ASYNC && need_resched())
    					|| fatal_signal_pending(current))
    			break;
    	}
    
    	return rc;
    }
    
    
    /* Compact all zones within a node */
    static void compact_node(int nid)
    {
    	pg_data_t *pgdat = NODE_DATA(nid);
    	int zoneid;
    	struct zone *zone;
    	struct compact_control cc = {
    		.order = -1,
    		.total_migrate_scanned = 0,
    		.total_free_scanned = 0,
    		.mode = MIGRATE_SYNC,
    		.ignore_skip_hint = true,
    		.whole_zone = true,
    		.gfp_mask = GFP_KERNEL,
    	};
    
    
    	for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
    
    		zone = &pgdat->node_zones[zoneid];
    		if (!populated_zone(zone))
    			continue;
    
    		cc.nr_freepages = 0;
    		cc.nr_migratepages = 0;
    		cc.zone = zone;
    		INIT_LIST_HEAD(&cc.freepages);
    		INIT_LIST_HEAD(&cc.migratepages);
    
    		compact_zone(&cc);
    
    		VM_BUG_ON(!list_empty(&cc.freepages));
    		VM_BUG_ON(!list_empty(&cc.migratepages));
    	}
    }
    
    /* Compact all nodes in the system */
    static void compact_nodes(void)
    {
    	int nid;
    
    	/* Flush pending updates to the LRU lists */
    	lru_add_drain_all();
    
    	for_each_online_node(nid)
    		compact_node(nid);
    }
    
    /* The written value is actually unused, all memory is compacted */
    int sysctl_compact_memory;
    
    /*
     * This is the entry point for compacting all nodes via
     * /proc/sys/vm/compact_memory
     */
    int sysctl_compaction_handler(struct ctl_table *table, int write,
    			void __user *buffer, size_t *length, loff_t *ppos)
    {
    	if (write)
    		compact_nodes();
    
    	return 0;
    }
    
    #if defined(CONFIG_SYSFS) && defined(CONFIG_NUMA)
    static ssize_t sysfs_compact_node(struct device *dev,
    			struct device_attribute *attr,
    			const char *buf, size_t count)
    {
    	int nid = dev->id;
    
    	if (nid >= 0 && nid < nr_node_ids && node_online(nid)) {
    		/* Flush pending updates to the LRU lists */
    		lru_add_drain_all();
    
    		compact_node(nid);
    	}
    
    	return count;
    }
    static DEVICE_ATTR(compact, 0200, NULL, sysfs_compact_node);
    
    int compaction_register_node(struct node *node)
    {
    	return device_create_file(&node->dev, &dev_attr_compact);
    }
    
    void compaction_unregister_node(struct node *node)
    {
    	return device_remove_file(&node->dev, &dev_attr_compact);
    }
    #endif /* CONFIG_SYSFS && CONFIG_NUMA */
    
    static inline bool kcompactd_work_requested(pg_data_t *pgdat)
    {
    	return pgdat->kcompactd_max_order > 0 || kthread_should_stop();
    }
    
    static bool kcompactd_node_suitable(pg_data_t *pgdat)
    {
    	int zoneid;
    	struct zone *zone;
    	enum zone_type classzone_idx = pgdat->kcompactd_classzone_idx;
    
    	for (zoneid = 0; zoneid <= classzone_idx; zoneid++) {
    		zone = &pgdat->node_zones[zoneid];
    
    		if (!populated_zone(zone))
    			continue;
    
    		if (compaction_suitable(zone, pgdat->kcompactd_max_order, 0,
    					classzone_idx) == COMPACT_CONTINUE)
    			return true;
    	}
    
    	return false;
    }
    
    static void kcompactd_do_work(pg_data_t *pgdat)
    {
    	/*
    	 * With no special task, compact all zones so that a page of requested
    	 * order is allocatable.
    	 */
    	int zoneid;
    	struct zone *zone;
    	struct compact_control cc = {
    		.order = pgdat->kcompactd_max_order,
    		.total_migrate_scanned = 0,
    		.total_free_scanned = 0,
    		.classzone_idx = pgdat->kcompactd_classzone_idx,
    		.mode = MIGRATE_SYNC_LIGHT,
    		.ignore_skip_hint = false,
    		.gfp_mask = GFP_KERNEL,
    	};
    	trace_mm_compaction_kcompactd_wake(pgdat->node_id, cc.order,
    							cc.classzone_idx);
    	count_compact_event(KCOMPACTD_WAKE);
    
    	for (zoneid = 0; zoneid <= cc.classzone_idx; zoneid++) {
    		int status;
    
    		zone = &pgdat->node_zones[zoneid];
    		if (!populated_zone(zone))
    			continue;
    
    		if (compaction_deferred(zone, cc.order))
    			continue;
    
    		if (compaction_suitable(zone, cc.order, 0, zoneid) !=
    							COMPACT_CONTINUE)
    			continue;
    
    		cc.nr_freepages = 0;
    		cc.nr_migratepages = 0;
    		cc.total_migrate_scanned = 0;
    		cc.total_free_scanned = 0;
    		cc.zone = zone;
    		INIT_LIST_HEAD(&cc.freepages);
    		INIT_LIST_HEAD(&cc.migratepages);
    
    		if (kthread_should_stop())
    			return;
    		status = compact_zone(&cc);
    
    		if (status == COMPACT_SUCCESS) {
    			compaction_defer_reset(zone, cc.order, false);
    		} else if (status == COMPACT_PARTIAL_SKIPPED || status == COMPACT_COMPLETE) {
    			/*
    			 * Buddy pages may become stranded on pcps that could
    			 * otherwise coalesce on the zone's free area for
    			 * order >= cc.order.  This is ratelimited by the
    			 * upcoming deferral.
    			 */
    			drain_all_pages(zone);
    
