text: /*
   *  linux/mm/vmscan.c
   *
   *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
   *
   *  Swap reorganised 29.12.95, Stephen Tweedie.
   *  kswapd added: 7.1.96  sct
   *  Removed kswapd_ctl limits, and swap out as many pages as needed
   *  to bring the system back to freepages.high: 2.4.97, Rik van Riel.
   *  Zone aware kswapd started 02/00, Kanoj Sarcar (kanoj@sgi.com).
   *  Multiqueue VM started 5.8.00, Rik van Riel.
   */
  
  #include <linux/mm.h>
  #include <linux/module.h>
  #include <linux/gfp.h>
  #include <linux/kernel_stat.h>
  #include <linux/swap.h>
  #include <linux/pagemap.h>
  #include <linux/init.h>
  #include <linux/highmem.h>
  #include <linux/vmstat.h>
  #include <linux/file.h>
  #include <linux/writeback.h>
  #include <linux/blkdev.h>
  #include <linux/buffer_head.h>	/* for try_to_release_page(),
  					buffer_heads_over_limit */
  #include <linux/mm_inline.h>
  #include <linux/backing-dev.h>
  #include <linux/rmap.h>
  #include <linux/topology.h>
  #include <linux/cpu.h>
  #include <linux/cpuset.h>
  #include <linux/compaction.h>
  #include <linux/notifier.h>
  #include <linux/rwsem.h>
  #include <linux/delay.h>
  #include <linux/kthread.h>
  #include <linux/freezer.h>
  #include <linux/memcontrol.h>
  #include <linux/delayacct.h>
  #include <linux/sysctl.h>
  #include <linux/oom.h>
  #include <linux/prefetch.h>
  
  #include <asm/tlbflush.h>
  #include <asm/div64.h>
  
  #include <linux/swapops.h>
  
  #include "internal.h"
  
  #define CREATE_TRACE_POINTS
  #include <trace/events/vmscan.h>
  
  struct scan_control {
  	/* Incremented by the number of inactive pages that were scanned */
  	unsigned long nr_scanned;
  
  	/* Number of pages freed so far during a call to shrink_zones() */
  	unsigned long nr_reclaimed;
  
  	/* How many pages shrink_list() should reclaim */
  	unsigned long nr_to_reclaim;
  
  	unsigned long hibernation_mode;
  
  	/* This context's GFP mask */
  	gfp_t gfp_mask;
  
  	int may_writepage;
  
  	/* Can mapped pages be reclaimed? */
  	int may_unmap;
  
  	/* Can pages be swapped as part of reclaim? */
  	int may_swap;
  
  	int order;
  
  	/* Scan (total_size >> priority) pages at once */
  	int priority;
  
  	/*
  	 * The memory cgroup that hit its limit and as a result is the
  	 * primary target of this reclaim invocation.
  	 */
  	struct mem_cgroup *target_mem_cgroup;
  
  	/*
  	 * Nodemask of nodes allowed by the caller. If NULL, all nodes
  	 * are scanned.
  	 */
  	nodemask_t	*nodemask;
  };
  
  #define lru_to_page(_head) (list_entry((_head)->prev, struct page, lru))
  
  #ifdef ARCH_HAS_PREFETCH
  #define prefetch_prev_lru_page(_page, _base, _field)			\
  	do {								\
  		if ((_page)->lru.prev != _base) {			\
  			struct page *prev;				\
  									\
  			prev = lru_to_page(&(_page->lru));		\
  			prefetch(&prev->_field);			\
  		}							\
  	} while (0)
  #else
  #define prefetch_prev_lru_page(_page, _base, _field) do { } while (0)
  #endif
  
  #ifdef ARCH_HAS_PREFETCHW
  #define prefetchw_prev_lru_page(_page, _base, _field)			\
  	do {								\
  		if ((_page)->lru.prev != _base) {			\
  			struct page *prev;				\
  									\
  			prev = lru_to_page(&(_page->lru));		\
  			prefetchw(&prev->_field);			\
  		}							\
  	} while (0)
  #else
  #define prefetchw_prev_lru_page(_page, _base, _field) do { } while (0)
  #endif
  
  /*
   * From 0 .. 100.  Higher means more swappy.
   */
  int vm_swappiness = 60;
  long vm_total_pages;	/* The total number of pages which the VM controls */
  
  static LIST_HEAD(shrinker_list);
  static DECLARE_RWSEM(shrinker_rwsem);
  
  #ifdef CONFIG_MEMCG
  static bool global_reclaim(struct scan_control *sc)
  {
  	return !sc->target_mem_cgroup;
  }
  #else
  static bool global_reclaim(struct scan_control *sc)
  {
  	return true;
  }
  #endif
  
  static unsigned long get_lru_size(struct lruvec *lruvec, enum lru_list lru)
  {
  	if (!mem_cgroup_disabled())
  		return mem_cgroup_get_lru_size(lruvec, lru);
  
  	return zone_page_state(lruvec_zone(lruvec), NR_LRU_BASE + lru);
  }
  
  /*
   * Add a shrinker callback to be called from the vm
   */
  void register_shrinker(struct shrinker *shrinker)
  {
  	atomic_long_set(&shrinker->nr_in_batch, 0);
  	down_write(&shrinker_rwsem);
  	list_add_tail(&shrinker->list, &shrinker_list);
  	up_write(&shrinker_rwsem);
  }
  EXPORT_SYMBOL(register_shrinker);
  
  /*
   * Remove one
   */
  void unregister_shrinker(struct shrinker *shrinker)
  {
  	down_write(&shrinker_rwsem);
  	list_del(&shrinker->list);
  	up_write(&shrinker_rwsem);
  }
  EXPORT_SYMBOL(unregister_shrinker);
  
  static inline int do_shrinker_shrink(struct shrinker *shrinker,
  				     struct shrink_control *sc,
  				     unsigned long nr_to_scan)
  {
  	sc->nr_to_scan = nr_to_scan;
  	return (*shrinker->shrink)(shrinker, sc);
  }
  
  #define SHRINK_BATCH 128
  /*
   * Call the shrink functions to age shrinkable caches
   *
   * Here we assume it costs one seek to replace a lru page and that it also
   * takes a seek to recreate a cache object.  With this in mind we age equal
   * percentages of the lru and ageable caches.  This should balance the seeks
   * generated by these structures.
   *
   * If the vm encountered mapped pages on the LRU it increase the pressure on
   * slab to avoid swapping.
   *
   * We do weird things to avoid (scanned*seeks*entries) overflowing 32 bits.
   *
   * `lru_pages' represents the number of on-LRU pages in all the zones which
   * are eligible for the caller's allocation attempt.  It is used for balancing
   * slab reclaim versus page reclaim.
   *
   * Returns the number of slab objects which we shrunk.
   */
  unsigned long shrink_slab(struct shrink_control *shrink,
  			  unsigned long nr_pages_scanned,
  			  unsigned long lru_pages)
  {
  	struct shrinker *shrinker;
  	unsigned long ret = 0;
  
  	if (nr_pages_scanned == 0)
  		nr_pages_scanned = SWAP_CLUSTER_MAX;
  
  	if (!down_read_trylock(&shrinker_rwsem)) {
  		/* Assume we'll be able to shrink next time */
  		ret = 1;
  		goto out;
  	}
  
  	list_for_each_entry(shrinker, &shrinker_list, list) {
  		unsigned long long delta;
  		long total_scan;
  		long max_pass;
  		int shrink_ret = 0;
  		long nr;
  		long new_nr;
  		long batch_size = shrinker->batch ? shrinker->batch
  						  : SHRINK_BATCH;
  
  		max_pass = do_shrinker_shrink(shrinker, shrink, 0);
  		if (max_pass <= 0)
  			continue;
  
  		/*
  		 * copy the current shrinker scan count into a local variable
  		 * and zero it so that other concurrent shrinker invocations
  		 * don't also do this scanning work.
  		 */
  		nr = atomic_long_xchg(&shrinker->nr_in_batch, 0);
  
  		total_scan = nr;
  		delta = (4 * nr_pages_scanned) / shrinker->seeks;
  		delta *= max_pass;
  		do_div(delta, lru_pages + 1);
  		total_scan += delta;
  		if (total_scan < 0) {
  			printk(KERN_ERR "shrink_slab: %pF negative objects to "
  			       "delete nr=%ld\n",
  			       shrinker->shrink, total_scan);
  			total_scan = max_pass;
  		}
  
  		/*
  		 * We need to avoid excessive windup on filesystem shrinkers
  		 * due to large numbers of GFP_NOFS allocations causing the
  		 * shrinkers to return -1 all the time. This results in a large
  		 * nr being built up so when a shrink that can do some work
  		 * comes along it empties the entire cache due to nr >>>
  		 * max_pass.  This is bad for sustaining a working set in
  		 * memory.
  		 *
  		 * Hence only allow the shrinker to scan the entire cache when
  		 * a large delta change is calculated directly.
  		 */
  		if (delta < max_pass / 4)
  			total_scan = min(total_scan, max_pass / 2);
  
  		/*
  		 * Avoid risking looping forever due to too large nr value:
  		 * never try to free more than twice the estimate number of
  		 * freeable entries.
  		 */
  		if (total_scan > max_pass * 2)
  			total_scan = max_pass * 2;
  
  		trace_mm_shrink_slab_start(shrinker, shrink, nr,
  					nr_pages_scanned, lru_pages,
  					max_pass, delta, total_scan);
  
  		while (total_scan >= batch_size) {
  			int nr_before;
  
  			nr_before = do_shrinker_shrink(shrinker, shrink, 0);
  			shrink_ret = do_shrinker_shrink(shrinker, shrink,
  							batch_size);
  			if (shrink_ret == -1)
  				break;
  			if (shrink_ret < nr_before)
  				ret += nr_before - shrink_ret;
  			count_vm_events(SLABS_SCANNED, batch_size);
  			total_scan -= batch_size;
  
  			cond_resched();
  		}
  
  		/*
  		 * move the unused scan count back into the shrinker in a
  		 * manner that handles concurrent updates. If we exhausted the
  		 * scan, there is no need to do an update.
  		 */
  		if (total_scan > 0)
  			new_nr = atomic_long_add_return(total_scan,
  					&shrinker->nr_in_batch);
  		else
  			new_nr = atomic_long_read(&shrinker->nr_in_batch);
  
  		trace_mm_shrink_slab_end(shrinker, shrink_ret, nr, new_nr);
  	}
  	up_read(&shrinker_rwsem);
  out:
  	cond_resched();
  	return ret;
  }
  
  static inline int is_page_cache_freeable(struct page *page)
  {
  	/*
  	 * A freeable page cache page is referenced only by the caller
  	 * that isolated the page, the page cache radix tree and
  	 * optional buffer heads at page->private.
  	 */
  	return page_count(page) - page_has_private(page) == 2;
  }
  
  static int may_write_to_queue(struct backing_dev_info *bdi,
  			      struct scan_control *sc)
  {
  	if (current->flags & PF_SWAPWRITE)
  		return 1;
  	if (!bdi_write_congested(bdi))
  		return 1;
  	if (bdi == current->backing_dev_info)
  		return 1;
  	return 0;
  }
  
  /*
   * We detected a synchronous write error writing a page out.  Probably
   * -ENOSPC.  We need to propagate that into the address_space for a subsequent
   * fsync(), msync() or close().
   *
   * The tricky part is that after writepage we cannot touch the mapping: nothing
   * prevents it from being freed up.  But we have a ref on the page and once
   * that page is locked, the mapping is pinned.
   *
   * We're allowed to run sleeping lock_page() here because we know the caller has
   * __GFP_FS.
   */
  static void handle_write_error(struct address_space *mapping,
  				struct page *page, int error)
  {
  	lock_page(page);
  	if (page_mapping(page) == mapping)
  		mapping_set_error(mapping, error);
  	unlock_page(page);
  }
  
  /* possible outcome of pageout() */
  typedef enum {
  	/* failed to write page out, page is locked */
  	PAGE_KEEP,
  	/* move page to the active list, page is locked */
  	PAGE_ACTIVATE,
  	/* page has been sent to the disk successfully, page is unlocked */
  	PAGE_SUCCESS,
  	/* page is clean and locked */
  	PAGE_CLEAN,
  } pageout_t;
  
  /*
   * pageout is called by shrink_page_list() for each dirty page.
   * Calls ->writepage().
   */
  static pageout_t pageout(struct page *page, struct address_space *mapping,
  			 struct scan_control *sc)
  {
  	/*
  	 * If the page is dirty, only perform writeback if that write
  	 * will be non-blocking.  To prevent this allocation from being
  	 * stalled by pagecache activity.  But note that there may be
  	 * stalls if we need to run get_block().  We could test
  	 * PagePrivate for that.
  	 *
  	 * If this process is currently in __generic_file_aio_write() against
  	 * this page's queue, we can perform writeback even if that
  	 * will block.
  	 *
  	 * If the page is swapcache, write it back even if that would
  	 * block, for some throttling. This happens by accident, because
  	 * swap_backing_dev_info is bust: it doesn't reflect the
  	 * congestion state of the swapdevs.  Easy to fix, if needed.
  	 */
  	if (!is_page_cache_freeable(page))
  		return PAGE_KEEP;
  	if (!mapping) {
  		/*
  		 * Some data journaling orphaned pages can have
  		 * page->mapping == NULL while being dirty with clean buffers.
  		 */
  		if (page_has_private(page)) {
  			if (try_to_free_buffers(page)) {
  				ClearPageDirty(page);
  				printk("%s: orphaned page\n", __func__);
  				return PAGE_CLEAN;
  			}
  		}
  		return PAGE_KEEP;
  	}
  	if (mapping->a_ops->writepage == NULL)
  		return PAGE_ACTIVATE;
  	if (!may_write_to_queue(mapping->backing_dev_info, sc))
  		return PAGE_KEEP;
  
  	if (clear_page_dirty_for_io(page)) {
  		int res;
  		struct writeback_control wbc = {
  			.sync_mode = WB_SYNC_NONE,
  			.nr_to_write = SWAP_CLUSTER_MAX,
  			.range_start = 0,
  			.range_end = LLONG_MAX,
  			.for_reclaim = 1,
  		};
  
