text: /*
   * 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 "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>
  
  static unsigned long release_freepages(struct list_head *freelist)
  {
  	struct page *page, *next;
  	unsigned long count = 0;
  
  	list_for_each_entry_safe(page, next, freelist, lru) {
  		list_del(&page->lru);
  		__free_page(page);
  		count++;
  	}
  
  	return count;
  }
  
  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);
  	}
  }
  
  static inline bool migrate_async_suitable(int migratetype)
  {
  	return is_migrate_cma(migratetype) || migratetype == MIGRATE_MOVABLE;
  }
  
  #ifdef CONFIG_COMPACTION
  /* 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 = 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;
  
  	if (cc->ignore_skip_hint)
  		return;
  
  	if (!page)
  		return;
  
  	if (!nr_isolated) {
  		unsigned long pfn = page_to_pfn(page);
  		set_pageblock_skip(page);
  
  		/* Update where compaction should restart */
  		if (migrate_scanner) {
  			if (!cc->finished_update_migrate &&
  			    pfn > zone->compact_cached_migrate_pfn)
  				zone->compact_cached_migrate_pfn = pfn;
  		} else {
  			if (!cc->finished_update_free &&
  			    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 */
  
  static inline bool should_release_lock(spinlock_t *lock)
  {
  	return need_resched() || spin_is_contended(lock);
  }
  
  /*
   * Compaction requires the taking of some coarse locks that are potentially
   * very heavily contended. Check if the process needs to be scheduled or
   * if the lock is contended. For async compaction, back out in the event
   * if contention is severe. For sync compaction, schedule.
   *
   * Returns true if the lock is held.
   * Returns false if the lock is released and compaction should abort
   */
  static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags,
  				      bool locked, struct compact_control *cc)
  {
  	if (should_release_lock(lock)) {
  		if (locked) {
  			spin_unlock_irqrestore(lock, *flags);
  			locked = false;
  		}
  
  		/* async aborts if taking too long or contended */
  		if (!cc->sync) {
  			cc->contended = true;
  			return false;
  		}
  
  		cond_resched();
  	}
  
  	if (!locked)
  		spin_lock_irqsave(lock, *flags);
  	return true;
  }
  
  static inline bool compact_trylock_irqsave(spinlock_t *lock,
  			unsigned long *flags, struct compact_control *cc)
  {
  	return compact_checklock_irqsave(lock, flags, false, cc);
  }
  
  /* Returns true if the page is within a block suitable for migration to */
  static bool suitable_migration_target(struct page *page)
  {
  	int migratetype = get_pageblock_migratetype(page);
  
  	/* Don't interfere with memory hot-remove or the min_free_kbytes blocks */
  	if (migratetype == MIGRATE_RESERVE)
  		return false;
  
  	if (is_migrate_isolate(migratetype))
  		return false;
  
  	/* If the page is a large free page, then allow migration */
  	if (PageBuddy(page) && page_order(page) >= pageblock_order)
  		return true;
  
  	/* If the block is MIGRATE_MOVABLE or MIGRATE_CMA, allow migration */
  	if (migrate_async_suitable(migratetype))
  		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 blockpfn,
  				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;
  	bool locked = false;
  
  	cursor = pfn_to_page(blockpfn);
  
  	/* Isolate free pages. */
  	for (; blockpfn < end_pfn; blockpfn++, cursor++) {
  		int isolated, i;
  		struct page *page = cursor;
  
  		nr_scanned++;
  		if (!pfn_valid_within(blockpfn))
  			goto isolate_fail;
  
  		if (!valid_page)
  			valid_page = page;
  		if (!PageBuddy(page))
  			goto isolate_fail;
  
  		/*
  		 * 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_checklock_irqsave(&cc->zone->lock, &flags,
  								locked, cc);
  		if (!locked)
  			break;
  
  		/* Recheck this is a suitable migration target under lock */
  		if (!strict && !suitable_migration_target(page))
  			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) {
  			blockpfn += isolated - 1;
  			cursor += isolated - 1;
  			continue;
  		}
  
  isolate_fail:
  		if (strict)
  			break;
  		else
  			continue;
  
  	}
  
  	trace_mm_compaction_isolate_freepages(nr_scanned, total_isolated);
  
  	/*
  	 * 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);
  
  	for (pfn = start_pfn; pfn < end_pfn; pfn += isolated) {
  		if (!pfn_valid(pfn) || cc->zone != page_zone(pfn_to_page(pfn)))
  			break;
  
