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32 results

core.c

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  • compaction.c 47.31 KiB
    /*
     * 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/swap.h>
    #include <linux/migrate.h>
    #include <linux/compaction.h>
    #include <linux/mm_inline.h>
    #include <linux/backing-dev.h>
    #include <linux/sysctl.h>
    #include <linux/sysfs.h>
    #include <linux/balloon_compaction.h>
    #include <linux/page-isolation.h>
    #include <linux/kasan.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
    #ifdef CONFIG_TRACEPOINTS
    static const char *const compaction_status_string[] = {
    	"deferred",
    	"skipped",
    	"continue",
    	"partial",
    	"complete",
    	"no_suitable_page",
    	"not_suitable_zone",
    };
    #endif
    
    #define CREATE_TRACE_POINTS
    #include <trace/events/compaction.h>
    
    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 map_pages(struct list_head *list)
    {
    	struct page *page;
    
    	list_for_each_entry(page, list, lru) {
    		arch_alloc_page(page, 0);
    		kernel_map_pages(page, 1, 1);
    		kasan_alloc_pages(page, 0);
    	}
    }
    
    static inline bool migrate_async_suitable(int migratetype)
    {
    	return is_migrate_cma(migratetype) || migratetype == MIGRATE_MOVABLE;
    }
    
    /*
     * Check that the whole (or subset of) a pageblock given by the interval of
     * [start_pfn, end_pfn) is valid and within the same zone, before scanning it
     * with the migration of free compaction scanner. The scanners then need to
     * use only pfn_valid_within() check for arches that allow holes within
     * pageblocks.
     *
     * Return struct page pointer of start_pfn, or NULL if checks were not passed.
     *
     * It's possible on some configurations to have a setup like node0 node1 node0
     * i.e. it's possible that all pages within a zones range of pages do not
     * belong to a single zone. We assume that a border between node0 and node1
     * can occur within a single pageblock, but not a node0 node1 node0
     * interleaving within a single pageblock. It is therefore sufficient to check
     * the first and last page of a pageblock and avoid checking each individual
     * page in a pageblock.
     */
    static struct page *pageblock_pfn_to_page(unsigned long start_pfn,
    				unsigned long end_pfn, struct zone *zone)
    {
    	struct page *start_page;
    	struct page *end_page;
    
    	/* end_pfn is one past the range we are checking */
    	end_pfn--;
    
    	if (!pfn_valid(start_pfn) || !pfn_valid(end_pfn))
    		return NULL;
    
    	start_page = pfn_to_page(start_pfn);
    
    	if (page_zone(start_page) != zone)
    		return NULL;
    
    	end_page = pfn_to_page(end_pfn);
    
    	/* This gives a shorter code than deriving page_zone(end_page) */
    	if (page_zone_id(start_page) != page_zone_id(end_page))
    		return NULL;
    
    	return start_page;
    }
    
    #ifdef CONFIG_COMPACTION
    
    /* 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);
    }
    
    /*
     * 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_cached_migrate_pfn[0] = start_pfn;
    	zone->compact_cached_migrate_pfn[1] = start_pfn;
    	zone->compact_cached_free_pfn = end_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();
    
    		if (!pfn_valid(pfn))
    			continue;
    
    		page = pfn_to_page(pfn);
    		if (zone != page_zone(page))
    			continue;
    
    		clear_pageblock_skip(page);
    	}
    }
    
    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);
    	}
    }
    
    /*
     * 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,
    			bool migrate_scanner)
    {
    	struct zone *zone = cc->zone;
    	unsigned long pfn;
    
    	if (cc->ignore_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 (migrate_scanner) {
    		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;
    	} else {
    		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 void update_pageblock_skip(struct compact_control *cc,
    			struct page *page, unsigned long nr_isolated,
    			bool migrate_scanner)
    {
    }
    #endif /* CONFIG_COMPACTION */
    