  		SetPageReclaim(page);
  		res = mapping->a_ops->writepage(page, &wbc);
  		if (res < 0)
  			handle_write_error(mapping, page, res);
  		if (res == AOP_WRITEPAGE_ACTIVATE) {
  			ClearPageReclaim(page);
  			return PAGE_ACTIVATE;
  		}
  
  		if (!PageWriteback(page)) {
  			/* synchronous write or broken a_ops? */
  			ClearPageReclaim(page);
  		}
  		trace_mm_vmscan_writepage(page, trace_reclaim_flags(page));
  		inc_zone_page_state(page, NR_VMSCAN_WRITE);
  		return PAGE_SUCCESS;
  	}
  
  	return PAGE_CLEAN;
  }
  
  /*
   * Same as remove_mapping, but if the page is removed from the mapping, it
   * gets returned with a refcount of 0.
   */
  static int __remove_mapping(struct address_space *mapping, struct page *page)
  {
  	BUG_ON(!PageLocked(page));
  	BUG_ON(mapping != page_mapping(page));
  
  	spin_lock_irq(&mapping->tree_lock);
  	/*
  	 * The non racy check for a busy page.
  	 *
  	 * Must be careful with the order of the tests. When someone has
  	 * a ref to the page, it may be possible that they dirty it then
  	 * drop the reference. So if PageDirty is tested before page_count
  	 * here, then the following race may occur:
  	 *
  	 * get_user_pages(&page);
  	 * [user mapping goes away]
  	 * write_to(page);
  	 *				!PageDirty(page)    [good]
  	 * SetPageDirty(page);
  	 * put_page(page);
  	 *				!page_count(page)   [good, discard it]
  	 *
  	 * [oops, our write_to data is lost]
  	 *
  	 * Reversing the order of the tests ensures such a situation cannot
  	 * escape unnoticed. The smp_rmb is needed to ensure the page->flags
  	 * load is not satisfied before that of page->_count.
  	 *
  	 * Note that if SetPageDirty is always performed via set_page_dirty,
  	 * and thus under tree_lock, then this ordering is not required.
  	 */
  	if (!page_freeze_refs(page, 2))
  		goto cannot_free;
  	/* note: atomic_cmpxchg in page_freeze_refs provides the smp_rmb */
  	if (unlikely(PageDirty(page))) {
  		page_unfreeze_refs(page, 2);
  		goto cannot_free;
  	}
  
  	if (PageSwapCache(page)) {
  		swp_entry_t swap = { .val = page_private(page) };
  		__delete_from_swap_cache(page);
  		spin_unlock_irq(&mapping->tree_lock);
  		swapcache_free(swap, page);
  	} else {
  		void (*freepage)(struct page *);
  
  		freepage = mapping->a_ops->freepage;
  
  		__delete_from_page_cache(page);
  		spin_unlock_irq(&mapping->tree_lock);
  		mem_cgroup_uncharge_cache_page(page);
  
  		if (freepage != NULL)
  			freepage(page);
  	}
  
  	return 1;
  
  cannot_free:
  	spin_unlock_irq(&mapping->tree_lock);
  	return 0;
  }
  
  /*
   * Attempt to detach a locked page from its ->mapping.  If it is dirty or if
   * someone else has a ref on the page, abort and return 0.  If it was
   * successfully detached, return 1.  Assumes the caller has a single ref on
   * this page.
   */
  int remove_mapping(struct address_space *mapping, struct page *page)
  {
  	if (__remove_mapping(mapping, page)) {
  		/*
  		 * Unfreezing the refcount with 1 rather than 2 effectively
  		 * drops the pagecache ref for us without requiring another
  		 * atomic operation.
  		 */
  		page_unfreeze_refs(page, 1);
  		return 1;
  	}
  	return 0;
  }
  
  /**
   * putback_lru_page - put previously isolated page onto appropriate LRU list
   * @page: page to be put back to appropriate lru list
   *
   * Add previously isolated @page to appropriate LRU list.
   * Page may still be unevictable for other reasons.
   *
   * lru_lock must not be held, interrupts must be enabled.
   */
  void putback_lru_page(struct page *page)
  {
  	int lru;
  	int active = !!TestClearPageActive(page);
  	int was_unevictable = PageUnevictable(page);
  
  	VM_BUG_ON(PageLRU(page));
  
  redo:
  	ClearPageUnevictable(page);
  
  	if (page_evictable(page)) {
  		/*
  		 * For evictable pages, we can use the cache.
  		 * In event of a race, worst case is we end up with an
  		 * unevictable page on [in]active list.
  		 * We know how to handle that.
  		 */
  		lru = active + page_lru_base_type(page);
  		lru_cache_add_lru(page, lru);
  	} else {
  		/*
  		 * Put unevictable pages directly on zone's unevictable
  		 * list.
  		 */
  		lru = LRU_UNEVICTABLE;
  		add_page_to_unevictable_list(page);
  		/*
  		 * When racing with an mlock or AS_UNEVICTABLE clearing
  		 * (page is unlocked) make sure that if the other thread
  		 * does not observe our setting of PG_lru and fails
  		 * isolation/check_move_unevictable_pages,
  		 * we see PG_mlocked/AS_UNEVICTABLE cleared below and move
  		 * the page back to the evictable list.
  		 *
  		 * The other side is TestClearPageMlocked() or shmem_lock().
  		 */
  		smp_mb();
  	}
  
  	/*
  	 * page's status can change while we move it among lru. If an evictable
  	 * page is on unevictable list, it never be freed. To avoid that,
  	 * check after we added it to the list, again.
  	 */
  	if (lru == LRU_UNEVICTABLE && page_evictable(page)) {
  		if (!isolate_lru_page(page)) {
  			put_page(page);
  			goto redo;
  		}
  		/* This means someone else dropped this page from LRU
  		 * So, it will be freed or putback to LRU again. There is
  		 * nothing to do here.
  		 */
  	}
  
  	if (was_unevictable && lru != LRU_UNEVICTABLE)
  		count_vm_event(UNEVICTABLE_PGRESCUED);
  	else if (!was_unevictable && lru == LRU_UNEVICTABLE)
  		count_vm_event(UNEVICTABLE_PGCULLED);
  
  	put_page(page);		/* drop ref from isolate */
  }
  
  enum page_references {
  	PAGEREF_RECLAIM,
  	PAGEREF_RECLAIM_CLEAN,
  	PAGEREF_KEEP,
  	PAGEREF_ACTIVATE,
  };
  
  static enum page_references page_check_references(struct page *page,
  						  struct scan_control *sc)
  {
  	int referenced_ptes, referenced_page;
  	unsigned long vm_flags;
  
  	referenced_ptes = page_referenced(page, 1, sc->target_mem_cgroup,
  					  &vm_flags);
  	referenced_page = TestClearPageReferenced(page);
  
  	/*
  	 * Mlock lost the isolation race with us.  Let try_to_unmap()
  	 * move the page to the unevictable list.
  	 */
  	if (vm_flags & VM_LOCKED)
  		return PAGEREF_RECLAIM;
  
  	if (referenced_ptes) {
  		if (PageSwapBacked(page))
  			return PAGEREF_ACTIVATE;
  		/*
  		 * All mapped pages start out with page table
  		 * references from the instantiating fault, so we need
  		 * to look twice if a mapped file page is used more
  		 * than once.
  		 *
  		 * Mark it and spare it for another trip around the
  		 * inactive list.  Another page table reference will
  		 * lead to its activation.
  		 *
  		 * Note: the mark is set for activated pages as well
  		 * so that recently deactivated but used pages are
  		 * quickly recovered.
  		 */
  		SetPageReferenced(page);
  
  		if (referenced_page || referenced_ptes > 1)
  			return PAGEREF_ACTIVATE;
  
  		/*
  		 * Activate file-backed executable pages after first usage.
  		 */
  		if (vm_flags & VM_EXEC)
  			return PAGEREF_ACTIVATE;
  
  		return PAGEREF_KEEP;
  	}
  
  	/* Reclaim if clean, defer dirty pages to writeback */
  	if (referenced_page && !PageSwapBacked(page))
  		return PAGEREF_RECLAIM_CLEAN;
  
  	return PAGEREF_RECLAIM;
  }
  
  /*
   * shrink_page_list() returns the number of reclaimed pages
   */
  static unsigned long shrink_page_list(struct list_head *page_list,
  				      struct zone *zone,
  				      struct scan_control *sc,
  				      enum ttu_flags ttu_flags,
  				      unsigned long *ret_nr_dirty,
  				      unsigned long *ret_nr_writeback,
  				      bool force_reclaim)
  {
  	LIST_HEAD(ret_pages);
  	LIST_HEAD(free_pages);
  	int pgactivate = 0;
  	unsigned long nr_dirty = 0;
  	unsigned long nr_congested = 0;
  	unsigned long nr_reclaimed = 0;
  	unsigned long nr_writeback = 0;
  
  	cond_resched();
  
  	mem_cgroup_uncharge_start();
  	while (!list_empty(page_list)) {
  		struct address_space *mapping;
  		struct page *page;
  		int may_enter_fs;
  		enum page_references references = PAGEREF_RECLAIM_CLEAN;
  
  		cond_resched();
  
  		page = lru_to_page(page_list);
  		list_del(&page->lru);
  
  		if (!trylock_page(page))
  			goto keep;
  
  		VM_BUG_ON(PageActive(page));
  		VM_BUG_ON(page_zone(page) != zone);
  
  		sc->nr_scanned++;
  
  		if (unlikely(!page_evictable(page)))
  			goto cull_mlocked;
  
  		if (!sc->may_unmap && page_mapped(page))
  			goto keep_locked;
  
  		/* Double the slab pressure for mapped and swapcache pages */
  		if (page_mapped(page) || PageSwapCache(page))
  			sc->nr_scanned++;
  
  		may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
  			(PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
  
  		if (PageWriteback(page)) {
  			/*
  			 * memcg doesn't have any dirty pages throttling so we
  			 * could easily OOM just because too many pages are in
  			 * writeback and there is nothing else to reclaim.
  			 *
  			 * Check __GFP_IO, certainly because a loop driver
  			 * thread might enter reclaim, and deadlock if it waits
  			 * on a page for which it is needed to do the write
  			 * (loop masks off __GFP_IO|__GFP_FS for this reason);
  			 * but more thought would probably show more reasons.
  			 *
  			 * Don't require __GFP_FS, since we're not going into
  			 * the FS, just waiting on its writeback completion.
  			 * Worryingly, ext4 gfs2 and xfs allocate pages with
  			 * grab_cache_page_write_begin(,,AOP_FLAG_NOFS), so
  			 * testing may_enter_fs here is liable to OOM on them.
  			 */
  			if (global_reclaim(sc) ||
  			    !PageReclaim(page) || !(sc->gfp_mask & __GFP_IO)) {
  				/*
  				 * This is slightly racy - end_page_writeback()
  				 * might have just cleared PageReclaim, then
  				 * setting PageReclaim here end up interpreted
  				 * as PageReadahead - but that does not matter
  				 * enough to care.  What we do want is for this
  				 * page to have PageReclaim set next time memcg
  				 * reclaim reaches the tests above, so it will
  				 * then wait_on_page_writeback() to avoid OOM;
  				 * and it's also appropriate in global reclaim.
  				 */
  				SetPageReclaim(page);
  				nr_writeback++;
  				goto keep_locked;
  			}
  			wait_on_page_writeback(page);
  		}
  
  		if (!force_reclaim)
  			references = page_check_references(page, sc);
  
  		switch (references) {
  		case PAGEREF_ACTIVATE:
  			goto activate_locked;
  		case PAGEREF_KEEP:
  			goto keep_locked;
  		case PAGEREF_RECLAIM:
  		case PAGEREF_RECLAIM_CLEAN:
  			; /* try to reclaim the page below */
  		}
  
  		/*
  		 * Anonymous process memory has backing store?
  		 * Try to allocate it some swap space here.
  		 */
  		if (PageAnon(page) && !PageSwapCache(page)) {
  			if (!(sc->gfp_mask & __GFP_IO))
  				goto keep_locked;
  			if (!add_to_swap(page))
  				goto activate_locked;
  			may_enter_fs = 1;
  		}
  
  		mapping = page_mapping(page);
  
  		/*
  		 * The page is mapped into the page tables of one or more
  		 * processes. Try to unmap it here.
  		 */
  		if (page_mapped(page) && mapping) {
  			switch (try_to_unmap(page, ttu_flags)) {
  			case SWAP_FAIL:
  				goto activate_locked;
  			case SWAP_AGAIN:
  				goto keep_locked;
  			case SWAP_MLOCK:
  				goto cull_mlocked;
  			case SWAP_SUCCESS:
  				; /* try to free the page below */
  			}
  		}
  
  		if (PageDirty(page)) {
  			nr_dirty++;
  
  			/*
  			 * Only kswapd can writeback filesystem pages to
  			 * avoid risk of stack overflow but do not writeback
  			 * unless under significant pressure.
  			 */
  			if (page_is_file_cache(page) &&
  					(!current_is_kswapd() ||
  					 sc->priority >= DEF_PRIORITY - 2)) {
  				/*
  				 * Immediately reclaim when written back.
  				 * Similar in principal to deactivate_page()
  				 * except we already have the page isolated
  				 * and know it's dirty
  				 */
  				inc_zone_page_state(page, NR_VMSCAN_IMMEDIATE);
  				SetPageReclaim(page);
  
  				goto keep_locked;
  			}
  
  			if (references == PAGEREF_RECLAIM_CLEAN)
  				goto keep_locked;
  			if (!may_enter_fs)
  				goto keep_locked;
  			if (!sc->may_writepage)
  				goto keep_locked;
  
  			/* Page is dirty, try to write it out here */
  			switch (pageout(page, mapping, sc)) {
  			case PAGE_KEEP:
  				nr_congested++;
  				goto keep_locked;
  			case PAGE_ACTIVATE:
  				goto activate_locked;
  			case PAGE_SUCCESS:
  				if (PageWriteback(page))
  					goto keep;
  				if (PageDirty(page))
  					goto keep;
  
  				/*
  				 * A synchronous write - probably a ramdisk.  Go
  				 * ahead and try to reclaim the page.
  				 */
  				if (!trylock_page(page))
  					goto keep;
  				if (PageDirty(page) || PageWriteback(page))
  					goto keep_locked;
  				mapping = page_mapping(page);
  			case PAGE_CLEAN:
  				; /* try to free the page below */
  			}
  		}
  
  		/*
  		 * If the page has buffers, try to free the buffer mappings
  		 * associated with this page. If we succeed we try to free
  		 * the page as well.
  		 *
  		 * We do this even if the page is PageDirty().
  		 * try_to_release_page() does not perform I/O, but it is
  		 * possible for a page to have PageDirty set, but it is actually
  		 * clean (all its buffers are clean).  This happens if the
  		 * buffers were written out directly, with submit_bh(). ext3
  		 * will do this, as well as the blockdev mapping.
  		 * try_to_release_page() will discover that cleanness and will
  		 * drop the buffers and mark the page clean - it can be freed.
  		 *
  		 * Rarely, pages can have buffers and no ->mapping.  These are
  		 * the pages which were not successfully invalidated in
  		 * truncate_complete_page().  We try to drop those buffers here
  		 * and if that worked, and the page is no longer mapped into
  		 * process address space (page_count == 1) it can be freed.
  		 * Otherwise, leave the page on the LRU so it is swappable.
  		 */
  		if (page_has_private(page)) {
  			if (!try_to_release_page(page, sc->gfp_mask))
  				goto activate_locked;
  			if (!mapping && page_count(page) == 1) {
  				unlock_page(page);
  				if (put_page_testzero(page))
  					goto free_it;
  				else {
  					/*
  					 * rare race with speculative reference.
  					 * the speculative reference will free
  					 * this page shortly, so we may
  					 * increment nr_reclaimed here (and
  					 * leave it off the LRU).
  					 */
  					nr_reclaimed++;
  					continue;
  				}
  			}
  		}
  
  		if (!mapping || !__remove_mapping(mapping, page))
  			goto keep_locked;
  
  		/*
  		 * At this point, we have no other references and there is
  		 * no way to pick any more up (removed from LRU, removed
  		 * from pagecache). Can use non-atomic bitops now (and
  		 * we obviously don't have to worry about waking up a process
  		 * waiting on the page lock, because there are no references.
  		 */
  		__clear_page_locked(page);
  free_it:
  		nr_reclaimed++;
  
