// 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 "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 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);
  	}
  }

  /*
   * 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->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 (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 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,

  			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 = true;

  			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 = true;

  		return true;
  	}

  	if (need_resched()) {

  		if (cc->mode == MIGRATE_ASYNC) {

  			cc->contended = true;

  			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 = true;

  			return 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) {
  			/*
  			 * 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, 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, false);

  	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 */

  	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;

  	/*
  	 * 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);

  		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;

  		}

  		/*

  		 * 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_trylock_irqsave(zone_lru_lock(zone),

  								&flags, cc);

  			if (!locked)
  				break;

  			/* 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++;

  		/*
  		 * Record where we could have freed pages by migration and not
  		 * yet flushed them to buddy allocator.
  		 * - this is the lowest page that was isolated and likely be
  		 * then freed by migration.
  		 */
  		if (!cc->last_migrated_pfn)
  			cc->last_migrated_pfn = low_pfn;

  		/* Avoid isolating too much */

  		if (cc->nr_migratepages == COMPACT_CLUSTER_MAX) {
  			++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;
  			cc->last_migrated_pfn = 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;

  	if (locked)

  		spin_unlock_irqrestore(zone_lru_lock(zone), 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);

  	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 ((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;
  }

  /*

   * 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);
  }
  
  /*

   * 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 = pageblock_start_pfn(cc->free_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);

  
  		/*

  		 * If we isolated enough freepages, or aborted due to lock
  		 * contention, terminate.

  		 */

  		if ((cc->nr_freepages >= cc->nr_migratepages)
  							|| cc->contended) {

  			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;

  		}

  	}

  	/* __isolate_free_page() does not map the pages */

  	map_pages(freelist);

  	/*

  	 * 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;

  }
  
  /*
   * 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;

  	/*
  	 * 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 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);

  	/*
  	 * Start at where we last stopped, or beginning of the zone as
  	 * initialized by compact_zone()
  	 */
  	low_pfn = cc->migrate_pfn;

  	block_start_pfn = pageblock_start_pfn(low_pfn);

  	if (block_start_pfn < zone->zone_start_pfn)
  		block_start_pfn = zone->zone_start_pfn;

  
  	/* 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;
  			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 */
  		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 (!suitable_migration_source(cc, page))

  			continue;
  
  		/* Perform the isolation */

  		low_pfn = isolate_migratepages_block(cc, low_pfn,
  						block_end_pfn, isolate_mode);

  		if (!low_pfn || cc->contended)

  			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 zone *zone,
  						struct compact_control *cc)

  {

  	unsigned int order;

  	const int migratetype = cc->migratetype;

  	if (cc->contended || fatal_signal_pending(current))

  		return COMPACT_CONTENDED;

  	/* Compaction run completes if the migrate and free scanner meet */

  	if (compact_scanners_met(cc)) {

  		/* Let the next compaction start anew. */

  		reset_cached_positions(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)

  			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;

  	if (cc->finishing_block) {
  		/*
  		 * We have finished the pageblock, but better check again that
  		 * we really succeeded.
  		 */
  		if (IS_ALIGNED(cc->migrate_pfn, pageblock_nr_pages))
  			cc->finishing_block = false;
  		else
  			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_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;
  			}
  
  			cc->finishing_block = true;
  			return COMPACT_CONTINUE;
  		}

  	}

  	return COMPACT_NO_SUITABLE_PAGE;
  }

  static enum compact_result compact_finished(struct zone *zone,

  			struct compact_control *cc)

  {
  	int ret;

  	ret = __compact_finished(zone, cc);

  	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_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 = zone->watermark[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 zone *zone, struct compact_control *cc)

  {

  	enum compact_result ret;

  	unsigned long start_pfn = zone->zone_start_pfn;

  	unsigned long end_pfn = zone_end_pfn(zone);

  	const bool sync = cc->mode != MIGRATE_ASYNC;

  	cc->migratetype = gfpflags_to_migratetype(cc->gfp_mask);

  	ret = compaction_suitable(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(zone, cc->order))

  		__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 (unless we explicitly
  	 * want to compact the whole zone), but check that it is initialised
  	 * by ensuring the values are within zone boundaries.

  	 */

  	if (cc->whole_zone) {

  		cc->migrate_pfn = start_pfn;

  		cc->free_pfn = pageblock_start_pfn(end_pfn - 1);
  	} else {
  		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 = pageblock_start_pfn(end_pfn - 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;
  		}

  		if (cc->migrate_pfn == start_pfn)
  			cc->whole_zone = true;
  	}

  	cc->last_migrated_pfn = 0;

  	trace_mm_compaction_begin(start_pfn, cc->migrate_pfn,
  				cc->free_pfn, end_pfn, sync);

  	migrate_prep_local();

  	while ((ret = compact_finished(zone, cc)) == COMPACT_CONTINUE) {

  		int err;

  		switch (isolate_migratepages(zone, cc)) {
  		case ISOLATE_ABORT:

  			ret = COMPACT_CONTENDED;

  			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 && !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 */
  				cc->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 && cc->last_migrated_pfn) {

  			int cpu;
  			unsigned long current_block_start =

  				block_start_pfn(cc->migrate_pfn, cc->order);

  			if (cc->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 */

  				cc->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 > zone->compact_cached_free_pfn)
  			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);