    /*
     * Compaction requires the taking of some coarse locks that are potentially
     * very heavily contended. For async compaction, back out if the lock cannot
     * be taken immediately. For sync compaction, spin on the lock if needed.
     *
     * Returns true if the lock is held
     * Returns false if the lock is not held and compaction should abort
     */
    static bool compact_trylock_irqsave(spinlock_t *lock, unsigned long *flags,
    						struct compact_control *cc)
    {
    	if (cc->mode == MIGRATE_ASYNC) {
    		if (!spin_trylock_irqsave(lock, *flags)) {
    			cc->contended = COMPACT_CONTENDED_LOCK;
    			return false;
    		}
    	} else {
    		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 = COMPACT_CONTENDED_SCHED;
    		return true;
    	}
    
    	if (need_resched()) {
    		if (cc->mode == MIGRATE_ASYNC) {
    			cc->contended = COMPACT_CONTENDED_SCHED;
    			return 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 = COMPACT_CONTENDED_SCHED;
    			return true;
    		}
    
    		cond_resched();
    	}
    
    	return false;
    }
    
    /* Returns true if the page is within a block suitable for migration to */
    static bool suitable_migration_target(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 the block is MIGRATE_MOVABLE or MIGRATE_CMA, allow migration */
    	if (migrate_async_suitable(get_pageblock_migratetype(page)))
    		return true;
    
    	/* Otherwise skip the block */
    	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;
    
    	cursor = pfn_to_page(blockpfn);
    
    	/* Isolate free pages. */
    	for (; blockpfn < end_pfn; blockpfn++, cursor++) {
    		int isolated, i;
    		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;
    		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) {
    			/*
    			 * The zone lock must be held to isolate freepages.
    			 * Unfortunately this is a very coarse lock and can be
    			 * heavily contended if there are parallel allocations
    			 * or parallel compactions. For async compaction do not
    			 * spin on the lock and we acquire the lock as late as
    			 * possible.
    			 */
    			locked = compact_trylock_irqsave(&cc->zone->lock,
    								&flags, cc);
    			if (!locked)
    				break;
    
    			/* Recheck this is a buddy page under lock */
    			if (!PageBuddy(page))
    				goto isolate_fail;
    		}
    
    		/* Found a free page, break it into order-0 pages */
    		isolated = split_free_page(page);
    		total_isolated += isolated;
    		for (i = 0; i < isolated; i++) {
    			list_add(&page->lru, freelist);
    			page++;
    		}
    
    		/* If a page was split, advance to the end of it */
    		if (isolated) {
    			cc->nr_freepages += isolated;
    			if (!strict &&
    				cc->nr_migratepages <= cc->nr_freepages) {
    				blockpfn += isolated;
    				break;
    			}
    
    			blockpfn += isolated - 1;
    			cursor += isolated - 1;
    			continue;
    		}
    
    isolate_fail:
    		if (strict)
    			break;
    		else
    			continue;
    
    	}
    
    	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;
    
    	if (locked)
    		spin_unlock_irqrestore(&cc->zone->lock, flags);
    
    	/* Update the pageblock-skip if the whole pageblock was scanned */
    	if (blockpfn == end_pfn)
    		update_pageblock_skip(cc, valid_page, total_isolated, false);
    
    	count_compact_events(COMPACTFREE_SCANNED, nr_scanned);
    	if (total_isolated)
    		count_compact_events(COMPACTISOLATED, total_isolated);
    	return total_isolated;
    }
    
    /**
     * isolate_freepages_range() - isolate free pages.
     * @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_end_pfn;
    	LIST_HEAD(freelist);
    
    	pfn = start_pfn;
    	block_end_pfn = ALIGN(pfn + 1, pageblock_nr_pages);
    
    	for (; pfn < end_pfn; pfn += isolated,
    				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_end_pfn = ALIGN(pfn + 1, pageblock_nr_pages);
    			block_end_pfn = min(block_end_pfn, end_pfn);
    		}
    
    		if (!pageblock_pfn_to_page(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).
    		 */
    	}
    
    	/* split_free_page does not map the pages */
    	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;
    }
    