  		/*
  		 * Is there need to periodically free_page_list? It would
  		 * appear not as the counts should be low
  		 */
  		list_add(&page->lru, &free_pages);
  		continue;
  
  cull_mlocked:
  		if (PageSwapCache(page))
  			try_to_free_swap(page);
  		unlock_page(page);
  		putback_lru_page(page);
  		continue;
  
  activate_locked:
  		/* Not a candidate for swapping, so reclaim swap space. */
  		if (PageSwapCache(page) && vm_swap_full())
  			try_to_free_swap(page);
  		VM_BUG_ON(PageActive(page));
  		SetPageActive(page);
  		pgactivate++;
  keep_locked:
  		unlock_page(page);
  keep:
  		list_add(&page->lru, &ret_pages);
  		VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
  	}
  
  	/*
  	 * Tag a zone as congested if all the dirty pages encountered were
  	 * backed by a congested BDI. In this case, reclaimers should just
  	 * back off and wait for congestion to clear because further reclaim
  	 * will encounter the same problem
  	 */
  	if (nr_dirty && nr_dirty == nr_congested && global_reclaim(sc))
  		zone_set_flag(zone, ZONE_CONGESTED);
  
  	free_hot_cold_page_list(&free_pages, 1);
  
  	list_splice(&ret_pages, page_list);
  	count_vm_events(PGACTIVATE, pgactivate);
  	mem_cgroup_uncharge_end();
  	*ret_nr_dirty += nr_dirty;
  	*ret_nr_writeback += nr_writeback;
  	return nr_reclaimed;
  }
  
  unsigned long reclaim_clean_pages_from_list(struct zone *zone,
  					    struct list_head *page_list)
  {
  	struct scan_control sc = {
  		.gfp_mask = GFP_KERNEL,
  		.priority = DEF_PRIORITY,
  		.may_unmap = 1,
  	};
  	unsigned long ret, dummy1, dummy2;
  	struct page *page, *next;
  	LIST_HEAD(clean_pages);
  
  	list_for_each_entry_safe(page, next, page_list, lru) {
  		if (page_is_file_cache(page) && !PageDirty(page)) {
  			ClearPageActive(page);
  			list_move(&page->lru, &clean_pages);
  		}
  	}
  
  	ret = shrink_page_list(&clean_pages, zone, &sc,
  				TTU_UNMAP|TTU_IGNORE_ACCESS,
  				&dummy1, &dummy2, true);
  	list_splice(&clean_pages, page_list);
  	__mod_zone_page_state(zone, NR_ISOLATED_FILE, -ret);
  	return ret;
  }
  
  /*
   * Attempt to remove the specified page from its LRU.  Only take this page
   * if it is of the appropriate PageActive status.  Pages which are being
   * freed elsewhere are also ignored.
   *
   * page:	page to consider
   * mode:	one of the LRU isolation modes defined above
   *
   * returns 0 on success, -ve errno on failure.
   */
  int __isolate_lru_page(struct page *page, isolate_mode_t mode)
  {
  	int ret = -EINVAL;
  
  	/* Only take pages on the LRU. */
  	if (!PageLRU(page))
  		return ret;
  
  	/* Compaction should not handle unevictable pages but CMA can do so */
  	if (PageUnevictable(page) && !(mode & ISOLATE_UNEVICTABLE))
  		return ret;
  
  	ret = -EBUSY;
  
  	/*
  	 * To minimise LRU disruption, the caller can indicate that it only
  	 * wants to isolate pages it will be able to operate on without
  	 * blocking - clean pages for the most part.
  	 *
  	 * ISOLATE_CLEAN means that only clean pages should be isolated. This
  	 * is used by reclaim when it is cannot write to backing storage
  	 *
  	 * ISOLATE_ASYNC_MIGRATE is used to indicate that it only wants to pages
  	 * that it is possible to migrate without blocking
  	 */
  	if (mode & (ISOLATE_CLEAN|ISOLATE_ASYNC_MIGRATE)) {
  		/* All the caller can do on PageWriteback is block */
  		if (PageWriteback(page))
  			return ret;
  
  		if (PageDirty(page)) {
  			struct address_space *mapping;
  
  			/* ISOLATE_CLEAN means only clean pages */
  			if (mode & ISOLATE_CLEAN)
  				return ret;
  
  			/*
  			 * Only pages without mappings or that have a
  			 * ->migratepage callback are possible to migrate
  			 * without blocking
  			 */
  			mapping = page_mapping(page);
  			if (mapping && !mapping->a_ops->migratepage)
  				return ret;
  		}
  	}
  
  	if ((mode & ISOLATE_UNMAPPED) && page_mapped(page))
  		return ret;
  
  	if (likely(get_page_unless_zero(page))) {
  		/*
  		 * Be careful not to clear PageLRU until after we're
  		 * sure the page is not being freed elsewhere -- the
  		 * page release code relies on it.
  		 */
  		ClearPageLRU(page);
  		ret = 0;
  	}
  
  	return ret;
  }
  
  /*
   * zone->lru_lock is heavily contended.  Some of the functions that
   * shrink the lists perform better by taking out a batch of pages
   * and working on them outside the LRU lock.
   *
   * For pagecache intensive workloads, this function is the hottest
   * spot in the kernel (apart from copy_*_user functions).
   *
   * Appropriate locks must be held before calling this function.
   *
   * @nr_to_scan:	The number of pages to look through on the list.
   * @lruvec:	The LRU vector to pull pages from.
   * @dst:	The temp list to put pages on to.
   * @nr_scanned:	The number of pages that were scanned.
   * @sc:		The scan_control struct for this reclaim session
   * @mode:	One of the LRU isolation modes
   * @lru:	LRU list id for isolating
   *
   * returns how many pages were moved onto *@dst.
   */
  static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
  		struct lruvec *lruvec, struct list_head *dst,
  		unsigned long *nr_scanned, struct scan_control *sc,
  		isolate_mode_t mode, enum lru_list lru)
  {
  	struct list_head *src = &lruvec->lists[lru];
  	unsigned long nr_taken = 0;
  	unsigned long scan;
  
  	for (scan = 0; scan < nr_to_scan && !list_empty(src); scan++) {
  		struct page *page;
  		int nr_pages;
  
  		page = lru_to_page(src);
  		prefetchw_prev_lru_page(page, src, flags);
  
  		VM_BUG_ON(!PageLRU(page));
  
  		switch (__isolate_lru_page(page, mode)) {
  		case 0:
  			nr_pages = hpage_nr_pages(page);
  			mem_cgroup_update_lru_size(lruvec, lru, -nr_pages);
  			list_move(&page->lru, dst);
  			nr_taken += nr_pages;
  			break;
  
  		case -EBUSY:
  			/* else it is being freed elsewhere */
  			list_move(&page->lru, src);
  			continue;
  
  		default:
  			BUG();
  		}
  	}
  
  	*nr_scanned = scan;
  	trace_mm_vmscan_lru_isolate(sc->order, nr_to_scan, scan,
  				    nr_taken, mode, is_file_lru(lru));
  	return nr_taken;
  }
  
  /**
   * isolate_lru_page - tries to isolate a page from its LRU list
   * @page: page to isolate from its LRU list
   *
   * Isolates a @page from an LRU list, clears PageLRU and adjusts the
   * vmstat statistic corresponding to whatever LRU list the page was on.
   *
   * Returns 0 if the page was removed from an LRU list.
   * Returns -EBUSY if the page was not on an LRU list.
   *
   * The returned page will have PageLRU() cleared.  If it was found on
   * the active list, it will have PageActive set.  If it was found on
   * the unevictable list, it will have the PageUnevictable bit set. That flag
   * may need to be cleared by the caller before letting the page go.
   *
   * The vmstat statistic corresponding to the list on which the page was
   * found will be decremented.
   *
   * Restrictions:
   * (1) Must be called with an elevated refcount on the page. This is a
   *     fundamentnal difference from isolate_lru_pages (which is called
   *     without a stable reference).
   * (2) the lru_lock must not be held.
   * (3) interrupts must be enabled.
   */
  int isolate_lru_page(struct page *page)
  {
  	int ret = -EBUSY;
  
  	VM_BUG_ON(!page_count(page));
  
  	if (PageLRU(page)) {
  		struct zone *zone = page_zone(page);
  		struct lruvec *lruvec;
  
  		spin_lock_irq(&zone->lru_lock);
  		lruvec = mem_cgroup_page_lruvec(page, zone);
  		if (PageLRU(page)) {
  			int lru = page_lru(page);
  			get_page(page);
  			ClearPageLRU(page);
  			del_page_from_lru_list(page, lruvec, lru);
  			ret = 0;
  		}
  		spin_unlock_irq(&zone->lru_lock);
  	}
  	return ret;
  }
  
  /*
   * Are there way too many processes in the direct reclaim path already?
   */
  static int too_many_isolated(struct zone *zone, int file,
  		struct scan_control *sc)
  {
  	unsigned long inactive, isolated;
  
  	if (current_is_kswapd())
  		return 0;
  
  	if (!global_reclaim(sc))
  		return 0;
  
  	if (file) {
  		inactive = zone_page_state(zone, NR_INACTIVE_FILE);
  		isolated = zone_page_state(zone, NR_ISOLATED_FILE);
  	} else {
  		inactive = zone_page_state(zone, NR_INACTIVE_ANON);
  		isolated = zone_page_state(zone, NR_ISOLATED_ANON);
  	}
  
  	return isolated > inactive;
  }
  
  static noinline_for_stack void
  putback_inactive_pages(struct lruvec *lruvec, struct list_head *page_list)
  {
  	struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
  	struct zone *zone = lruvec_zone(lruvec);
  	LIST_HEAD(pages_to_free);
  
  	/*
  	 * Put back any unfreeable pages.
  	 */
  	while (!list_empty(page_list)) {
  		struct page *page = lru_to_page(page_list);
  		int lru;
  
  		VM_BUG_ON(PageLRU(page));
  		list_del(&page->lru);
  		if (unlikely(!page_evictable(page))) {
  			spin_unlock_irq(&zone->lru_lock);
  			putback_lru_page(page);
  			spin_lock_irq(&zone->lru_lock);
  			continue;
  		}
  
  		lruvec = mem_cgroup_page_lruvec(page, zone);
  
  		SetPageLRU(page);
  		lru = page_lru(page);
  		add_page_to_lru_list(page, lruvec, lru);
  
  		if (is_active_lru(lru)) {
  			int file = is_file_lru(lru);
  			int numpages = hpage_nr_pages(page);
  			reclaim_stat->recent_rotated[file] += numpages;
  		}
  		if (put_page_testzero(page)) {
  			__ClearPageLRU(page);
  			__ClearPageActive(page);
  			del_page_from_lru_list(page, lruvec, lru);
  
  			if (unlikely(PageCompound(page))) {
  				spin_unlock_irq(&zone->lru_lock);
  				(*get_compound_page_dtor(page))(page);
  				spin_lock_irq(&zone->lru_lock);
  			} else
  				list_add(&page->lru, &pages_to_free);
  		}
  	}
  
  	/*
  	 * To save our caller's stack, now use input list for pages to free.
  	 */
  	list_splice(&pages_to_free, page_list);
  }
  
  /*
   * shrink_inactive_list() is a helper for shrink_zone().  It returns the number
   * of reclaimed pages
   */
  static noinline_for_stack unsigned long
  shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
  		     struct scan_control *sc, enum lru_list lru)
  {
  	LIST_HEAD(page_list);
  	unsigned long nr_scanned;
  	unsigned long nr_reclaimed = 0;
  	unsigned long nr_taken;
  	unsigned long nr_dirty = 0;
  	unsigned long nr_writeback = 0;
  	isolate_mode_t isolate_mode = 0;
  	int file = is_file_lru(lru);
  	struct zone *zone = lruvec_zone(lruvec);
  	struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
  
  	while (unlikely(too_many_isolated(zone, file, sc))) {
  		congestion_wait(BLK_RW_ASYNC, HZ/10);
  
  		/* We are about to die and free our memory. Return now. */
  		if (fatal_signal_pending(current))
  			return SWAP_CLUSTER_MAX;
  	}
  
  	lru_add_drain();
  
  	if (!sc->may_unmap)
  		isolate_mode |= ISOLATE_UNMAPPED;
  	if (!sc->may_writepage)
  		isolate_mode |= ISOLATE_CLEAN;
  
  	spin_lock_irq(&zone->lru_lock);
  
  	nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &page_list,
  				     &nr_scanned, sc, isolate_mode, lru);
  
  	__mod_zone_page_state(zone, NR_LRU_BASE + lru, -nr_taken);
  	__mod_zone_page_state(zone, NR_ISOLATED_ANON + file, nr_taken);
  
  	if (global_reclaim(sc)) {
  		zone->pages_scanned += nr_scanned;
  		if (current_is_kswapd())
  			__count_zone_vm_events(PGSCAN_KSWAPD, zone, nr_scanned);
  		else
  			__count_zone_vm_events(PGSCAN_DIRECT, zone, nr_scanned);
  	}
  	spin_unlock_irq(&zone->lru_lock);
  
  	if (nr_taken == 0)
  		return 0;
  
  	nr_reclaimed = shrink_page_list(&page_list, zone, sc, TTU_UNMAP,
  					&nr_dirty, &nr_writeback, false);
  
  	spin_lock_irq(&zone->lru_lock);
  
  	reclaim_stat->recent_scanned[file] += nr_taken;
  
  	if (global_reclaim(sc)) {
  		if (current_is_kswapd())
  			__count_zone_vm_events(PGSTEAL_KSWAPD, zone,
  					       nr_reclaimed);
  		else
  			__count_zone_vm_events(PGSTEAL_DIRECT, zone,
  					       nr_reclaimed);
  	}
  
  	putback_inactive_pages(lruvec, &page_list);
  
  	__mod_zone_page_state(zone, NR_ISOLATED_ANON + file, -nr_taken);
  
  	spin_unlock_irq(&zone->lru_lock);
  
  	free_hot_cold_page_list(&page_list, 1);
  
  	/*
  	 * If reclaim is isolating dirty pages under writeback, it implies
  	 * that the long-lived page allocation rate is exceeding the page
  	 * laundering rate. Either the global limits are not being effective
  	 * at throttling processes due to the page distribution throughout
  	 * zones or there is heavy usage of a slow backing device. The
  	 * only option is to throttle from reclaim context which is not ideal
  	 * as there is no guarantee the dirtying process is throttled in the
  	 * same way balance_dirty_pages() manages.
  	 *
  	 * This scales the number of dirty pages that must be under writeback
  	 * before throttling depending on priority. It is a simple backoff
  	 * function that has the most effect in the range DEF_PRIORITY to
  	 * DEF_PRIORITY-2 which is the priority reclaim is considered to be
  	 * in trouble and reclaim is considered to be in trouble.
  	 *
  	 * DEF_PRIORITY   100% isolated pages must be PageWriteback to throttle
  	 * DEF_PRIORITY-1  50% must be PageWriteback
  	 * DEF_PRIORITY-2  25% must be PageWriteback, kswapd in trouble
  	 * ...
  	 * DEF_PRIORITY-6 For SWAP_CLUSTER_MAX isolated pages, throttle if any
  	 *                     isolated page is PageWriteback
  	 */
  	if (nr_writeback && nr_writeback >=
  			(nr_taken >> (DEF_PRIORITY - sc->priority)))
  		wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);
  
  	trace_mm_vmscan_lru_shrink_inactive(zone->zone_pgdat->node_id,
  		zone_idx(zone),
  		nr_scanned, nr_reclaimed,
  		sc->priority,
  		trace_shrink_flags(file));
  	return nr_reclaimed;
  }
  