  		/*
  		 * On subsequent iterations ALIGN() is actually not needed,
  		 * but we keep it that we not to complicate the code.
  		 */
  		block_end_pfn = ALIGN(pfn + 1, pageblock_nr_pages);
  		block_end_pfn = min(block_end_pfn, end_pfn);
  
  		isolated = isolate_freepages_block(cc, 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, bool locked, struct compact_control *cc)
  {
  	struct page *page;
  	unsigned int count[2] = { 0, };
  
  	list_for_each_entry(page, &cc->migratepages, lru)
  		count[!!page_is_file_cache(page)]++;
  
  	/* If locked we can use the interrupt unsafe versions */
  	if (locked) {
  		__mod_zone_page_state(zone, NR_ISOLATED_ANON, count[0]);
  		__mod_zone_page_state(zone, NR_ISOLATED_FILE, count[1]);
  	} else {
  		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_range() - isolate all migrate-able pages in range.
   * @zone:	Zone pages are in.
   * @cc:		Compaction control structure.
   * @low_pfn:	The first PFN of the range.
   * @end_pfn:	The one-past-the-last PFN of the range.
   * @unevictable: true if it allows to isolate unevictable pages
   *
   * Isolate all pages that can be migrated from the range specified by
   * [low_pfn, end_pfn).  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 then end_pfn).
   *
   * Assumes that cc->migratepages is empty and cc->nr_migratepages is
   * zero.
   *
   * Apart from cc->migratepages and cc->nr_migratetypes this function
   * does not modify any cc's fields, in particular it does not modify
   * (or read for that matter) cc->migrate_pfn.
   */
  unsigned long
  isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
  		unsigned long low_pfn, unsigned long end_pfn, bool unevictable)
  {
  	unsigned long last_pageblock_nr = 0, pageblock_nr;
  	unsigned long nr_scanned = 0, nr_isolated = 0;
  	struct list_head *migratelist = &cc->migratepages;
  	isolate_mode_t mode = 0;
  	struct lruvec *lruvec;
  	unsigned long flags;
  	bool locked = false;
  	struct page *page = NULL, *valid_page = NULL;
  
  	/*
  	 * 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->sync)
  			return 0;
  
  		congestion_wait(BLK_RW_ASYNC, HZ/10);
  
  		if (fatal_signal_pending(current))
  			return 0;
  	}
  
  	/* Time to isolate some pages for migration */
  	cond_resched();
  	for (; low_pfn < end_pfn; low_pfn++) {
  		/* give a chance to irqs before checking need_resched() */
  		if (locked && !((low_pfn+1) % SWAP_CLUSTER_MAX)) {
  			if (should_release_lock(&zone->lru_lock)) {
  				spin_unlock_irqrestore(&zone->lru_lock, flags);
  				locked = false;
  			}
  		}
  
  		/*
  		 * migrate_pfn does not necessarily start aligned to a
  		 * pageblock. Ensure that pfn_valid is called when moving
  		 * into a new MAX_ORDER_NR_PAGES range in case of large
  		 * memory holes within the zone
  		 */
  		if ((low_pfn & (MAX_ORDER_NR_PAGES - 1)) == 0) {
  			if (!pfn_valid(low_pfn)) {
  				low_pfn += MAX_ORDER_NR_PAGES - 1;
  				continue;
  			}
  		}
  
  		if (!pfn_valid_within(low_pfn))
  			continue;
  		nr_scanned++;
  
  		/*
  		 * Get the page and ensure the page is within the same zone.
  		 * See the comment in isolate_freepages about overlapping
  		 * nodes. It is deliberate that the new zone lock is not taken
  		 * as memory compaction should not move pages between nodes.
  		 */
  		page = pfn_to_page(low_pfn);
  		if (page_zone(page) != zone)
  			continue;
  
  		if (!valid_page)
  			valid_page = page;
  
  		/* If isolation recently failed, do not retry */
  		pageblock_nr = low_pfn >> pageblock_order;
  		if (!isolation_suitable(cc, page))
  			goto next_pageblock;
  
  		/* Skip if free */
  		if (PageBuddy(page))
  			continue;
  
  		/*
  		 * For async migration, also only scan in MOVABLE blocks. Async
  		 * migration is optimistic to see if the minimum amount of work
  		 * satisfies the allocation
  		 */
  		if (!cc->sync && last_pageblock_nr != pageblock_nr &&
  		    !migrate_async_suitable(get_pageblock_migratetype(page))) {
  			cc->finished_update_migrate = true;
  			goto next_pageblock;
  		}
  