    /* Update the number of anon and file isolated pages in the zone */
    static void acct_isolated(struct zone *zone, struct compact_control *cc)
    {
    	struct page *page;
    	unsigned int count[2] = { 0, };
    
    	if (list_empty(&cc->migratepages))
    		return;
    
    	list_for_each_entry(page, &cc->migratepages, lru)
    		count[!!page_is_file_cache(page)]++;
    
    	mod_zone_page_state(zone, NR_ISOLATED_ANON, count[0]);
    	mod_zone_page_state(zone, NR_ISOLATED_FILE, count[1]);
    }
    
    /* 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 = zone_page_state(zone, NR_INACTIVE_FILE) +
    					zone_page_state(zone, NR_INACTIVE_ANON);
    	active = zone_page_state(zone, NR_ACTIVE_FILE) +
    					zone_page_state(zone, NR_ACTIVE_ANON);
    	isolated = zone_page_state(zone, NR_ISOLATED_FILE) +
    					zone_page_state(zone, 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 list_head *migratelist = &cc->migratepages;
    	struct lruvec *lruvec;
    	unsigned long flags = 0;
    	bool locked = false;
    	struct page *page = NULL, *valid_page = NULL;
    	unsigned long start_pfn = low_pfn;
    
    	/*
    	 * 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;
    
    	/* Time to isolate some pages for migration */
    	for (; low_pfn < end_pfn; low_pfn++) {
    		/*
    		 * 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, flags,
    								&locked, cc))
    			break;
    
    		if (!pfn_valid_within(low_pfn))
    			continue;
    		nr_scanned++;
    
    		page = pfn_to_page(low_pfn);
    
    		if (!valid_page)
    			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;
    		}
    
    		/*
    		 * Check may be lockless but that's ok as we recheck later.
    		 * It's possible to migrate LRU pages and balloon pages
    		 * Skip any other type of page
    		 */
    		if (!PageLRU(page)) {
    			if (unlikely(balloon_page_movable(page))) {
    				if (balloon_page_isolate(page)) {
    					/* Successfully isolated */
    					goto isolate_success;
    				}
    			}
    			continue;
    		}
    
    		/*
    		 * PageLRU is set. lru_lock normally excludes isolation
    		 * splitting and collapsing (collapsing has already happened
    		 * if PageLRU is set) but the lock is not necessarily taken
    		 * here and it is wasteful to take it just to check transhuge.
    		 * Check TransHuge without lock and skip the whole pageblock if
    		 * it's either a transhuge or hugetlbfs page, as calling
    		 * compound_order() without preventing THP from splitting the
    		 * page underneath us may return surprising results.
    		 */
    		if (PageTransHuge(page)) {
    			if (!locked)
    				low_pfn = ALIGN(low_pfn + 1,
    						pageblock_nr_pages) - 1;
    			else
    				low_pfn += (1 << compound_order(page)) - 1;
    
    			continue;
    		}
    
    		/*
    		 * 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))
    			continue;
    
    		/* If we already hold the lock, we can skip some rechecking */
    		if (!locked) {
    			locked = compact_trylock_irqsave(&zone->lru_lock,
    								&flags, cc);
    			if (!locked)
    				break;
    
    			/* Recheck PageLRU and PageTransHuge under lock */
    			if (!PageLRU(page))
    				continue;
    			if (PageTransHuge(page)) {
    				low_pfn += (1 << compound_order(page)) - 1;
    				continue;
    			}
    		}
    
    		lruvec = mem_cgroup_page_lruvec(page, zone);
    
    		/* Try isolate the page */
    		if (__isolate_lru_page(page, isolate_mode) != 0)
    			continue;
    
    		VM_BUG_ON_PAGE(PageTransCompound(page), page);
    
    		/* Successfully isolated */
    		del_page_from_lru_list(page, lruvec, page_lru(page));
    
    isolate_success:
    		list_add(&page->lru, migratelist);
    		cc->nr_migratepages++;
    		nr_isolated++;
    
    		/* Avoid isolating too much */
    		if (cc->nr_migratepages == COMPACT_CLUSTER_MAX) {
    			++low_pfn;
    			break;
    		}
    	}
    