  /*
   * This moves pages from the active list to the inactive list.
   *
   * We move them the other way if the page is referenced by one or more
   * processes, from rmap.
   *
   * If the pages are mostly unmapped, the processing is fast and it is
   * appropriate to hold zone->lru_lock across the whole operation.  But if
   * the pages are mapped, the processing is slow (page_referenced()) so we
   * should drop zone->lru_lock around each page.  It's impossible to balance
   * this, so instead we remove the pages from the LRU while processing them.
   * It is safe to rely on PG_active against the non-LRU pages in here because
   * nobody will play with that bit on a non-LRU page.
   *
   * The downside is that we have to touch page->_count against each page.
   * But we had to alter page->flags anyway.
   */
  
  static void move_active_pages_to_lru(struct lruvec *lruvec,
  				     struct list_head *list,
  				     struct list_head *pages_to_free,
  				     enum lru_list lru)
  {
  	struct zone *zone = lruvec_zone(lruvec);
  	unsigned long pgmoved = 0;
  	struct page *page;
  	int nr_pages;
  
  	while (!list_empty(list)) {
  		page = lru_to_page(list);
  		lruvec = mem_cgroup_page_lruvec(page, zone);
  
  		VM_BUG_ON(PageLRU(page));
  		SetPageLRU(page);
  
  		nr_pages = hpage_nr_pages(page);
  		mem_cgroup_update_lru_size(lruvec, lru, nr_pages);
  		list_move(&page->lru, &lruvec->lists[lru]);
  		pgmoved += nr_pages;
  
  		if (put_page_testzero(page)) {
  			__ClearPageLRU(page);
  			__ClearPageActive(page);
  			del_page_from_lru_list(page, lruvec, lru);
  
  			if (unlikely(PageCompound(page))) {
  				spin_unlock_irq(&zone->lru_lock);
  				(*get_compound_page_dtor(page))(page);
  				spin_lock_irq(&zone->lru_lock);
  			} else
  				list_add(&page->lru, pages_to_free);
  		}
  	}
  	__mod_zone_page_state(zone, NR_LRU_BASE + lru, pgmoved);
  	if (!is_active_lru(lru))
  		__count_vm_events(PGDEACTIVATE, pgmoved);
  }
  
  static void shrink_active_list(unsigned long nr_to_scan,
  			       struct lruvec *lruvec,
  			       struct scan_control *sc,
  			       enum lru_list lru)
  {
  	unsigned long nr_taken;
  	unsigned long nr_scanned;
  	unsigned long vm_flags;
  	LIST_HEAD(l_hold);	/* The pages which were snipped off */
  	LIST_HEAD(l_active);
  	LIST_HEAD(l_inactive);
  	struct page *page;
  	struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
  	unsigned long nr_rotated = 0;
  	isolate_mode_t isolate_mode = 0;
  	int file = is_file_lru(lru);
  	struct zone *zone = lruvec_zone(lruvec);
  
  	lru_add_drain();
  
  	if (!sc->may_unmap)
  		isolate_mode |= ISOLATE_UNMAPPED;
  	if (!sc->may_writepage)
  		isolate_mode |= ISOLATE_CLEAN;
  
  	spin_lock_irq(&zone->lru_lock);
  
  	nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &l_hold,
  				     &nr_scanned, sc, isolate_mode, lru);
  	if (global_reclaim(sc))
  		zone->pages_scanned += nr_scanned;
  
  	reclaim_stat->recent_scanned[file] += nr_taken;
  
  	__count_zone_vm_events(PGREFILL, zone, nr_scanned);
  	__mod_zone_page_state(zone, NR_LRU_BASE + lru, -nr_taken);
  	__mod_zone_page_state(zone, NR_ISOLATED_ANON + file, nr_taken);
  	spin_unlock_irq(&zone->lru_lock);
  
  	while (!list_empty(&l_hold)) {
  		cond_resched();
  		page = lru_to_page(&l_hold);
  		list_del(&page->lru);
  
  		if (unlikely(!page_evictable(page))) {
  			putback_lru_page(page);
  			continue;
  		}
  
  		if (unlikely(buffer_heads_over_limit)) {
  			if (page_has_private(page) && trylock_page(page)) {
  				if (page_has_private(page))
  					try_to_release_page(page, 0);
  				unlock_page(page);
  			}
  		}
  
  		if (page_referenced(page, 0, sc->target_mem_cgroup,
  				    &vm_flags)) {
  			nr_rotated += hpage_nr_pages(page);
  			/*
  			 * Identify referenced, file-backed active pages and
  			 * give them one more trip around the active list. So
  			 * that executable code get better chances to stay in
  			 * memory under moderate memory pressure.  Anon pages
  			 * are not likely to be evicted by use-once streaming
  			 * IO, plus JVM can create lots of anon VM_EXEC pages,
  			 * so we ignore them here.
  			 */
  			if ((vm_flags & VM_EXEC) && page_is_file_cache(page)) {
  				list_add(&page->lru, &l_active);
  				continue;
  			}
  		}
  
  		ClearPageActive(page);	/* we are de-activating */
  		list_add(&page->lru, &l_inactive);
  	}
  
  	/*
  	 * Move pages back to the lru list.
  	 */
  	spin_lock_irq(&zone->lru_lock);
  	/*
  	 * Count referenced pages from currently used mappings as rotated,
  	 * even though only some of them are actually re-activated.  This
  	 * helps balance scan pressure between file and anonymous pages in
  	 * get_scan_ratio.
  	 */
  	reclaim_stat->recent_rotated[file] += nr_rotated;
  
  	move_active_pages_to_lru(lruvec, &l_active, &l_hold, lru);
  	move_active_pages_to_lru(lruvec, &l_inactive, &l_hold, lru - LRU_ACTIVE);
  	__mod_zone_page_state(zone, NR_ISOLATED_ANON + file, -nr_taken);
  	spin_unlock_irq(&zone->lru_lock);
  
  	free_hot_cold_page_list(&l_hold, 1);
  }
  
  #ifdef CONFIG_SWAP
  static int inactive_anon_is_low_global(struct zone *zone)
  {
  	unsigned long active, inactive;
  
  	active = zone_page_state(zone, NR_ACTIVE_ANON);
  	inactive = zone_page_state(zone, NR_INACTIVE_ANON);
  
  	if (inactive * zone->inactive_ratio < active)
  		return 1;
  
  	return 0;
  }
  
  /**
   * inactive_anon_is_low - check if anonymous pages need to be deactivated
   * @lruvec: LRU vector to check
   *
   * Returns true if the zone does not have enough inactive anon pages,
   * meaning some active anon pages need to be deactivated.
   */
  static int inactive_anon_is_low(struct lruvec *lruvec)
  {
  	/*
  	 * If we don't have swap space, anonymous page deactivation
  	 * is pointless.
  	 */
  	if (!total_swap_pages)
  		return 0;
  
  	if (!mem_cgroup_disabled())
  		return mem_cgroup_inactive_anon_is_low(lruvec);
  
  	return inactive_anon_is_low_global(lruvec_zone(lruvec));
  }
  #else
  static inline int inactive_anon_is_low(struct lruvec *lruvec)
  {
  	return 0;
  }
  #endif
  
  static int inactive_file_is_low_global(struct zone *zone)
  {
  	unsigned long active, inactive;
  
  	active = zone_page_state(zone, NR_ACTIVE_FILE);
  	inactive = zone_page_state(zone, NR_INACTIVE_FILE);
  
  	return (active > inactive);
  }
  
  /**
   * inactive_file_is_low - check if file pages need to be deactivated
   * @lruvec: LRU vector to check
   *
   * When the system is doing streaming IO, memory pressure here
   * ensures that active file pages get deactivated, until more
   * than half of the file pages are on the inactive list.
   *
   * Once we get to that situation, protect the system's working
   * set from being evicted by disabling active file page aging.
   *
   * This uses a different ratio than the anonymous pages, because
   * the page cache uses a use-once replacement algorithm.
   */
  static int inactive_file_is_low(struct lruvec *lruvec)
  {
  	if (!mem_cgroup_disabled())
  		return mem_cgroup_inactive_file_is_low(lruvec);
  
  	return inactive_file_is_low_global(lruvec_zone(lruvec));
  }
  
  static int inactive_list_is_low(struct lruvec *lruvec, enum lru_list lru)
  {
  	if (is_file_lru(lru))
  		return inactive_file_is_low(lruvec);
  	else
  		return inactive_anon_is_low(lruvec);
  }
  
  static unsigned long shrink_list(enum lru_list lru, unsigned long nr_to_scan,
  				 struct lruvec *lruvec, struct scan_control *sc)
  {
  	if (is_active_lru(lru)) {
  		if (inactive_list_is_low(lruvec, lru))
  			shrink_active_list(nr_to_scan, lruvec, sc, lru);
  		return 0;
  	}
  
  	return shrink_inactive_list(nr_to_scan, lruvec, sc, lru);
  }
  
  static int vmscan_swappiness(struct scan_control *sc)
  {
  	if (global_reclaim(sc))
  		return vm_swappiness;
  	return mem_cgroup_swappiness(sc->target_mem_cgroup);
  }
  
  /*
   * Determine how aggressively the anon and file LRU lists should be
   * scanned.  The relative value of each set of LRU lists is determined
   * by looking at the fraction of the pages scanned we did rotate back
   * onto the active list instead of evict.
   *
   * nr[0] = anon inactive pages to scan; nr[1] = anon active pages to scan
   * nr[2] = file inactive pages to scan; nr[3] = file active pages to scan
   */
  static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
  			   unsigned long *nr)
  {
  	unsigned long anon, file, free;
  	unsigned long anon_prio, file_prio;
  	unsigned long ap, fp;
  	struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
  	u64 fraction[2], denominator;
  	enum lru_list lru;
  	int noswap = 0;
  	bool force_scan = false;
  	struct zone *zone = lruvec_zone(lruvec);
  
  	/*
  	 * If the zone or memcg is small, nr[l] can be 0.  This
  	 * results in no scanning on this priority and a potential
  	 * priority drop.  Global direct reclaim can go to the next
  	 * zone and tends to have no problems. Global kswapd is for
  	 * zone balancing and it needs to scan a minimum amount. When
  	 * reclaiming for a memcg, a priority drop can cause high
  	 * latencies, so it's better to scan a minimum amount there as
  	 * well.
  	 */
  	if (current_is_kswapd() && zone->all_unreclaimable)
  		force_scan = true;
  	if (!global_reclaim(sc))
  		force_scan = true;
  
  	/* If we have no swap space, do not bother scanning anon pages. */
  	if (!sc->may_swap || (nr_swap_pages <= 0)) {
  		noswap = 1;
  		fraction[0] = 0;
  		fraction[1] = 1;
  		denominator = 1;
  		goto out;
  	}
  
  	anon  = get_lru_size(lruvec, LRU_ACTIVE_ANON) +
  		get_lru_size(lruvec, LRU_INACTIVE_ANON);
  	file  = get_lru_size(lruvec, LRU_ACTIVE_FILE) +
  		get_lru_size(lruvec, LRU_INACTIVE_FILE);
  
  	if (global_reclaim(sc)) {
  		free  = zone_page_state(zone, NR_FREE_PAGES);
  		/* If we have very few page cache pages,
  		   force-scan anon pages. */
  		if (unlikely(file + free <= high_wmark_pages(zone))) {
  			fraction[0] = 1;
  			fraction[1] = 0;
  			denominator = 1;
  			goto out;
  		}
  	}
  
  	/*
  	 * With swappiness at 100, anonymous and file have the same priority.
  	 * This scanning priority is essentially the inverse of IO cost.
  	 */
  	anon_prio = vmscan_swappiness(sc);
  	file_prio = 200 - anon_prio;
  
  	/*
  	 * OK, so we have swap space and a fair amount of page cache
  	 * pages.  We use the recently rotated / recently scanned
  	 * ratios to determine how valuable each cache is.
  	 *
  	 * Because workloads change over time (and to avoid overflow)
  	 * we keep these statistics as a floating average, which ends
  	 * up weighing recent references more than old ones.
  	 *
  	 * anon in [0], file in [1]
  	 */
  	spin_lock_irq(&zone->lru_lock);
  	if (unlikely(reclaim_stat->recent_scanned[0] > anon / 4)) {
  		reclaim_stat->recent_scanned[0] /= 2;
  		reclaim_stat->recent_rotated[0] /= 2;
  	}
  
  	if (unlikely(reclaim_stat->recent_scanned[1] > file / 4)) {
  		reclaim_stat->recent_scanned[1] /= 2;
  		reclaim_stat->recent_rotated[1] /= 2;
  	}
  
  	/*
  	 * The amount of pressure on anon vs file pages is inversely
  	 * proportional to the fraction of recently scanned pages on
  	 * each list that were recently referenced and in active use.
  	 */
  	ap = anon_prio * (reclaim_stat->recent_scanned[0] + 1);
  	ap /= reclaim_stat->recent_rotated[0] + 1;
  
  	fp = file_prio * (reclaim_stat->recent_scanned[1] + 1);
  	fp /= reclaim_stat->recent_rotated[1] + 1;
  	spin_unlock_irq(&zone->lru_lock);
  
  	fraction[0] = ap;
  	fraction[1] = fp;
  	denominator = ap + fp + 1;
  out:
  	for_each_evictable_lru(lru) {
  		int file = is_file_lru(lru);
  		unsigned long scan;
  
  		scan = get_lru_size(lruvec, lru);
  		if (sc->priority || noswap || !vmscan_swappiness(sc)) {
  			scan >>= sc->priority;
  			if (!scan && force_scan)
  				scan = SWAP_CLUSTER_MAX;
  			scan = div64_u64(scan * fraction[file], denominator);
  		}
  		nr[lru] = scan;
  	}
  }
  
  /* Use reclaim/compaction for costly allocs or under memory pressure */
  static bool in_reclaim_compaction(struct scan_control *sc)
  {
  	if (COMPACTION_BUILD && sc->order &&
  			(sc->order > PAGE_ALLOC_COSTLY_ORDER ||
  			 sc->priority < DEF_PRIORITY - 2))
  		return true;
  
  	return false;
  }
  
  /*
   * Reclaim/compaction is used for high-order allocation requests. It reclaims
   * order-0 pages before compacting the zone. should_continue_reclaim() returns
   * true if more pages should be reclaimed such that when the page allocator
   * calls try_to_compact_zone() that it will have enough free pages to succeed.
   * It will give up earlier than that if there is difficulty reclaiming pages.
   */
  static inline bool should_continue_reclaim(struct lruvec *lruvec,
  					unsigned long nr_reclaimed,
  					unsigned long nr_scanned,
  					struct scan_control *sc)
  {
  	unsigned long pages_for_compaction;
  	unsigned long inactive_lru_pages;
  
  	/* If not in reclaim/compaction mode, stop */
  	if (!in_reclaim_compaction(sc))
  		return false;
  