  		/*
  		 * 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 (locked && balloon_page_isolate(page)) {
  					/* Successfully isolated */
  					cc->finished_update_migrate = true;
  					list_add(&page->lru, migratelist);
  					cc->nr_migratepages++;
  					nr_isolated++;
  					goto check_compact_cluster;
  				}
  			}
  			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)
  				goto next_pageblock;
  			low_pfn += (1 << compound_order(page)) - 1;
  			continue;
  		}
  
  		/* Check if it is ok to still hold the lock */
  		locked = compact_checklock_irqsave(&zone->lru_lock, &flags,
  								locked, cc);
  		if (!locked || fatal_signal_pending(current))
  			break;
  
  		/* Recheck PageLRU and PageTransHuge under lock */
  		if (!PageLRU(page))
  			continue;
  		if (PageTransHuge(page)) {
  			low_pfn += (1 << compound_order(page)) - 1;
  			continue;
  		}
  
  		if (!cc->sync)
  			mode |= ISOLATE_ASYNC_MIGRATE;
  
  		if (unevictable)
  			mode |= ISOLATE_UNEVICTABLE;
  
  		lruvec = mem_cgroup_page_lruvec(page, zone);
  
  		/* Try isolate the page */
  		if (__isolate_lru_page(page, mode) != 0)
  			continue;
  
  		VM_BUG_ON(PageTransCompound(page));
  
  		/* Successfully isolated */
  		cc->finished_update_migrate = true;
  		del_page_from_lru_list(page, lruvec, page_lru(page));
  		list_add(&page->lru, migratelist);
  		cc->nr_migratepages++;
  		nr_isolated++;
  
  check_compact_cluster:
  		/* Avoid isolating too much */
  		if (cc->nr_migratepages == COMPACT_CLUSTER_MAX) {
  			++low_pfn;
  			break;
  		}
  
  		continue;
  
  next_pageblock:
  		low_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages) - 1;
  		last_pageblock_nr = pageblock_nr;
  	}
  
  	acct_isolated(zone, locked, cc);
  
  	if (locked)
  		spin_unlock_irqrestore(&zone->lru_lock, flags);
  
  	/* Update the pageblock-skip if the whole pageblock was scanned */
  	if (low_pfn == end_pfn)
  		update_pageblock_skip(cc, valid_page, nr_isolated, true);
  
  	trace_mm_compaction_isolate_migratepages(nr_scanned, nr_isolated);
  
  	count_compact_events(COMPACTMIGRATE_SCANNED, nr_scanned);
  	if (nr_isolated)
  		count_compact_events(COMPACTISOLATED, nr_isolated);
  
  	return low_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 zone *zone,
  				struct compact_control *cc)
  {
  	struct page *page;
  	unsigned long high_pfn, low_pfn, pfn, z_end_pfn, end_pfn;
  	int nr_freepages = cc->nr_freepages;
  	struct list_head *freelist = &cc->freepages;
  
  	/*
  	 * Initialise the free scanner. The starting point is where we last
  	 * scanned from (or the end of the zone if starting). The low point
  	 * is the end of the pageblock the migration scanner is using.
  	 */
  	pfn = cc->free_pfn;
  	low_pfn = cc->migrate_pfn + pageblock_nr_pages;
  
  	/*
  	 * Take care that if the migration scanner is at the end of the zone
  	 * that the free scanner does not accidentally move to the next zone
  	 * in the next isolation cycle.
  	 */
  	high_pfn = min(low_pfn, pfn);
  
  	z_end_pfn = zone_end_pfn(zone);
  
  	/*
  	 * 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 (; pfn > low_pfn && cc->nr_migratepages > nr_freepages;
  					pfn -= pageblock_nr_pages) {
  		unsigned long isolated;
  
  		/*
  		 * This can iterate a massively long zone without finding any
  		 * suitable migration targets, so periodically check if we need
  		 * to schedule.
  		 */
  		cond_resched();
  
  		if (!pfn_valid(pfn))
  			continue;
  
  		/*
  		 * Check for overlapping nodes/zones. 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.
  		 */
  		page = pfn_to_page(pfn);
  		if (page_zone(page) != zone)
  			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 */
  		isolated = 0;
  
  		/*
  		 * As pfn may not start aligned, pfn+pageblock_nr_page
  		 * may cross a MAX_ORDER_NR_PAGES boundary and miss
  		 * a pfn_valid check. Ensure isolate_freepages_block()
  		 * only scans within a pageblock
  		 */
  		end_pfn = ALIGN(pfn + 1, pageblock_nr_pages);
  		end_pfn = min(end_pfn, z_end_pfn);
  		isolated = isolate_freepages_block(cc, pfn, end_pfn,
  						   freelist, false);
  		nr_freepages += isolated;
  