    	/*
    	 * The PageBuddy() check could have potentially brought us outside
    	 * the range to be scanned.
    	 */
    	if (unlikely(low_pfn > end_pfn))
    		low_pfn = end_pfn;
    
    	if (locked)
    		spin_unlock_irqrestore(&zone->lru_lock, flags);
    
    	/*
    	 * Update the pageblock-skip information and cached scanner pfn,
    	 * if the whole pageblock was scanned without isolating any page.
    	 */
    	if (low_pfn == end_pfn)
    		update_pageblock_skip(cc, valid_page, nr_isolated, true);
    
    	trace_mm_compaction_isolate_migratepages(start_pfn, low_pfn,
    						nr_scanned, nr_isolated);
    
    	count_compact_events(COMPACTMIGRATE_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_end_pfn;
    
    	/* Scan block by block. First and last block may be incomplete */
    	pfn = start_pfn;
    	block_end_pfn = ALIGN(pfn + 1, pageblock_nr_pages);
    
    	for (; pfn < end_pfn; pfn = block_end_pfn,
    				block_end_pfn += pageblock_nr_pages) {
    
    		block_end_pfn = min(block_end_pfn, end_pfn);
    
    		if (!pageblock_pfn_to_page(pfn, block_end_pfn, cc->zone))
    			continue;
    
    		pfn = isolate_migratepages_block(cc, pfn, block_end_pfn,
    							ISOLATE_UNEVICTABLE);
    
    		/*
    		 * In case of fatal failure, release everything that might
    		 * have been isolated in the previous iteration, and signal
    		 * the failure back to caller.
    		 */
    		if (!pfn) {
    			putback_movable_pages(&cc->migratepages);
    			cc->nr_migratepages = 0;
    			break;
    		}
    
    		if (cc->nr_migratepages == COMPACT_CLUSTER_MAX)
    			break;
    	}
    	acct_isolated(cc->zone, cc);
    
    	return pfn;
    }
    
    #endif /* CONFIG_COMPACTION || CONFIG_CMA */
    #ifdef CONFIG_COMPACTION
    /*
     * 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;
    
    	/*
    	 * 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 = cc->free_pfn & ~(pageblock_nr_pages-1);
    	block_end_pfn = min(block_start_pfn + pageblock_nr_pages,
    						zone_end_pfn(zone));
    	low_pfn = ALIGN(cc->migrate_pfn + 1, pageblock_nr_pages);
    
    	/*
    	 * 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 &&
    			cc->nr_migratepages > cc->nr_freepages;
    				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(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);
    
    		/*
    		 * Remember where the free scanner should restart next time,
    		 * which is where isolate_freepages_block() left off.
    		 * But if it scanned the whole pageblock, isolate_start_pfn
    		 * now points at block_end_pfn, which is the start of the next
    		 * pageblock.
    		 * In that case we will however want to restart at the start
    		 * of the previous pageblock.
    		 */
    		cc->free_pfn = (isolate_start_pfn < block_end_pfn) ?
    				isolate_start_pfn :
    				block_start_pfn - pageblock_nr_pages;
    
    		/*
    		 * isolate_freepages_block() might have aborted due to async
    		 * compaction being contended
    		 */
    		if (cc->contended)
    			break;
    	}
    
    	/* split_free_page does not map the pages */
    	map_pages(freelist);
    
    	/*
    	 * If we crossed the migrate scanner, we want to keep it that way
    	 * so that compact_finished() may detect this
    	 */
    	if (block_start_pfn < low_pfn)
    		cc->free_pfn = cc->migrate_pfn;
    }
    
    /*
     * 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,
    					int **result)
    {
    	struct compact_control *cc = (struct compact_control *)data;
    	struct page *freepage;
    
    	/*
    	 * Isolate free pages if necessary, and if we are not aborting due to
    	 * contention.
    	 */
    	if (list_empty(&cc->freepages)) {
    		if (!cc->contended)
    			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;
    
    /*
     * 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 low_pfn, end_pfn;
    	struct page *page;
    	const isolate_mode_t isolate_mode =
    		(sysctl_compact_unevictable_allowed ? ISOLATE_UNEVICTABLE : 0) |
    		(cc->mode == MIGRATE_ASYNC ? ISOLATE_ASYNC_MIGRATE : 0);
    