    			/*
    			 * We use sync migration mode here, so we defer like
    			 * sync direct compaction does.
    			 */
    			defer_compaction(zone, cc.order);
    		}
    
    		count_compact_events(KCOMPACTD_MIGRATE_SCANNED,
    				     cc.total_migrate_scanned);
    		count_compact_events(KCOMPACTD_FREE_SCANNED,
    				     cc.total_free_scanned);
    
    		VM_BUG_ON(!list_empty(&cc.freepages));
    		VM_BUG_ON(!list_empty(&cc.migratepages));
    	}
    
    	/*
    	 * Regardless of success, we are done until woken up next. But remember
    	 * the requested order/classzone_idx in case it was higher/tighter than
    	 * our current ones
    	 */
    	if (pgdat->kcompactd_max_order <= cc.order)
    		pgdat->kcompactd_max_order = 0;
    	if (pgdat->kcompactd_classzone_idx >= cc.classzone_idx)
    		pgdat->kcompactd_classzone_idx = pgdat->nr_zones - 1;
    }
    
    void wakeup_kcompactd(pg_data_t *pgdat, int order, int classzone_idx)
    {
    	if (!order)
    		return;
    
    	if (pgdat->kcompactd_max_order < order)
    		pgdat->kcompactd_max_order = order;
    
    	if (pgdat->kcompactd_classzone_idx > classzone_idx)
    		pgdat->kcompactd_classzone_idx = classzone_idx;
    
    	/*
    	 * Pairs with implicit barrier in wait_event_freezable()
    	 * such that wakeups are not missed.
    	 */
    	if (!wq_has_sleeper(&pgdat->kcompactd_wait))
    		return;
    
    	if (!kcompactd_node_suitable(pgdat))
    		return;
    
    	trace_mm_compaction_wakeup_kcompactd(pgdat->node_id, order,
    							classzone_idx);
    	wake_up_interruptible(&pgdat->kcompactd_wait);
    }
    
    /*
     * The background compaction daemon, started as a kernel thread
     * from the init process.
     */
    static int kcompactd(void *p)
    {
    	pg_data_t *pgdat = (pg_data_t*)p;
    	struct task_struct *tsk = current;
    
    	const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
    
    	if (!cpumask_empty(cpumask))
    		set_cpus_allowed_ptr(tsk, cpumask);
    
    	set_freezable();
    
    	pgdat->kcompactd_max_order = 0;
    	pgdat->kcompactd_classzone_idx = pgdat->nr_zones - 1;
    
    	while (!kthread_should_stop()) {
    		unsigned long pflags;
    
    		trace_mm_compaction_kcompactd_sleep(pgdat->node_id);
    		wait_event_freezable(pgdat->kcompactd_wait,
    				kcompactd_work_requested(pgdat));
    
    		psi_memstall_enter(&pflags);
    		kcompactd_do_work(pgdat);
    		psi_memstall_leave(&pflags);
    	}
    
    	return 0;
    }
    
    /*
     * This kcompactd start function will be called by init and node-hot-add.
     * On node-hot-add, kcompactd will moved to proper cpus if cpus are hot-added.
     */
    int kcompactd_run(int nid)
    {
    	pg_data_t *pgdat = NODE_DATA(nid);
    	int ret = 0;
    
    	if (pgdat->kcompactd)
    		return 0;
    
    	pgdat->kcompactd = kthread_run(kcompactd, pgdat, "kcompactd%d", nid);
    	if (IS_ERR(pgdat->kcompactd)) {
    		pr_err("Failed to start kcompactd on node %d\n", nid);
    		ret = PTR_ERR(pgdat->kcompactd);
    		pgdat->kcompactd = NULL;
    	}
    	return ret;
    }
    
    /*
     * Called by memory hotplug when all memory in a node is offlined. Caller must
     * hold mem_hotplug_begin/end().
     */
    void kcompactd_stop(int nid)
    {
    	struct task_struct *kcompactd = NODE_DATA(nid)->kcompactd;
    
    	if (kcompactd) {
    		kthread_stop(kcompactd);
    		NODE_DATA(nid)->kcompactd = NULL;
    	}
    }
    
    /*
     * It's optimal to keep kcompactd 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 kcompactd_cpu_online(unsigned int cpu)
    {
    	int nid;
    
    	for_each_node_state(nid, N_MEMORY) {
    		pg_data_t *pgdat = NODE_DATA(nid);
    		const struct cpumask *mask;
    
    		mask = cpumask_of_node(pgdat->node_id);
    
    		if (cpumask_any_and(cpu_online_mask, mask) < nr_cpu_ids)
    			/* One of our CPUs online: restore mask */
    			set_cpus_allowed_ptr(pgdat->kcompactd, mask);
    	}
    	return 0;
    }
    
    static int __init kcompactd_init(void)
    {
    	int nid;
    	int ret;
    
    	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
    					"mm/compaction:online",
    					kcompactd_cpu_online, NULL);
    	if (ret < 0) {
    		pr_err("kcompactd: failed to register hotplug callbacks.\n");
    		return ret;
    	}
    
    	for_each_node_state(nid, N_MEMORY)
    		kcompactd_run(nid);
    	return 0;
    }
    subsys_initcall(kcompactd_init)
    
    #endif /* CONFIG_COMPACTION */