  	/* Consider stopping depending on scan and reclaim activity */
  	if (sc->gfp_mask & __GFP_REPEAT) {
  		/*
  		 * For __GFP_REPEAT allocations, stop reclaiming if the
  		 * full LRU list has been scanned and we are still failing
  		 * to reclaim pages. This full LRU scan is potentially
  		 * expensive but a __GFP_REPEAT caller really wants to succeed
  		 */
  		if (!nr_reclaimed && !nr_scanned)
  			return false;
  	} else {
  		/*
  		 * For non-__GFP_REPEAT allocations which can presumably
  		 * fail without consequence, stop if we failed to reclaim
  		 * any pages from the last SWAP_CLUSTER_MAX number of
  		 * pages that were scanned. This will return to the
  		 * caller faster at the risk reclaim/compaction and
  		 * the resulting allocation attempt fails
  		 */
  		if (!nr_reclaimed)
  			return false;
  	}
  
  	/*
  	 * If we have not reclaimed enough pages for compaction and the
  	 * inactive lists are large enough, continue reclaiming
  	 */
  	pages_for_compaction = (2UL << sc->order);
  	inactive_lru_pages = get_lru_size(lruvec, LRU_INACTIVE_FILE);
  	if (nr_swap_pages > 0)
  		inactive_lru_pages += get_lru_size(lruvec, LRU_INACTIVE_ANON);
  	if (sc->nr_reclaimed < pages_for_compaction &&
  			inactive_lru_pages > pages_for_compaction)
  		return true;
  
  	/* If compaction would go ahead or the allocation would succeed, stop */
  	switch (compaction_suitable(lruvec_zone(lruvec), sc->order)) {
  	case COMPACT_PARTIAL:
  	case COMPACT_CONTINUE:
  		return false;
  	default:
  		return true;
  	}
  }
  
  /*
   * This is a basic per-zone page freer.  Used by both kswapd and direct reclaim.
   */
  static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
  {
  	unsigned long nr[NR_LRU_LISTS];
  	unsigned long nr_to_scan;
  	enum lru_list lru;
  	unsigned long nr_reclaimed, nr_scanned;
  	unsigned long nr_to_reclaim = sc->nr_to_reclaim;
  	struct blk_plug plug;
  
  restart:
  	nr_reclaimed = 0;
  	nr_scanned = sc->nr_scanned;
  	get_scan_count(lruvec, sc, nr);
  
  	blk_start_plug(&plug);
  	while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
  					nr[LRU_INACTIVE_FILE]) {
  		for_each_evictable_lru(lru) {
  			if (nr[lru]) {
  				nr_to_scan = min_t(unsigned long,
  						   nr[lru], SWAP_CLUSTER_MAX);
  				nr[lru] -= nr_to_scan;
  
  				nr_reclaimed += shrink_list(lru, nr_to_scan,
  							    lruvec, sc);
  			}
  		}
  		/*
  		 * On large memory systems, scan >> priority can become
  		 * really large. This is fine for the starting priority;
  		 * we want to put equal scanning pressure on each zone.
  		 * However, if the VM has a harder time of freeing pages,
  		 * with multiple processes reclaiming pages, the total
  		 * freeing target can get unreasonably large.
  		 */
  		if (nr_reclaimed >= nr_to_reclaim &&
  		    sc->priority < DEF_PRIORITY)
  			break;
  	}
  	blk_finish_plug(&plug);
  	sc->nr_reclaimed += nr_reclaimed;
  
  	/*
  	 * Even if we did not try to evict anon pages at all, we want to
  	 * rebalance the anon lru active/inactive ratio.
  	 */
  	if (inactive_anon_is_low(lruvec))
  		shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
  				   sc, LRU_ACTIVE_ANON);
  
  	/* reclaim/compaction might need reclaim to continue */
  	if (should_continue_reclaim(lruvec, nr_reclaimed,
  				    sc->nr_scanned - nr_scanned, sc))
  		goto restart;
  
  	throttle_vm_writeout(sc->gfp_mask);
  }
  
  static void shrink_zone(struct zone *zone, struct scan_control *sc)
  {
  	struct mem_cgroup *root = sc->target_mem_cgroup;
  	struct mem_cgroup_reclaim_cookie reclaim = {
  		.zone = zone,
  		.priority = sc->priority,
  	};
  	struct mem_cgroup *memcg;
  
  	memcg = mem_cgroup_iter(root, NULL, &reclaim);
  	do {
  		struct lruvec *lruvec = mem_cgroup_zone_lruvec(zone, memcg);
  
  		shrink_lruvec(lruvec, sc);
  
  		/*
  		 * Limit reclaim has historically picked one memcg and
  		 * scanned it with decreasing priority levels until
  		 * nr_to_reclaim had been reclaimed.  This priority
  		 * cycle is thus over after a single memcg.
  		 *
  		 * Direct reclaim and kswapd, on the other hand, have
  		 * to scan all memory cgroups to fulfill the overall
  		 * scan target for the zone.
  		 */
  		if (!global_reclaim(sc)) {
  			mem_cgroup_iter_break(root, memcg);
  			break;
  		}
  		memcg = mem_cgroup_iter(root, memcg, &reclaim);
  	} while (memcg);
  }
  
  /* Returns true if compaction should go ahead for a high-order request */
  static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
  {
  	unsigned long balance_gap, watermark;
  	bool watermark_ok;
  
  	/* Do not consider compaction for orders reclaim is meant to satisfy */
  	if (sc->order <= PAGE_ALLOC_COSTLY_ORDER)
  		return false;
  
  	/*
  	 * Compaction takes time to run and there are potentially other
  	 * callers using the pages just freed. Continue reclaiming until
  	 * there is a buffer of free pages available to give compaction
  	 * a reasonable chance of completing and allocating the page
  	 */
  	balance_gap = min(low_wmark_pages(zone),
  		(zone->present_pages + KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
  			KSWAPD_ZONE_BALANCE_GAP_RATIO);
  	watermark = high_wmark_pages(zone) + balance_gap + (2UL << sc->order);
  	watermark_ok = zone_watermark_ok_safe(zone, 0, watermark, 0, 0);
  
  	/*
  	 * If compaction is deferred, reclaim up to a point where
  	 * compaction will have a chance of success when re-enabled
  	 */
  	if (compaction_deferred(zone, sc->order))
  		return watermark_ok;
  
  	/* If compaction is not ready to start, keep reclaiming */
  	if (!compaction_suitable(zone, sc->order))
  		return false;
  
  	return watermark_ok;
  }
  
  /*
   * This is the direct reclaim path, for page-allocating processes.  We only
   * try to reclaim pages from zones which will satisfy the caller's allocation
   * request.
   *
   * We reclaim from a zone even if that zone is over high_wmark_pages(zone).
   * Because:
   * a) The caller may be trying to free *extra* pages to satisfy a higher-order
   *    allocation or
   * b) The target zone may be at high_wmark_pages(zone) but the lower zones
   *    must go *over* high_wmark_pages(zone) to satisfy the `incremental min'
   *    zone defense algorithm.
   *
   * If a zone is deemed to be full of pinned pages then just give it a light
   * scan then give up on it.
   *
   * This function returns true if a zone is being reclaimed for a costly
   * high-order allocation and compaction is ready to begin. This indicates to
   * the caller that it should consider retrying the allocation instead of
   * further reclaim.
   */
  static bool shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
  {
  	struct zoneref *z;
  	struct zone *zone;
  	unsigned long nr_soft_reclaimed;
  	unsigned long nr_soft_scanned;
  	bool aborted_reclaim = false;
  
  	/*
  	 * If the number of buffer_heads in the machine exceeds the maximum
  	 * allowed level, force direct reclaim to scan the highmem zone as
  	 * highmem pages could be pinning lowmem pages storing buffer_heads
  	 */
  	if (buffer_heads_over_limit)
  		sc->gfp_mask |= __GFP_HIGHMEM;
  
  	for_each_zone_zonelist_nodemask(zone, z, zonelist,
  					gfp_zone(sc->gfp_mask), sc->nodemask) {
  		if (!populated_zone(zone))
  			continue;
  		/*
  		 * Take care memory controller reclaiming has small influence
  		 * to global LRU.
  		 */
  		if (global_reclaim(sc)) {
  			if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
  				continue;
  			if (zone->all_unreclaimable &&
  					sc->priority != DEF_PRIORITY)
  				continue;	/* Let kswapd poll it */
  			if (COMPACTION_BUILD) {
  				/*
  				 * If we already have plenty of memory free for
  				 * compaction in this zone, don't free any more.
  				 * Even though compaction is invoked for any
  				 * non-zero order, only frequent costly order
  				 * reclamation is disruptive enough to become a
  				 * noticeable problem, like transparent huge
  				 * page allocations.
  				 */
  				if (compaction_ready(zone, sc)) {
  					aborted_reclaim = true;
  					continue;
  				}
  			}
  			/*
  			 * This steals pages from memory cgroups over softlimit
  			 * and returns the number of reclaimed pages and
  			 * scanned pages. This works for global memory pressure
  			 * and balancing, not for a memcg's limit.
  			 */
  			nr_soft_scanned = 0;
  			nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone,
  						sc->order, sc->gfp_mask,
  						&nr_soft_scanned);
  			sc->nr_reclaimed += nr_soft_reclaimed;
  			sc->nr_scanned += nr_soft_scanned;
  			/* need some check for avoid more shrink_zone() */
  		}
  
  		shrink_zone(zone, sc);
  	}
  
  	return aborted_reclaim;
  }
  
  static bool zone_reclaimable(struct zone *zone)
  {
  	return zone->pages_scanned < zone_reclaimable_pages(zone) * 6;
  }
  
  /* All zones in zonelist are unreclaimable? */
  static bool all_unreclaimable(struct zonelist *zonelist,
  		struct scan_control *sc)
  {
  	struct zoneref *z;
  	struct zone *zone;
  
  	for_each_zone_zonelist_nodemask(zone, z, zonelist,
  			gfp_zone(sc->gfp_mask), sc->nodemask) {
  		if (!populated_zone(zone))
  			continue;
  		if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
  			continue;
  		if (!zone->all_unreclaimable)
  			return false;
  	}
  
  	return true;
  }
  
  /*
   * This is the main entry point to direct page reclaim.
   *
   * If a full scan of the inactive list fails to free enough memory then we
   * are "out of memory" and something needs to be killed.
   *
   * If the caller is !__GFP_FS then the probability of a failure is reasonably
   * high - the zone may be full of dirty or under-writeback pages, which this
   * caller can't do much about.  We kick the writeback threads and take explicit
   * naps in the hope that some of these pages can be written.  But if the
   * allocating task holds filesystem locks which prevent writeout this might not
   * work, and the allocation attempt will fail.
   *
   * returns:	0, if no pages reclaimed
   * 		else, the number of pages reclaimed
   */
  static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
  					struct scan_control *sc,
  					struct shrink_control *shrink)
  {
  	unsigned long total_scanned = 0;
  	struct reclaim_state *reclaim_state = current->reclaim_state;
  	struct zoneref *z;
  	struct zone *zone;
  	unsigned long writeback_threshold;
  	bool aborted_reclaim;
  
  	delayacct_freepages_start();
  
  	if (global_reclaim(sc))
  		count_vm_event(ALLOCSTALL);
  
  	do {
  		sc->nr_scanned = 0;
  		aborted_reclaim = shrink_zones(zonelist, sc);
  
  		/*
  		 * Don't shrink slabs when reclaiming memory from
  		 * over limit cgroups
  		 */
  		if (global_reclaim(sc)) {
  			unsigned long lru_pages = 0;
  			for_each_zone_zonelist(zone, z, zonelist,
  					gfp_zone(sc->gfp_mask)) {
  				if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
  					continue;
  
  				lru_pages += zone_reclaimable_pages(zone);
  			}
  
  			shrink_slab(shrink, sc->nr_scanned, lru_pages);
  			if (reclaim_state) {
  				sc->nr_reclaimed += reclaim_state->reclaimed_slab;
  				reclaim_state->reclaimed_slab = 0;
  			}
  		}
  		total_scanned += sc->nr_scanned;
  		if (sc->nr_reclaimed >= sc->nr_to_reclaim)
  			goto out;
  
  		/*
  		 * Try to write back as many pages as we just scanned.  This
  		 * tends to cause slow streaming writers to write data to the
  		 * disk smoothly, at the dirtying rate, which is nice.   But
  		 * that's undesirable in laptop mode, where we *want* lumpy
  		 * writeout.  So in laptop mode, write out the whole world.
  		 */
  		writeback_threshold = sc->nr_to_reclaim + sc->nr_to_reclaim / 2;
  		if (total_scanned > writeback_threshold) {
  			wakeup_flusher_threads(laptop_mode ? 0 : total_scanned,
  						WB_REASON_TRY_TO_FREE_PAGES);
  			sc->may_writepage = 1;
  		}
  
  		/* Take a nap, wait for some writeback to complete */
  		if (!sc->hibernation_mode && sc->nr_scanned &&
  		    sc->priority < DEF_PRIORITY - 2) {
  			struct zone *preferred_zone;
  
  			first_zones_zonelist(zonelist, gfp_zone(sc->gfp_mask),
  						&cpuset_current_mems_allowed,
  						&preferred_zone);
  			wait_iff_congested(preferred_zone, BLK_RW_ASYNC, HZ/10);
  		}
  	} while (--sc->priority >= 0);
  
  out:
  	delayacct_freepages_end();
  
  	if (sc->nr_reclaimed)
  		return sc->nr_reclaimed;
  
  	/*
  	 * As hibernation is going on, kswapd is freezed so that it can't mark
  	 * the zone into all_unreclaimable. Thus bypassing all_unreclaimable
  	 * check.
  	 */
  	if (oom_killer_disabled)
  		return 0;
  
  	/* Aborted reclaim to try compaction? don't OOM, then */
  	if (aborted_reclaim)
  		return 1;
  
  	/* top priority shrink_zones still had more to do? don't OOM, then */
  	if (global_reclaim(sc) && !all_unreclaimable(zonelist, sc))
  		return 1;
  
  	return 0;
  }
  
  static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
  {
  	struct zone *zone;
  	unsigned long pfmemalloc_reserve = 0;
  	unsigned long free_pages = 0;
  	int i;
  	bool wmark_ok;
  
  	for (i = 0; i <= ZONE_NORMAL; i++) {
  		zone = &pgdat->node_zones[i];
  		pfmemalloc_reserve += min_wmark_pages(zone);
  		free_pages += zone_page_state(zone, NR_FREE_PAGES);
  	}
  
  	wmark_ok = free_pages > pfmemalloc_reserve / 2;
  
  	/* kswapd must be awake if processes are being throttled */
  	if (!wmark_ok && waitqueue_active(&pgdat->kswapd_wait)) {
  		pgdat->classzone_idx = min(pgdat->classzone_idx,
  						(enum zone_type)ZONE_NORMAL);
  		wake_up_interruptible(&pgdat->kswapd_wait);
  	}
  
  	return wmark_ok;
  }
  
  /*
   * Throttle direct reclaimers if backing storage is backed by the network
   * and the PFMEMALLOC reserve for the preferred node is getting dangerously
   * depleted. kswapd will continue to make progress and wake the processes
   * when the low watermark is reached.
   *
   * Returns true if a fatal signal was delivered during throttling. If this
   * happens, the page allocator should not consider triggering the OOM killer.
   */
  static bool throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
  					nodemask_t *nodemask)
  {
  	struct zone *zone;
  	int high_zoneidx = gfp_zone(gfp_mask);
  	pg_data_t *pgdat;
  