    	/*
    	 * Start at where we last stopped, or beginning of the zone as
    	 * initialized by compact_zone()
    	 */
    	low_pfn = cc->migrate_pfn;
    
    	/* Only scan within a pageblock boundary */
    	end_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages);
    
    	/*
    	 * Iterate over whole pageblocks until we find the first suitable.
    	 * Do not cross the free scanner.
    	 */
    	for (; end_pfn <= cc->free_pfn;
    			low_pfn = end_pfn, 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(low_pfn, end_pfn, zone);
    		if (!page)
    			continue;
    
    		/* If isolation recently failed, do not retry */
    		if (!isolation_suitable(cc, page))
    			continue;
    
    		/*
    		 * For async compaction, also only scan in MOVABLE blocks.
    		 * Async compaction is optimistic to see if the minimum amount
    		 * of work satisfies the allocation.
    		 */
    		if (cc->mode == MIGRATE_ASYNC &&
    		    !migrate_async_suitable(get_pageblock_migratetype(page)))
    			continue;
    
    		/* Perform the isolation */
    		low_pfn = isolate_migratepages_block(cc, low_pfn, end_pfn,
    								isolate_mode);
    
    		if (!low_pfn || cc->contended) {
    			acct_isolated(zone, cc);
    			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;
    	}
    
    	acct_isolated(zone, cc);
    	/*
    	 * Record where migration scanner will be restarted. If we end up in
    	 * the same pageblock as the free scanner, make the scanners fully
    	 * meet so that compact_finished() terminates compaction.
    	 */
    	cc->migrate_pfn = (end_pfn <= cc->free_pfn) ? low_pfn : cc->free_pfn;
    
    	return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
    }
    
    static int __compact_finished(struct zone *zone, struct compact_control *cc,
    			    const int migratetype)
    {
    	unsigned int order;
    	unsigned long watermark;
    
    	if (cc->contended || fatal_signal_pending(current))
    		return COMPACT_PARTIAL;
    
    	/* Compaction run completes if the migrate and free scanner meet */
    	if (cc->free_pfn <= cc->migrate_pfn) {
    		/* Let the next compaction start anew. */
    		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 = zone_end_pfn(zone);
    
    		/*
    		 * Mark that the PG_migrate_skip information should be cleared
    		 * by kswapd when it goes to sleep. kswapd does not set the
    		 * flag itself as the decision to be clear should be directly
    		 * based on an allocation request.
    		 */
    		if (!current_is_kswapd())
    			zone->compact_blockskip_flush = true;
    
    		return COMPACT_COMPLETE;
    	}
    
    	/*
    	 * order == -1 is expected when compacting via
    	 * /proc/sys/vm/compact_memory
    	 */
    	if (cc->order == -1)
    		return COMPACT_CONTINUE;
    
    	/* Compaction run is not finished if the watermark is not met */
    	watermark = low_wmark_pages(zone);
    
    	if (!zone_watermark_ok(zone, cc->order, watermark, cc->classzone_idx,
    							cc->alloc_flags))
    		return COMPACT_CONTINUE;
    
    	/* Direct compactor: Is a suitable page free? */
    	for (order = cc->order; order < MAX_ORDER; order++) {
    		struct free_area *area = &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_PARTIAL;
    
    #ifdef CONFIG_CMA
    		/* MIGRATE_MOVABLE can fallback on MIGRATE_CMA */
    		if (migratetype == MIGRATE_MOVABLE &&
    			!list_empty(&area->free_list[MIGRATE_CMA]))
    			return COMPACT_PARTIAL;
    #endif
    		/*
    		 * Job done if allocation would steal freepages from
    		 * other migratetype buddy lists.
    		 */
    		if (find_suitable_fallback(area, order, migratetype,
    						true, &can_steal) != -1)
    			return COMPACT_PARTIAL;
    	}
    
    	return COMPACT_NO_SUITABLE_PAGE;
    }
    
    static int compact_finished(struct zone *zone, struct compact_control *cc,
    			    const int migratetype)
    {
    	int ret;
    
    	ret = __compact_finished(zone, cc, migratetype);
    	trace_mm_compaction_finished(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_PARTIAL  - If the allocation would succeed without compaction
     *   COMPACT_CONTINUE - If compaction should run now
     */
    static unsigned long __compaction_suitable(struct zone *zone, int order,
    					int alloc_flags, int classzone_idx)
    {
    	int fragindex;
    	unsigned long watermark;
    