  		/*
  		 * Record the highest PFN we isolated pages from. When next
  		 * looking for free pages, the search will restart here as
  		 * page migration may have returned some pages to the allocator
  		 */
  		if (isolated) {
  			cc->finished_update_free = true;
  			high_pfn = max(high_pfn, pfn);
  		}
  	}
  
  	/* split_free_page does not map the pages */
  	map_pages(freelist);
  
  	cc->free_pfn = high_pfn;
  	cc->nr_freepages = nr_freepages;
  }
  
  /*
   * 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 */
  	if (list_empty(&cc->freepages)) {
  		isolate_freepages(cc->zone, 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;
  }
  
  /*
   * We cannot control nr_migratepages and nr_freepages fully when migration is
   * running as migrate_pages() has no knowledge of compact_control. When
   * migration is complete, we count the number of pages on the lists by hand.
   */
  static void update_nr_listpages(struct compact_control *cc)
  {
  	int nr_migratepages = 0;
  	int nr_freepages = 0;
  	struct page *page;
  
  	list_for_each_entry(page, &cc->migratepages, lru)
  		nr_migratepages++;
  	list_for_each_entry(page, &cc->freepages, lru)
  		nr_freepages++;
  
  	cc->nr_migratepages = nr_migratepages;
  	cc->nr_freepages = 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;
  
  /*
   * Isolate all pages that can be migrated from the block pointed to by
   * the migrate scanner within compact_control.
   */
  static isolate_migrate_t isolate_migratepages(struct zone *zone,
  					struct compact_control *cc)
  {
  	unsigned long low_pfn, end_pfn;
  
  	/* Do not scan outside zone boundaries */
  	low_pfn = max(cc->migrate_pfn, zone->zone_start_pfn);
  
  	/* Only scan within a pageblock boundary */
  	end_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages);
  
  	/* Do not cross the free scanner or scan within a memory hole */
  	if (end_pfn > cc->free_pfn || !pfn_valid(low_pfn)) {
  		cc->migrate_pfn = end_pfn;
  		return ISOLATE_NONE;
  	}
  
  	/* Perform the isolation */
  	low_pfn = isolate_migratepages_range(zone, cc, low_pfn, end_pfn, false);
  	if (!low_pfn || cc->contended)
  		return ISOLATE_ABORT;
  
  	cc->migrate_pfn = low_pfn;
  
  	return ISOLATE_SUCCESS;
  }
  
  static int compact_finished(struct zone *zone,
  			    struct compact_control *cc)
  {
  	unsigned int order;
  	unsigned long watermark;
  
  	if (fatal_signal_pending(current))
  		return COMPACT_PARTIAL;
  
  	/* Compaction run completes if the migrate and free scanner meet */
  	if (cc->free_pfn <= cc->migrate_pfn) {
  		/*
  		 * 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);
  	watermark += (1 << cc->order);
  
  	if (!zone_watermark_ok(zone, cc->order, watermark, 0, 0))
  		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];
  
  		/* Job done if page is free of the right migratetype */
  		if (!list_empty(&area->free_list[cc->migratetype]))
  			return COMPACT_PARTIAL;
  
  		/* Job done if allocation would set block type */
  		if (cc->order >= pageblock_order && area->nr_free)
  			return COMPACT_PARTIAL;
  	}
  
  	return COMPACT_CONTINUE;
  }
  
  /*
   * 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
   */
  unsigned long compaction_suitable(struct zone *zone, int order)
  {
  	int fragindex;
  	unsigned long watermark;
  
  	/*
  	 * order == -1 is expected when compacting via
  	 * /proc/sys/vm/compact_memory
  	 */
  	if (order == -1)
  		return COMPACT_CONTINUE;
  
  	/*
  	 * 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 = low_wmark_pages(zone) + (2UL << order);
  	if (!zone_watermark_ok(zone, 0, watermark, 0, 0))
  		return COMPACT_SKIPPED;
  
  	/*
  	 * fragmentation index determines if allocation failures are due to
  	 * low memory or external fragmentation
  	 *
  	 * index of -1000 implies allocations might succeed depending on
  	 * watermarks
  	 * 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_SKIPPED;
  
  	if (fragindex == -1000 && zone_watermark_ok(zone, order, watermark,
  	    0, 0))
  		return COMPACT_PARTIAL;
  
  	return COMPACT_CONTINUE;
  }
  
  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);
  
  	ret = compaction_suitable(zone, cc->order);
  	switch (ret) {
  	case COMPACT_PARTIAL:
  	case COMPACT_SKIPPED:
  		/* Compaction is likely to fail */
  		return ret;
  	case COMPACT_CONTINUE:
  		/* Fall through to compaction */
  		;
  	}
  