  	/*
  	 * Kernel threads should not be throttled as they may be indirectly
  	 * responsible for cleaning pages necessary for reclaim to make forward
  	 * progress. kjournald for example may enter direct reclaim while
  	 * committing a transaction where throttling it could forcing other
  	 * processes to block on log_wait_commit().
  	 */
  	if (current->flags & PF_KTHREAD)
  		goto out;
  
  	/*
  	 * If a fatal signal is pending, this process should not throttle.
  	 * It should return quickly so it can exit and free its memory
  	 */
  	if (fatal_signal_pending(current))
  		goto out;
  
  	/* Check if the pfmemalloc reserves are ok */
  	first_zones_zonelist(zonelist, high_zoneidx, NULL, &zone);
  	pgdat = zone->zone_pgdat;
  	if (pfmemalloc_watermark_ok(pgdat))
  		goto out;
  
  	/* Account for the throttling */
  	count_vm_event(PGSCAN_DIRECT_THROTTLE);
  
  	/*
  	 * If the caller cannot enter the filesystem, it's possible that it
  	 * is due to the caller holding an FS lock or performing a journal
  	 * transaction in the case of a filesystem like ext[3|4]. In this case,
  	 * it is not safe to block on pfmemalloc_wait as kswapd could be
  	 * blocked waiting on the same lock. Instead, throttle for up to a
  	 * second before continuing.
  	 */
  	if (!(gfp_mask & __GFP_FS)) {
  		wait_event_interruptible_timeout(pgdat->pfmemalloc_wait,
  			pfmemalloc_watermark_ok(pgdat), HZ);
  
  		goto check_pending;
  	}
  
  	/* Throttle until kswapd wakes the process */
  	wait_event_killable(zone->zone_pgdat->pfmemalloc_wait,
  		pfmemalloc_watermark_ok(pgdat));
  
  check_pending:
  	if (fatal_signal_pending(current))
  		return true;
  
  out:
  	return false;
  }
  
  unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
  				gfp_t gfp_mask, nodemask_t *nodemask)
  {
  	unsigned long nr_reclaimed;
  	struct scan_control sc = {
  		.gfp_mask = gfp_mask,
  		.may_writepage = !laptop_mode,
  		.nr_to_reclaim = SWAP_CLUSTER_MAX,
  		.may_unmap = 1,
  		.may_swap = 1,
  		.order = order,
  		.priority = DEF_PRIORITY,
  		.target_mem_cgroup = NULL,
  		.nodemask = nodemask,
  	};
  	struct shrink_control shrink = {
  		.gfp_mask = sc.gfp_mask,
  	};
  
  	/*
  	 * Do not enter reclaim if fatal signal was delivered while throttled.
  	 * 1 is returned so that the page allocator does not OOM kill at this
  	 * point.
  	 */
  	if (throttle_direct_reclaim(gfp_mask, zonelist, nodemask))
  		return 1;
  
  	trace_mm_vmscan_direct_reclaim_begin(order,
  				sc.may_writepage,
  				gfp_mask);
  
  	nr_reclaimed = do_try_to_free_pages(zonelist, &sc, &shrink);
  
  	trace_mm_vmscan_direct_reclaim_end(nr_reclaimed);
  
  	return nr_reclaimed;
  }
  
  #ifdef CONFIG_MEMCG
  
  unsigned long mem_cgroup_shrink_node_zone(struct mem_cgroup *memcg,
  						gfp_t gfp_mask, bool noswap,
  						struct zone *zone,
  						unsigned long *nr_scanned)
  {
  	struct scan_control sc = {
  		.nr_scanned = 0,
  		.nr_to_reclaim = SWAP_CLUSTER_MAX,
  		.may_writepage = !laptop_mode,
  		.may_unmap = 1,
  		.may_swap = !noswap,
  		.order = 0,
  		.priority = 0,
  		.target_mem_cgroup = memcg,
  	};
  	struct lruvec *lruvec = mem_cgroup_zone_lruvec(zone, memcg);
  
  	sc.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
  			(GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK);
  
  	trace_mm_vmscan_memcg_softlimit_reclaim_begin(sc.order,
  						      sc.may_writepage,
  						      sc.gfp_mask);
  
  	/*
  	 * NOTE: Although we can get the priority field, using it
  	 * here is not a good idea, since it limits the pages we can scan.
  	 * if we don't reclaim here, the shrink_zone from balance_pgdat
  	 * will pick up pages from other mem cgroup's as well. We hack
  	 * the priority and make it zero.
  	 */
  	shrink_lruvec(lruvec, &sc);
  
  	trace_mm_vmscan_memcg_softlimit_reclaim_end(sc.nr_reclaimed);
  
  	*nr_scanned = sc.nr_scanned;
  	return sc.nr_reclaimed;
  }
  
  unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
  					   gfp_t gfp_mask,
  					   bool noswap)
  {
  	struct zonelist *zonelist;
  	unsigned long nr_reclaimed;
  	int nid;
  	struct scan_control sc = {
  		.may_writepage = !laptop_mode,
  		.may_unmap = 1,
  		.may_swap = !noswap,
  		.nr_to_reclaim = SWAP_CLUSTER_MAX,
  		.order = 0,
  		.priority = DEF_PRIORITY,
  		.target_mem_cgroup = memcg,
  		.nodemask = NULL, /* we don't care the placement */
  		.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
  				(GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK),
  	};
  	struct shrink_control shrink = {
  		.gfp_mask = sc.gfp_mask,
  	};
  
  	/*
  	 * Unlike direct reclaim via alloc_pages(), memcg's reclaim doesn't
  	 * take care of from where we get pages. So the node where we start the
  	 * scan does not need to be the current node.
  	 */
  	nid = mem_cgroup_select_victim_node(memcg);
  
  	zonelist = NODE_DATA(nid)->node_zonelists;
  
  	trace_mm_vmscan_memcg_reclaim_begin(0,
  					    sc.may_writepage,
  					    sc.gfp_mask);
  
  	nr_reclaimed = do_try_to_free_pages(zonelist, &sc, &shrink);
  
  	trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);
  
  	return nr_reclaimed;
  }
  #endif
  
  static void age_active_anon(struct zone *zone, struct scan_control *sc)
  {
  	struct mem_cgroup *memcg;
  
  	if (!total_swap_pages)
  		return;
  
  	memcg = mem_cgroup_iter(NULL, NULL, NULL);
  	do {
  		struct lruvec *lruvec = mem_cgroup_zone_lruvec(zone, memcg);
  
  		if (inactive_anon_is_low(lruvec))
  			shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
  					   sc, LRU_ACTIVE_ANON);
  
  		memcg = mem_cgroup_iter(NULL, memcg, NULL);
  	} while (memcg);
  }
  
  static bool zone_balanced(struct zone *zone, int order,
  			  unsigned long balance_gap, int classzone_idx)
  {
  	if (!zone_watermark_ok_safe(zone, order, high_wmark_pages(zone) +
  				    balance_gap, classzone_idx, 0))
  		return false;
  
  	if (COMPACTION_BUILD && order && !compaction_suitable(zone, order))
  		return false;
  
  	return true;
  }
  
  /*
   * pgdat_balanced is used when checking if a node is balanced for high-order
   * allocations. Only zones that meet watermarks and are in a zone allowed
   * by the callers classzone_idx are added to balanced_pages. The total of
   * balanced pages must be at least 25% of the zones allowed by classzone_idx
   * for the node to be considered balanced. Forcing all zones to be balanced
   * for high orders can cause excessive reclaim when there are imbalanced zones.
   * The choice of 25% is due to
   *   o a 16M DMA zone that is balanced will not balance a zone on any
   *     reasonable sized machine
   *   o On all other machines, the top zone must be at least a reasonable
   *     percentage of the middle zones. For example, on 32-bit x86, highmem
   *     would need to be at least 256M for it to be balance a whole node.
   *     Similarly, on x86-64 the Normal zone would need to be at least 1G
   *     to balance a node on its own. These seemed like reasonable ratios.
   */
  static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced_pages,
  						int classzone_idx)
  {
  	unsigned long present_pages = 0;
  	int i;
  
  	for (i = 0; i <= classzone_idx; i++)
  		present_pages += pgdat->node_zones[i].present_pages;
  
  	/* A special case here: if zone has no page, we think it's balanced */
  	return balanced_pages >= (present_pages >> 2);
  }
  
  /*
   * Prepare kswapd for sleeping. This verifies that there are no processes
   * waiting in throttle_direct_reclaim() and that watermarks have been met.
   *
   * Returns true if kswapd is ready to sleep
   */
  static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
  					int classzone_idx)
  {
  	int i;
  	unsigned long balanced = 0;
  	bool all_zones_ok = true;
  
  	/* If a direct reclaimer woke kswapd within HZ/10, it's premature */
  	if (remaining)
  		return false;
  
  	/*
  	 * There is a potential race between when kswapd checks its watermarks
  	 * and a process gets throttled. There is also a potential race if
  	 * processes get throttled, kswapd wakes, a large process exits therby
  	 * balancing the zones that causes kswapd to miss a wakeup. If kswapd
  	 * is going to sleep, no process should be sleeping on pfmemalloc_wait
  	 * so wake them now if necessary. If necessary, processes will wake
  	 * kswapd and get throttled again
  	 */
  	if (waitqueue_active(&pgdat->pfmemalloc_wait)) {
  		wake_up(&pgdat->pfmemalloc_wait);
  		return false;
  	}
  
  	/* Check the watermark levels */
  	for (i = 0; i <= classzone_idx; i++) {
  		struct zone *zone = pgdat->node_zones + i;
  
  		if (!populated_zone(zone))
  			continue;
  
  		/*
  		 * balance_pgdat() skips over all_unreclaimable after
  		 * DEF_PRIORITY. Effectively, it considers them balanced so
  		 * they must be considered balanced here as well if kswapd
  		 * is to sleep
  		 */
  		if (zone->all_unreclaimable) {
  			balanced += zone->present_pages;
  			continue;
  		}
  
  		if (!zone_balanced(zone, order, 0, i))
  			all_zones_ok = false;
  		else
  			balanced += zone->present_pages;
  	}
  
  	/*
  	 * For high-order requests, the balanced zones must contain at least
  	 * 25% of the nodes pages for kswapd to sleep. For order-0, all zones
  	 * must be balanced
  	 */
  	if (order)
  		return pgdat_balanced(pgdat, balanced, classzone_idx);
  	else
  		return all_zones_ok;
  }
  
  /*
   * For kswapd, balance_pgdat() will work across all this node's zones until
   * they are all at high_wmark_pages(zone).
   *
   * Returns the final order kswapd was reclaiming at
   *
   * There is special handling here for zones which are full of pinned pages.
   * This can happen if the pages are all mlocked, or if they are all used by
   * device drivers (say, ZONE_DMA).  Or if they are all in use by hugetlb.
   * What we do is to detect the case where all pages in the zone have been
   * scanned twice and there has been zero successful reclaim.  Mark the zone as
   * dead and from now on, only perform a short scan.  Basically we're polling
   * the zone for when the problem goes away.
   *
   * kswapd scans the zones in the highmem->normal->dma direction.  It skips
   * zones which have free_pages > high_wmark_pages(zone), but once a zone is
   * found to have free_pages <= high_wmark_pages(zone), we scan that zone and the
   * lower zones regardless of the number of free pages in the lower zones. This
   * interoperates with the page allocator fallback scheme to ensure that aging
   * of pages is balanced across the zones.
   */
  static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
  							int *classzone_idx)
  {
  	int all_zones_ok;
  	unsigned long balanced;
  	int i;
  	int end_zone = 0;	/* Inclusive.  0 = ZONE_DMA */
  	unsigned long total_scanned;
  	struct reclaim_state *reclaim_state = current->reclaim_state;
  	unsigned long nr_soft_reclaimed;
  	unsigned long nr_soft_scanned;
  	struct scan_control sc = {
  		.gfp_mask = GFP_KERNEL,
  		.may_unmap = 1,
  		.may_swap = 1,
  		/*
  		 * kswapd doesn't want to be bailed out while reclaim. because
  		 * we want to put equal scanning pressure on each zone.
  		 */
  		.nr_to_reclaim = ULONG_MAX,
  		.order = order,
  		.target_mem_cgroup = NULL,
  	};
  	struct shrink_control shrink = {
  		.gfp_mask = sc.gfp_mask,
  	};
  loop_again:
  	total_scanned = 0;
  	sc.priority = DEF_PRIORITY;
  	sc.nr_reclaimed = 0;
  	sc.may_writepage = !laptop_mode;
  	count_vm_event(PAGEOUTRUN);
  
  	do {
  		unsigned long lru_pages = 0;
  		int has_under_min_watermark_zone = 0;
  
  		all_zones_ok = 1;
  		balanced = 0;
  
  		/*
  		 * Scan in the highmem->dma direction for the highest
  		 * zone which needs scanning
  		 */
  		for (i = pgdat->nr_zones - 1; i >= 0; i--) {
  			struct zone *zone = pgdat->node_zones + i;
  
  			if (!populated_zone(zone))
  				continue;
  
  			if (zone->all_unreclaimable &&
  			    sc.priority != DEF_PRIORITY)
  				continue;
  
  			/*
  			 * Do some background aging of the anon list, to give
  			 * pages a chance to be referenced before reclaiming.
  			 */
  			age_active_anon(zone, &sc);
  
  			/*
  			 * If the number of buffer_heads in the machine
  			 * exceeds the maximum allowed level and this node
  			 * has a highmem zone, force kswapd to reclaim from
  			 * it to relieve lowmem pressure.
  			 */
  			if (buffer_heads_over_limit && is_highmem_idx(i)) {
  				end_zone = i;
  				break;
  			}
  
  			if (!zone_balanced(zone, order, 0, 0)) {
  				end_zone = i;
  				break;
  			} else {
  				/* If balanced, clear the congested flag */
  				zone_clear_flag(zone, ZONE_CONGESTED);
  			}
  		}
  		if (i < 0)
  			goto out;
  
  		for (i = 0; i <= end_zone; i++) {
  			struct zone *zone = pgdat->node_zones + i;
  
  			lru_pages += zone_reclaimable_pages(zone);
  		}
  
  		/*
  		 * Now scan the zone in the dma->highmem direction, stopping
  		 * at the last zone which needs scanning.
  		 *
  		 * We do this because the page allocator works in the opposite
  		 * direction.  This prevents the page allocator from allocating
  		 * pages behind kswapd's direction of progress, which would
  		 * cause too much scanning of the lower zones.
  		 */
  		for (i = 0; i <= end_zone; i++) {
  			struct zone *zone = pgdat->node_zones + i;
  			int nr_slab, testorder;
  			unsigned long balance_gap;
  
  			if (!populated_zone(zone))
  				continue;
  
  			if (zone->all_unreclaimable &&
  			    sc.priority != DEF_PRIORITY)
  				continue;
  
  			sc.nr_scanned = 0;
  
  			nr_soft_scanned = 0;
  			/*
  			 * Call soft limit reclaim before calling shrink_zone.
  			 */
  			nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone,
  							order, sc.gfp_mask,
  							&nr_soft_scanned);
  			sc.nr_reclaimed += nr_soft_reclaimed;
  			total_scanned += nr_soft_scanned;
  