    	/*
    	 * order == -1 is expected when compacting via
    	 * /proc/sys/vm/compact_memory
    	 */
    	if (order == -1)
    		return COMPACT_CONTINUE;
    
    	watermark = low_wmark_pages(zone);
    	/*
    	 * 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_PARTIAL;
    
    	/*
    	 * Watermarks for order-0 must be met for compaction. Note the 2UL.
    	 * This is because during migration, copies of pages need to be
    	 * allocated and for a short time, the footprint is higher
    	 */
    	watermark += (2UL << order);
    	if (!zone_watermark_ok(zone, 0, watermark, classzone_idx, alloc_flags))
    		return COMPACT_SKIPPED;
    
    	/*
    	 * 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.
    	 */
    	fragindex = fragmentation_index(zone, order);
    	if (fragindex >= 0 && fragindex <= sysctl_extfrag_threshold)
    		return COMPACT_NOT_SUITABLE_ZONE;
    
    	return COMPACT_CONTINUE;
    }
    
    unsigned long compaction_suitable(struct zone *zone, int order,
    					int alloc_flags, int classzone_idx)
    {
    	unsigned long ret;
    
    	ret = __compaction_suitable(zone, order, alloc_flags, classzone_idx);
    	trace_mm_compaction_suitable(zone, order, ret);
    	if (ret == COMPACT_NOT_SUITABLE_ZONE)
    		ret = COMPACT_SKIPPED;
    
    	return ret;
    }
    
    static int compact_zone(struct zone *zone, struct compact_control *cc)
    {
    	int ret;
    	unsigned long start_pfn = zone->zone_start_pfn;
    	unsigned long end_pfn = zone_end_pfn(zone);
    	const int migratetype = gfpflags_to_migratetype(cc->gfp_mask);
    	const bool sync = cc->mode != MIGRATE_ASYNC;
    	unsigned long last_migrated_pfn = 0;
    
    	ret = compaction_suitable(zone, cc->order, cc->alloc_flags,
    							cc->classzone_idx);
    	switch (ret) {
    	case COMPACT_PARTIAL:
    	case COMPACT_SKIPPED:
    		/* Compaction is likely to fail */
    		return ret;
    	case COMPACT_CONTINUE:
    		/* Fall through to compaction */
    		;
    	}
    
    	/*
    	 * Clear pageblock skip if there were failures recently and compaction
    	 * is about to be retried after being deferred. kswapd does not do
    	 * this reset as it'll reset the cached information when going to sleep.
    	 */
    	if (compaction_restarting(zone, cc->order) && !current_is_kswapd())
    		__reset_isolation_suitable(zone);
    
    	/*
    	 * Setup to move all movable pages to the end of the zone. Used cached
    	 * information on where the scanners should start but check that it
    	 * is initialised by ensuring the values are within zone boundaries.
    	 */
    	cc->migrate_pfn = zone->compact_cached_migrate_pfn[sync];
    	cc->free_pfn = zone->compact_cached_free_pfn;
    	if (cc->free_pfn < start_pfn || cc->free_pfn > end_pfn) {
    		cc->free_pfn = end_pfn & ~(pageblock_nr_pages-1);
    		zone->compact_cached_free_pfn = cc->free_pfn;
    	}
    	if (cc->migrate_pfn < start_pfn || cc->migrate_pfn > end_pfn) {
    		cc->migrate_pfn = start_pfn;
    		zone->compact_cached_migrate_pfn[0] = cc->migrate_pfn;
    		zone->compact_cached_migrate_pfn[1] = cc->migrate_pfn;
    	}
    