  	/*
  	 * 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;
  	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 = cc->migrate_pfn;
  	}
  
  	/*
  	 * 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);
  
  	migrate_prep_local();
  
  	while ((ret = compact_finished(zone, cc)) == COMPACT_CONTINUE) {
  		unsigned long nr_migrate, nr_remaining;
  		int err;
  
  		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:
  			continue;
  		case ISOLATE_SUCCESS:
  			;
  		}
  
  		nr_migrate = cc->nr_migratepages;
  		err = migrate_pages(&cc->migratepages, compaction_alloc,
  				(unsigned long)cc,
  				cc->sync ? MIGRATE_SYNC_LIGHT : MIGRATE_ASYNC,
  				MR_COMPACTION);
  		update_nr_listpages(cc);
  		nr_remaining = cc->nr_migratepages;
  
  		trace_mm_compaction_migratepages(nr_migrate - nr_remaining,
  						nr_remaining);
  
  		/* Release isolated pages not migrated */
  		if (err) {
  			putback_movable_pages(&cc->migratepages);
  			cc->nr_migratepages = 0;
  			if (err == -ENOMEM) {
  				ret = COMPACT_PARTIAL;
  				goto out;
  			}
  		}
  	}
  
  out:
  	/* Release free pages and check accounting */
  	cc->nr_freepages -= release_freepages(&cc->freepages);
  	VM_BUG_ON(cc->nr_freepages != 0);
  
  	return ret;
  }
  
  static unsigned long compact_zone_order(struct zone *zone,
  				 int order, gfp_t gfp_mask,
  				 bool sync, bool *contended)
  {
  	unsigned long ret;
  	struct compact_control cc = {
  		.nr_freepages = 0,
  		.nr_migratepages = 0,
  		.order = order,
  		.migratetype = allocflags_to_migratetype(gfp_mask),
  		.zone = zone,
  		.sync = sync,
  	};
  	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
   * @zonelist: The zonelist used for the current allocation
   * @order: The order of the current allocation
   * @gfp_mask: The GFP mask of the current allocation
   * @nodemask: The allowed nodes to allocate from
   * @sync: Whether migration is synchronous or not
   * @contended: Return value that is true if compaction was aborted due to lock contention
   * @page: Optionally capture a free page of the requested order during compaction
   *
   * This is the main entry point for direct page compaction.
   */
  unsigned long try_to_compact_pages(struct zonelist *zonelist,
  			int order, gfp_t gfp_mask, nodemask_t *nodemask,
  			bool sync, bool *contended)
  {
  	enum zone_type high_zoneidx = gfp_zone(gfp_mask);
  	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_SKIPPED;
  	int alloc_flags = 0;
  
  	/* Check if the GFP flags allow compaction */
  	if (!order || !may_enter_fs || !may_perform_io)
  		return rc;
  
  	count_compact_event(COMPACTSTALL);
  
  #ifdef CONFIG_CMA
  	if (allocflags_to_migratetype(gfp_mask) == MIGRATE_MOVABLE)
  		alloc_flags |= ALLOC_CMA;
  #endif
  	/* Compact each zone in the list */
  	for_each_zone_zonelist_nodemask(zone, z, zonelist, high_zoneidx,
  								nodemask) {
  		int status;
  
  		status = compact_zone_order(zone, order, gfp_mask, sync,
  						contended);
  		rc = max(status, rc);
  
  		/* If a normal allocation would succeed, stop compacting */
  		if (zone_watermark_ok(zone, order, low_wmark_pages(zone), 0,
  				      alloc_flags))
  			break;
  	}
  
  	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);
  
  		if (cc->order == -1 || !compaction_deferred(zone, cc->order))
  			compact_zone(zone, cc);
  
  		if (cc->order > 0) {
  			int ok = zone_watermark_ok(zone, cc->order,
  						low_wmark_pages(zone), 0, 0);
  			if (ok && cc->order >= zone->compact_order_failed)
  				zone->compact_order_failed = cc->order + 1;
  			/* Currently async compaction is never deferred. */
  			else if (!ok && cc->sync)
  				defer_compaction(zone, cc->order);
  		}
  
  		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,
  		.sync = false,
  	};
  
  	if (!order)
  		return;
  
  	__compact_pgdat(pgdat, &cc);
  }
  
  static void compact_node(int nid)
  {
  	struct compact_control cc = {
  		.order = -1,
  		.sync = 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)
  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 */
  