  			/*
  			 * We put equal pressure on every zone, unless
  			 * one zone has way too many pages free
  			 * already. The "too many pages" is defined
  			 * as the high wmark plus a "gap" where the
  			 * gap is either the low watermark or 1%
  			 * of the zone, whichever is smaller.
  			 */
  			balance_gap = min(low_wmark_pages(zone),
  				(zone->present_pages +
  					KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
  				KSWAPD_ZONE_BALANCE_GAP_RATIO);
  			/*
  			 * Kswapd reclaims only single pages with compaction
  			 * enabled. Trying too hard to reclaim until contiguous
  			 * free pages have become available can hurt performance
  			 * by evicting too much useful data from memory.
  			 * Do not reclaim more than needed for compaction.
  			 */
  			testorder = order;
  			if (COMPACTION_BUILD && order &&
  					compaction_suitable(zone, order) !=
  						COMPACT_SKIPPED)
  				testorder = 0;
  
  			if ((buffer_heads_over_limit && is_highmem_idx(i)) ||
  			    !zone_balanced(zone, testorder,
  					   balance_gap, end_zone)) {
  				shrink_zone(zone, &sc);
  
  				reclaim_state->reclaimed_slab = 0;
  				nr_slab = shrink_slab(&shrink, sc.nr_scanned, lru_pages);
  				sc.nr_reclaimed += reclaim_state->reclaimed_slab;
  				total_scanned += sc.nr_scanned;
  
  				if (nr_slab == 0 && !zone_reclaimable(zone))
  					zone->all_unreclaimable = 1;
  			}
  
  			/*
  			 * If we've done a decent amount of scanning and
  			 * the reclaim ratio is low, start doing writepage
  			 * even in laptop mode
  			 */
  			if (total_scanned > SWAP_CLUSTER_MAX * 2 &&
  			    total_scanned > sc.nr_reclaimed + sc.nr_reclaimed / 2)
  				sc.may_writepage = 1;
  
  			if (zone->all_unreclaimable) {
  				if (end_zone && end_zone == i)
  					end_zone--;
  				continue;
  			}
  
  			if (!zone_balanced(zone, testorder, 0, end_zone)) {
  				all_zones_ok = 0;
  				/*
  				 * We are still under min water mark.  This
  				 * means that we have a GFP_ATOMIC allocation
  				 * failure risk. Hurry up!
  				 */
  				if (!zone_watermark_ok_safe(zone, order,
  					    min_wmark_pages(zone), end_zone, 0))
  					has_under_min_watermark_zone = 1;
  			} else {
  				/*
  				 * If a zone reaches its high watermark,
  				 * consider it to be no longer congested. It's
  				 * possible there are dirty pages backed by
  				 * congested BDIs but as pressure is relieved,
  				 * speculatively avoid congestion waits
  				 */
  				zone_clear_flag(zone, ZONE_CONGESTED);
  				if (i <= *classzone_idx)
  					balanced += zone->present_pages;
  			}
  
  		}
  
  		/*
  		 * If the low watermark is met there is no need for processes
  		 * to be throttled on pfmemalloc_wait as they should not be
  		 * able to safely make forward progress. Wake them
  		 */
  		if (waitqueue_active(&pgdat->pfmemalloc_wait) &&
  				pfmemalloc_watermark_ok(pgdat))
  			wake_up(&pgdat->pfmemalloc_wait);
  
  		if (all_zones_ok || (order && pgdat_balanced(pgdat, balanced, *classzone_idx)))
  			break;		/* kswapd: all done */
  		/*
  		 * OK, kswapd is getting into trouble.  Take a nap, then take
  		 * another pass across the zones.
  		 */
  		if (total_scanned && (sc.priority < DEF_PRIORITY - 2)) {
  			if (has_under_min_watermark_zone)
  				count_vm_event(KSWAPD_SKIP_CONGESTION_WAIT);
  			else
  				congestion_wait(BLK_RW_ASYNC, HZ/10);
  		}
  
  		/*
  		 * We do this so kswapd doesn't build up large priorities for
  		 * example when it is freeing in parallel with allocators. It
  		 * matches the direct reclaim path behaviour in terms of impact
  		 * on zone->*_priority.
  		 */
  		if (sc.nr_reclaimed >= SWAP_CLUSTER_MAX)
  			break;
  	} while (--sc.priority >= 0);
  out:
  
  	/*
  	 * order-0: All zones must meet high watermark for a balanced node
  	 * high-order: Balanced zones must make up at least 25% of the node
  	 *             for the node to be balanced
  	 */
  	if (!(all_zones_ok || (order && pgdat_balanced(pgdat, balanced, *classzone_idx)))) {
  		cond_resched();
  
  		try_to_freeze();
  
  		/*
  		 * Fragmentation may mean that the system cannot be
  		 * rebalanced for high-order allocations in all zones.
  		 * At this point, if nr_reclaimed < SWAP_CLUSTER_MAX,
  		 * it means the zones have been fully scanned and are still
  		 * not balanced. For high-order allocations, there is
  		 * little point trying all over again as kswapd may
  		 * infinite loop.
  		 *
  		 * Instead, recheck all watermarks at order-0 as they
  		 * are the most important. If watermarks are ok, kswapd will go
  		 * back to sleep. High-order users can still perform direct
  		 * reclaim if they wish.
  		 */
  		if (sc.nr_reclaimed < SWAP_CLUSTER_MAX)
  			order = sc.order = 0;
  
  		goto loop_again;
  	}
  
  	/*
  	 * If kswapd was reclaiming at a higher order, it has the option of
  	 * sleeping without all zones being balanced. Before it does, it must
  	 * ensure that the watermarks for order-0 on *all* zones are met and
  	 * that the congestion flags are cleared. The congestion flag must
  	 * be cleared as kswapd is the only mechanism that clears the flag
  	 * and it is potentially going to sleep here.
  	 */
  	if (order) {
  		int zones_need_compaction = 1;
  
  		for (i = 0; i <= end_zone; i++) {
  			struct zone *zone = pgdat->node_zones + i;
  
  			if (!populated_zone(zone))
  				continue;
  
  			/* Check if the memory needs to be defragmented. */
  			if (zone_watermark_ok(zone, order,
  				    low_wmark_pages(zone), *classzone_idx, 0))
  				zones_need_compaction = 0;
  		}
  
  		if (zones_need_compaction)
  			compact_pgdat(pgdat, order);
  	}
  
  	/*
  	 * Return the order we were reclaiming at so prepare_kswapd_sleep()
  	 * makes a decision on the order we were last reclaiming at. However,
  	 * if another caller entered the allocator slow path while kswapd
  	 * was awake, order will remain at the higher level
  	 */
  	*classzone_idx = end_zone;
  	return order;
  }
  
  static void kswapd_try_to_sleep(pg_data_t *pgdat, int order, int classzone_idx)
  {
  	long remaining = 0;
  	DEFINE_WAIT(wait);
  
  	if (freezing(current) || kthread_should_stop())
  		return;
  
  	prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
  
  	/* Try to sleep for a short interval */
  	if (prepare_kswapd_sleep(pgdat, order, remaining, classzone_idx)) {
  		remaining = schedule_timeout(HZ/10);
  		finish_wait(&pgdat->kswapd_wait, &wait);
  		prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
  	}
  
  	/*
  	 * After a short sleep, check if it was a premature sleep. If not, then
  	 * go fully to sleep until explicitly woken up.
  	 */
  	if (prepare_kswapd_sleep(pgdat, order, remaining, classzone_idx)) {
  		trace_mm_vmscan_kswapd_sleep(pgdat->node_id);
  
  		/*
  		 * vmstat counters are not perfectly accurate and the estimated
  		 * value for counters such as NR_FREE_PAGES can deviate from the
  		 * true value by nr_online_cpus * threshold. To avoid the zone
  		 * watermarks being breached while under pressure, we reduce the
  		 * per-cpu vmstat threshold while kswapd is awake and restore
  		 * them before going back to sleep.
  		 */
  		set_pgdat_percpu_threshold(pgdat, calculate_normal_threshold);
  
  		/*
  		 * Compaction records what page blocks it recently failed to
  		 * isolate pages from and skips them in the future scanning.
  		 * When kswapd is going to sleep, it is reasonable to assume
  		 * that pages and compaction may succeed so reset the cache.
  		 */
  		reset_isolation_suitable(pgdat);
  
  		if (!kthread_should_stop())
  			schedule();
  
  		set_pgdat_percpu_threshold(pgdat, calculate_pressure_threshold);
  	} else {
  		if (remaining)
  			count_vm_event(KSWAPD_LOW_WMARK_HIT_QUICKLY);
  		else
  			count_vm_event(KSWAPD_HIGH_WMARK_HIT_QUICKLY);
  	}
  	finish_wait(&pgdat->kswapd_wait, &wait);
  }
  
  /*
   * The background pageout daemon, started as a kernel thread
   * from the init process.
   *
   * This basically trickles out pages so that we have _some_
   * free memory available even if there is no other activity
   * that frees anything up. This is needed for things like routing
   * etc, where we otherwise might have all activity going on in
   * asynchronous contexts that cannot page things out.
   *
   * If there are applications that are active memory-allocators
   * (most normal use), this basically shouldn't matter.
   */
  static int kswapd(void *p)
  {
  	unsigned long order, new_order;
  	unsigned balanced_order;
  	int classzone_idx, new_classzone_idx;
  	int balanced_classzone_idx;
  	pg_data_t *pgdat = (pg_data_t*)p;
  	struct task_struct *tsk = current;
  
  	struct reclaim_state reclaim_state = {
  		.reclaimed_slab = 0,
  	};
  	const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
  
  	lockdep_set_current_reclaim_state(GFP_KERNEL);
  
  	if (!cpumask_empty(cpumask))
  		set_cpus_allowed_ptr(tsk, cpumask);
  	current->reclaim_state = &reclaim_state;
  
  	/*
  	 * Tell the memory management that we're a "memory allocator",
  	 * and that if we need more memory we should get access to it
  	 * regardless (see "__alloc_pages()"). "kswapd" should
  	 * never get caught in the normal page freeing logic.
  	 *
  	 * (Kswapd normally doesn't need memory anyway, but sometimes
  	 * you need a small amount of memory in order to be able to
  	 * page out something else, and this flag essentially protects
  	 * us from recursively trying to free more memory as we're
  	 * trying to free the first piece of memory in the first place).
  	 */
  	tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
  	set_freezable();
  
  	order = new_order = 0;
  	balanced_order = 0;
  	classzone_idx = new_classzone_idx = pgdat->nr_zones - 1;
  	balanced_classzone_idx = classzone_idx;
  	for ( ; ; ) {
  		int ret;
  
  		/*
  		 * If the last balance_pgdat was unsuccessful it's unlikely a
  		 * new request of a similar or harder type will succeed soon
  		 * so consider going to sleep on the basis we reclaimed at
  		 */
  		if (balanced_classzone_idx >= new_classzone_idx &&
  					balanced_order == new_order) {
  			new_order = pgdat->kswapd_max_order;
  			new_classzone_idx = pgdat->classzone_idx;
  			pgdat->kswapd_max_order =  0;
  			pgdat->classzone_idx = pgdat->nr_zones - 1;
  		}
  
  		if (order < new_order || classzone_idx > new_classzone_idx) {
  			/*
  			 * Don't sleep if someone wants a larger 'order'
  			 * allocation or has tigher zone constraints
  			 */
  			order = new_order;
  			classzone_idx = new_classzone_idx;
  		} else {
  			kswapd_try_to_sleep(pgdat, balanced_order,
  						balanced_classzone_idx);
  			order = pgdat->kswapd_max_order;
  			classzone_idx = pgdat->classzone_idx;
  			new_order = order;
  			new_classzone_idx = classzone_idx;
  			pgdat->kswapd_max_order = 0;
  			pgdat->classzone_idx = pgdat->nr_zones - 1;
  		}
  
  		ret = try_to_freeze();
  		if (kthread_should_stop())
  			break;
  
  		/*
  		 * We can speed up thawing tasks if we don't call balance_pgdat
  		 * after returning from the refrigerator
  		 */
  		if (!ret) {
  			trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
  			balanced_classzone_idx = classzone_idx;
  			balanced_order = balance_pgdat(pgdat, order,
  						&balanced_classzone_idx);
  		}
  	}
  
  	current->reclaim_state = NULL;
  	return 0;
  }
  
  /*
   * A zone is low on free memory, so wake its kswapd task to service it.
   */
  void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
  {
  	pg_data_t *pgdat;
  
  	if (!populated_zone(zone))
  		return;
  
  	if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
  		return;
  	pgdat = zone->zone_pgdat;
  	if (pgdat->kswapd_max_order < order) {
  		pgdat->kswapd_max_order = order;
  		pgdat->classzone_idx = min(pgdat->classzone_idx, classzone_idx);
  	}
  	if (!waitqueue_active(&pgdat->kswapd_wait))
  		return;
  	if (zone_watermark_ok_safe(zone, order, low_wmark_pages(zone), 0, 0))
  		return;
  
  	trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, zone_idx(zone), order);
  	wake_up_interruptible(&pgdat->kswapd_wait);
  }
  