    	trace_mm_compaction_begin(start_pfn, cc->migrate_pfn,
    				cc->free_pfn, end_pfn, sync);
    
    	migrate_prep_local();
    
    	while ((ret = compact_finished(zone, cc, migratetype)) ==
    						COMPACT_CONTINUE) {
    		int err;
    		unsigned long isolate_start_pfn = cc->migrate_pfn;
    
    		switch (isolate_migratepages(zone, cc)) {
    		case ISOLATE_ABORT:
    			ret = COMPACT_PARTIAL;
    			putback_movable_pages(&cc->migratepages);
    			cc->nr_migratepages = 0;
    			goto out;
    		case ISOLATE_NONE:
    			/*
    			 * 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:
    			;
    		}
    
    		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 && cc->free_pfn > cc->migrate_pfn) {
    				ret = COMPACT_PARTIAL;
    				goto out;
    			}
    		}
    
    		/*
    		 * Record where we could have freed pages by migration and not
    		 * yet flushed them to buddy allocator. We use the pfn that
    		 * isolate_migratepages() started from in this loop iteration
    		 * - this is the lowest page that could have been isolated and
    		 * then freed by migration.
    		 */
    		if (!last_migrated_pfn)
    			last_migrated_pfn = isolate_start_pfn;
    
    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 =
    				cc->migrate_pfn & ~((1UL << cc->order) - 1);
    
    			if (last_migrated_pfn < current_block_start) {
    				cpu = get_cpu();
    				lru_add_drain_cpu(cpu);
    				drain_local_pages(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_nr_pages-1);
    		/*
    		 * Only go back, not forward. The cached pfn might have been
    		 * already reset to zone end in compact_finished()
    		 */
    		if (free_pfn > zone->compact_cached_free_pfn)
    			zone->compact_cached_free_pfn = free_pfn;
    	}
    
    	trace_mm_compaction_end(start_pfn, cc->migrate_pfn,
    				cc->free_pfn, end_pfn, sync, ret);
    
    	return ret;
    }
    
    static unsigned long compact_zone_order(struct zone *zone, int order,
    		gfp_t gfp_mask, enum migrate_mode mode, int *contended,
    		int alloc_flags, int classzone_idx)
    {
    	unsigned long ret;
    	struct compact_control cc = {
    		.nr_freepages = 0,
    		.nr_migratepages = 0,
    		.order = order,
    		.gfp_mask = gfp_mask,
    		.zone = zone,
    		.mode = mode,
    		.alloc_flags = alloc_flags,
    		.classzone_idx = classzone_idx,
    	};
    	INIT_LIST_HEAD(&cc.freepages);
    	INIT_LIST_HEAD(&cc.migratepages);
    
    	ret = compact_zone(zone, &cc);
    
    	VM_BUG_ON(!list_empty(&cc.freepages));
    	VM_BUG_ON(!list_empty(&cc.migratepages));
    
    	*contended = cc.contended;
    	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
     * @mode: The migration mode for async, sync light, or sync migration
     * @contended: Return value that determines if compaction was aborted due to
     *	       need_resched() or lock contention
     *
     * This is the main entry point for direct page compaction.
     */
    unsigned long try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
    			int alloc_flags, const struct alloc_context *ac,
    			enum migrate_mode mode, int *contended)
    {
    	int may_enter_fs = gfp_mask & __GFP_FS;
    	int may_perform_io = gfp_mask & __GFP_IO;
    	struct zoneref *z;
    	struct zone *zone;
    	int rc = COMPACT_DEFERRED;
    	int all_zones_contended = COMPACT_CONTENDED_LOCK; /* init for &= op */
    
    	*contended = COMPACT_CONTENDED_NONE;
    
    	/* Check if the GFP flags allow compaction */
    	if (!order || !may_enter_fs || !may_perform_io)
    		return COMPACT_SKIPPED;
    
    	trace_mm_compaction_try_to_compact_pages(order, gfp_mask, mode);
    
    	/* Compact each zone in the list */
    	for_each_zone_zonelist_nodemask(zone, z, ac->zonelist, ac->high_zoneidx,
    								ac->nodemask) {
    		int status;
    		int zone_contended;
    
    		if (compaction_deferred(zone, order))
    			continue;
    
    		status = compact_zone_order(zone, order, gfp_mask, mode,
    				&zone_contended, alloc_flags,
    				ac->classzone_idx);
    		rc = max(status, rc);
    		/*
    		 * It takes at least one zone that wasn't lock contended
    		 * to clear all_zones_contended.
    		 */
    		all_zones_contended &= zone_contended;
    
    		/* If a normal allocation would succeed, stop compacting */
    		if (zone_watermark_ok(zone, order, low_wmark_pages(zone),
    					ac->classzone_idx, alloc_flags)) {
    			/*
    			 * 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);
    			/*
    			 * It is possible that async compaction aborted due to
    			 * need_resched() and the watermarks were ok thanks to
    			 * somebody else freeing memory. The allocation can
    			 * however still fail so we better signal the
    			 * need_resched() contention anyway (this will not
    			 * prevent the allocation attempt).
    			 */
    			if (zone_contended == COMPACT_CONTENDED_SCHED)
    				*contended = COMPACT_CONTENDED_SCHED;
    
    			goto break_loop;
    		}
    
    		if (mode != MIGRATE_ASYNC && status == COMPACT_COMPLETE) {
    			/*
    			 * 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 and signal need_resched()
    		 * contention.
    		 */
    		if ((zone_contended == COMPACT_CONTENDED_SCHED)
    					|| fatal_signal_pending(current)) {
    			*contended = COMPACT_CONTENDED_SCHED;
    			goto break_loop;
    		}
    
    		continue;
    break_loop:
    		/*
    		 * We might not have tried all the zones, so  be conservative
    		 * and assume they are not all lock contended.
    		 */
    		all_zones_contended = 0;
    		break;
    	}
    
    	/*
    	 * If at least one zone wasn't deferred or skipped, we report if all
    	 * zones that were tried were lock contended.
    	 */
    	if (rc > COMPACT_SKIPPED && all_zones_contended)
    		*contended = COMPACT_CONTENDED_LOCK;
    
    	return rc;
    }
    
    
    /* Compact all zones within a node */
    static void __compact_pgdat(pg_data_t *pgdat, struct compact_control *cc)
    {
    	int zoneid;
    	struct zone *zone;
    
    	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);
    
    		/*
    		 * When called via /proc/sys/vm/compact_memory
    		 * this makes sure we compact the whole zone regardless of
    		 * cached scanner positions.
    		 */
    		if (cc->order == -1)
    			__reset_isolation_suitable(zone);
    
    		if (cc->order == -1 || !compaction_deferred(zone, cc->order))
    			compact_zone(zone, cc);
    
    		if (cc->order > 0) {
    			if (zone_watermark_ok(zone, cc->order,
    						low_wmark_pages(zone), 0, 0))
    				compaction_defer_reset(zone, cc->order, false);
    		}
    
    		VM_BUG_ON(!list_empty(&cc->freepages));
    		VM_BUG_ON(!list_empty(&cc->migratepages));
    	}
    }
    
    void compact_pgdat(pg_data_t *pgdat, int order)
    {
    	struct compact_control cc = {
    		.order = order,
    		.mode = MIGRATE_ASYNC,
    	};
    
    	if (!order)
    		return;
    
    	__compact_pgdat(pgdat, &cc);
    }
    
    static void compact_node(int nid)
    {
    	struct compact_control cc = {
    		.order = -1,
    		.mode = MIGRATE_SYNC,
    		.ignore_skip_hint = true,
    	};
    
    	__compact_pgdat(NODE_DATA(nid), &cc);
    }
    
    /* 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 */
    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;
    }
    
    int sysctl_extfrag_handler(struct ctl_table *table, int write,
    			void __user *buffer, size_t *length, loff_t *ppos)
    {
    	proc_dointvec_minmax(table, write, buffer, length, ppos);
    
    	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, S_IWUSR, 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 */
    
    #endif /* CONFIG_COMPACTION */