  /*
   * The reclaimable count would be mostly accurate.
   * The less reclaimable pages may be
   * - mlocked pages, which will be moved to unevictable list when encountered
   * - mapped pages, which may require several travels to be reclaimed
   * - dirty pages, which is not "instantly" reclaimable
   */
  unsigned long global_reclaimable_pages(void)
  {
  	int nr;
  
  	nr = global_page_state(NR_ACTIVE_FILE) +
  	     global_page_state(NR_INACTIVE_FILE);
  
  	if (nr_swap_pages > 0)
  		nr += global_page_state(NR_ACTIVE_ANON) +
  		      global_page_state(NR_INACTIVE_ANON);
  
  	return nr;
  }
  
  unsigned long zone_reclaimable_pages(struct zone *zone)
  {
  	int nr;
  
  	nr = zone_page_state(zone, NR_ACTIVE_FILE) +
  	     zone_page_state(zone, NR_INACTIVE_FILE);
  
  	if (nr_swap_pages > 0)
  		nr += zone_page_state(zone, NR_ACTIVE_ANON) +
  		      zone_page_state(zone, NR_INACTIVE_ANON);
  
  	return nr;
  }
  
  #ifdef CONFIG_HIBERNATION
  /*
   * Try to free `nr_to_reclaim' of memory, system-wide, and return the number of
   * freed pages.
   *
   * Rather than trying to age LRUs the aim is to preserve the overall
   * LRU order by reclaiming preferentially
   * inactive > active > active referenced > active mapped
   */
  unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
  {
  	struct reclaim_state reclaim_state;
  	struct scan_control sc = {
  		.gfp_mask = GFP_HIGHUSER_MOVABLE,
  		.may_swap = 1,
  		.may_unmap = 1,
  		.may_writepage = 1,
  		.nr_to_reclaim = nr_to_reclaim,
  		.hibernation_mode = 1,
  		.order = 0,
  		.priority = DEF_PRIORITY,
  	};
  	struct shrink_control shrink = {
  		.gfp_mask = sc.gfp_mask,
  	};
  	struct zonelist *zonelist = node_zonelist(numa_node_id(), sc.gfp_mask);
  	struct task_struct *p = current;
  	unsigned long nr_reclaimed;
  
  	p->flags |= PF_MEMALLOC;
  	lockdep_set_current_reclaim_state(sc.gfp_mask);
  	reclaim_state.reclaimed_slab = 0;
  	p->reclaim_state = &reclaim_state;
  
  	nr_reclaimed = do_try_to_free_pages(zonelist, &sc, &shrink);
  
  	p->reclaim_state = NULL;
  	lockdep_clear_current_reclaim_state();
  	p->flags &= ~PF_MEMALLOC;
  
  	return nr_reclaimed;
  }
  #endif /* CONFIG_HIBERNATION */
  
  /* It's optimal to keep kswapds on the same CPUs as their memory, but
     not required for correctness.  So if the last cpu in a node goes
     away, we get changed to run anywhere: as the first one comes back,
     restore their cpu bindings. */
  static int __devinit cpu_callback(struct notifier_block *nfb,
  				  unsigned long action, void *hcpu)
  {
  	int nid;
  
  	if (action == CPU_ONLINE || action == CPU_ONLINE_FROZEN) {
  		for_each_node_state(nid, N_HIGH_MEMORY) {
  			pg_data_t *pgdat = NODE_DATA(nid);
  			const struct cpumask *mask;
  
  			mask = cpumask_of_node(pgdat->node_id);
  
  			if (cpumask_any_and(cpu_online_mask, mask) < nr_cpu_ids)
  				/* One of our CPUs online: restore mask */
  				set_cpus_allowed_ptr(pgdat->kswapd, mask);
  		}
  	}
  	return NOTIFY_OK;
  }
  
  /*
   * This kswapd start function will be called by init and node-hot-add.
   * On node-hot-add, kswapd will moved to proper cpus if cpus are hot-added.
   */
  int kswapd_run(int nid)
  {
  	pg_data_t *pgdat = NODE_DATA(nid);
  	int ret = 0;
  
  	if (pgdat->kswapd)
  		return 0;
  
  	pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid);
  	if (IS_ERR(pgdat->kswapd)) {
  		/* failure at boot is fatal */
  		BUG_ON(system_state == SYSTEM_BOOTING);
  		pgdat->kswapd = NULL;
  		pr_err("Failed to start kswapd on node %d\n", nid);
  		ret = PTR_ERR(pgdat->kswapd);
  	}
  	return ret;
  }
  
  /*
   * Called by memory hotplug when all memory in a node is offlined.  Caller must
   * hold lock_memory_hotplug().
   */
  void kswapd_stop(int nid)
  {
  	struct task_struct *kswapd = NODE_DATA(nid)->kswapd;
  
  	if (kswapd) {
  		kthread_stop(kswapd);
  		NODE_DATA(nid)->kswapd = NULL;
  	}
  }
  
  static int __init kswapd_init(void)
  {
  	int nid;
  
  	swap_setup();
  	for_each_node_state(nid, N_HIGH_MEMORY)
   		kswapd_run(nid);
  	hotcpu_notifier(cpu_callback, 0);
  	return 0;
  }
  
  module_init(kswapd_init)
  
  #ifdef CONFIG_NUMA
  /*
   * Zone reclaim mode
   *
   * If non-zero call zone_reclaim when the number of free pages falls below
   * the watermarks.
   */
  int zone_reclaim_mode __read_mostly;
  
  #define RECLAIM_OFF 0
  #define RECLAIM_ZONE (1<<0)	/* Run shrink_inactive_list on the zone */
  #define RECLAIM_WRITE (1<<1)	/* Writeout pages during reclaim */
  #define RECLAIM_SWAP (1<<2)	/* Swap pages out during reclaim */
  
  /*
   * Priority for ZONE_RECLAIM. This determines the fraction of pages
   * of a node considered for each zone_reclaim. 4 scans 1/16th of
   * a zone.
   */
  #define ZONE_RECLAIM_PRIORITY 4
  
  /*
   * Percentage of pages in a zone that must be unmapped for zone_reclaim to
   * occur.
   */
  int sysctl_min_unmapped_ratio = 1;
  
  /*
   * If the number of slab pages in a zone grows beyond this percentage then
   * slab reclaim needs to occur.
   */
  int sysctl_min_slab_ratio = 5;
  
  static inline unsigned long zone_unmapped_file_pages(struct zone *zone)
  {
  	unsigned long file_mapped = zone_page_state(zone, NR_FILE_MAPPED);
  	unsigned long file_lru = zone_page_state(zone, NR_INACTIVE_FILE) +
  		zone_page_state(zone, NR_ACTIVE_FILE);
  
  	/*
  	 * It's possible for there to be more file mapped pages than
  	 * accounted for by the pages on the file LRU lists because
  	 * tmpfs pages accounted for as ANON can also be FILE_MAPPED
  	 */
  	return (file_lru > file_mapped) ? (file_lru - file_mapped) : 0;
  }
  
  /* Work out how many page cache pages we can reclaim in this reclaim_mode */
  static long zone_pagecache_reclaimable(struct zone *zone)
  {
  	long nr_pagecache_reclaimable;
  	long delta = 0;
  
  	/*
  	 * If RECLAIM_SWAP is set, then all file pages are considered
  	 * potentially reclaimable. Otherwise, we have to worry about
  	 * pages like swapcache and zone_unmapped_file_pages() provides
  	 * a better estimate
  	 */
  	if (zone_reclaim_mode & RECLAIM_SWAP)
  		nr_pagecache_reclaimable = zone_page_state(zone, NR_FILE_PAGES);
  	else
  		nr_pagecache_reclaimable = zone_unmapped_file_pages(zone);
  
  	/* If we can't clean pages, remove dirty pages from consideration */
  	if (!(zone_reclaim_mode & RECLAIM_WRITE))
  		delta += zone_page_state(zone, NR_FILE_DIRTY);
  
  	/* Watch for any possible underflows due to delta */
  	if (unlikely(delta > nr_pagecache_reclaimable))
  		delta = nr_pagecache_reclaimable;
  
  	return nr_pagecache_reclaimable - delta;
  }
  
  /*
   * Try to free up some pages from this zone through reclaim.
   */
  static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
  {
  	/* Minimum pages needed in order to stay on node */
  	const unsigned long nr_pages = 1 << order;
  	struct task_struct *p = current;
  	struct reclaim_state reclaim_state;
  	struct scan_control sc = {
  		.may_writepage = !!(zone_reclaim_mode & RECLAIM_WRITE),
  		.may_unmap = !!(zone_reclaim_mode & RECLAIM_SWAP),
  		.may_swap = 1,
  		.nr_to_reclaim = max_t(unsigned long, nr_pages,
  				       SWAP_CLUSTER_MAX),
  		.gfp_mask = gfp_mask,
  		.order = order,
  		.priority = ZONE_RECLAIM_PRIORITY,
  	};
  	struct shrink_control shrink = {
  		.gfp_mask = sc.gfp_mask,
  	};
  	unsigned long nr_slab_pages0, nr_slab_pages1;
  
  	cond_resched();
  	/*
  	 * We need to be able to allocate from the reserves for RECLAIM_SWAP
  	 * and we also need to be able to write out pages for RECLAIM_WRITE
  	 * and RECLAIM_SWAP.
  	 */
  	p->flags |= PF_MEMALLOC | PF_SWAPWRITE;
  	lockdep_set_current_reclaim_state(gfp_mask);
  	reclaim_state.reclaimed_slab = 0;
  	p->reclaim_state = &reclaim_state;
  
  	if (zone_pagecache_reclaimable(zone) > zone->min_unmapped_pages) {
  		/*
  		 * Free memory by calling shrink zone with increasing
  		 * priorities until we have enough memory freed.
  		 */
  		do {
  			shrink_zone(zone, &sc);
  		} while (sc.nr_reclaimed < nr_pages && --sc.priority >= 0);
  	}
  
  	nr_slab_pages0 = zone_page_state(zone, NR_SLAB_RECLAIMABLE);
  	if (nr_slab_pages0 > zone->min_slab_pages) {
  		/*
  		 * shrink_slab() does not currently allow us to determine how
  		 * many pages were freed in this zone. So we take the current
  		 * number of slab pages and shake the slab until it is reduced
  		 * by the same nr_pages that we used for reclaiming unmapped
  		 * pages.
  		 *
  		 * Note that shrink_slab will free memory on all zones and may
  		 * take a long time.
  		 */
  		for (;;) {
  			unsigned long lru_pages = zone_reclaimable_pages(zone);
  
  			/* No reclaimable slab or very low memory pressure */
  			if (!shrink_slab(&shrink, sc.nr_scanned, lru_pages))
  				break;
  
  			/* Freed enough memory */
  			nr_slab_pages1 = zone_page_state(zone,
  							NR_SLAB_RECLAIMABLE);
  			if (nr_slab_pages1 + nr_pages <= nr_slab_pages0)
  				break;
  		}
  
  		/*
  		 * Update nr_reclaimed by the number of slab pages we
  		 * reclaimed from this zone.
  		 */
  		nr_slab_pages1 = zone_page_state(zone, NR_SLAB_RECLAIMABLE);
  		if (nr_slab_pages1 < nr_slab_pages0)
  			sc.nr_reclaimed += nr_slab_pages0 - nr_slab_pages1;
  	}
  
  	p->reclaim_state = NULL;
  	current->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE);
  	lockdep_clear_current_reclaim_state();
  	return sc.nr_reclaimed >= nr_pages;
  }
  
  int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
  {
  	int node_id;
  	int ret;
  
  	/*
  	 * Zone reclaim reclaims unmapped file backed pages and
  	 * slab pages if we are over the defined limits.
  	 *
  	 * A small portion of unmapped file backed pages is needed for
  	 * file I/O otherwise pages read by file I/O will be immediately
  	 * thrown out if the zone is overallocated. So we do not reclaim
  	 * if less than a specified percentage of the zone is used by
  	 * unmapped file backed pages.
  	 */
  	if (zone_pagecache_reclaimable(zone) <= zone->min_unmapped_pages &&
  	    zone_page_state(zone, NR_SLAB_RECLAIMABLE) <= zone->min_slab_pages)
  		return ZONE_RECLAIM_FULL;
  
  	if (zone->all_unreclaimable)
  		return ZONE_RECLAIM_FULL;
  
  	/*
  	 * Do not scan if the allocation should not be delayed.
  	 */
  	if (!(gfp_mask & __GFP_WAIT) || (current->flags & PF_MEMALLOC))
  		return ZONE_RECLAIM_NOSCAN;
  
  	/*
  	 * Only run zone reclaim on the local zone or on zones that do not
  	 * have associated processors. This will favor the local processor
  	 * over remote processors and spread off node memory allocations
  	 * as wide as possible.
  	 */
  	node_id = zone_to_nid(zone);
  	if (node_state(node_id, N_CPU) && node_id != numa_node_id())
  		return ZONE_RECLAIM_NOSCAN;
  
  	if (zone_test_and_set_flag(zone, ZONE_RECLAIM_LOCKED))
  		return ZONE_RECLAIM_NOSCAN;
  
  	ret = __zone_reclaim(zone, gfp_mask, order);
  	zone_clear_flag(zone, ZONE_RECLAIM_LOCKED);
  
  	if (!ret)
  		count_vm_event(PGSCAN_ZONE_RECLAIM_FAILED);
  
  	return ret;
  }
  #endif
  
  /*
   * page_evictable - test whether a page is evictable
   * @page: the page to test
   *
   * Test whether page is evictable--i.e., should be placed on active/inactive
   * lists vs unevictable list.
   *
   * Reasons page might not be evictable:
   * (1) page's mapping marked unevictable
   * (2) page is part of an mlocked VMA
   *
   */
  int page_evictable(struct page *page)
  {
  	return !mapping_unevictable(page_mapping(page)) && !PageMlocked(page);
  }
  
  #ifdef CONFIG_SHMEM
  /**
   * check_move_unevictable_pages - check pages for evictability and move to appropriate zone lru list
   * @pages:	array of pages to check
   * @nr_pages:	number of pages to check
   *
   * Checks pages for evictability and moves them to the appropriate lru list.
   *
   * This function is only used for SysV IPC SHM_UNLOCK.
   */
  void check_move_unevictable_pages(struct page **pages, int nr_pages)
  {
  	struct lruvec *lruvec;
  	struct zone *zone = NULL;
  	int pgscanned = 0;
  	int pgrescued = 0;
  	int i;
  
  	for (i = 0; i < nr_pages; i++) {
  		struct page *page = pages[i];
  		struct zone *pagezone;
  
  		pgscanned++;
  		pagezone = page_zone(page);
  		if (pagezone != zone) {
  			if (zone)
  				spin_unlock_irq(&zone->lru_lock);
  			zone = pagezone;
  			spin_lock_irq(&zone->lru_lock);
  		}
  		lruvec = mem_cgroup_page_lruvec(page, zone);
  
  		if (!PageLRU(page) || !PageUnevictable(page))
  			continue;
  
  		if (page_evictable(page)) {
  			enum lru_list lru = page_lru_base_type(page);
  
  			VM_BUG_ON(PageActive(page));
  			ClearPageUnevictable(page);
  			del_page_from_lru_list(page, lruvec, LRU_UNEVICTABLE);
  			add_page_to_lru_list(page, lruvec, lru);
  			pgrescued++;
  		}
  	}
  
  	if (zone) {
  		__count_vm_events(UNEVICTABLE_PGRESCUED, pgrescued);
  		__count_vm_events(UNEVICTABLE_PGSCANNED, pgscanned);
  		spin_unlock_irq(&zone->lru_lock);
  	}
  }
  #endif /* CONFIG_SHMEM */
  
  static void warn_scan_unevictable_pages(void)
  {
  	printk_once(KERN_WARNING
  		    "%s: The scan_unevictable_pages sysctl/node-interface has been "
  		    "disabled for lack of a legitimate use case.  If you have "
  		    "one, please send an email to linux-mm@kvack.org.\n",
  		    current->comm);
  }
  
  /*
   * scan_unevictable_pages [vm] sysctl handler.  On demand re-scan of
   * all nodes' unevictable lists for evictable pages
   */
  unsigned long scan_unevictable_pages;
  
  int scan_unevictable_handler(struct ctl_table *table, int write,
  			   void __user *buffer,
  			   size_t *length, loff_t *ppos)
  {
  	warn_scan_unevictable_pages();
  	proc_doulongvec_minmax(table, write, buffer, length, ppos);
  	scan_unevictable_pages = 0;
  	return 0;
  }
  
  #ifdef CONFIG_NUMA
  /*
   * per node 'scan_unevictable_pages' attribute.  On demand re-scan of
   * a specified node's per zone unevictable lists for evictable pages.
   */
  
  static ssize_t read_scan_unevictable_node(struct device *dev,
  					  struct device_attribute *attr,
  					  char *buf)
  {
  	warn_scan_unevictable_pages();
  	return sprintf(buf, "0\n");	/* always zero; should fit... */
  }
  
  static ssize_t write_scan_unevictable_node(struct device *dev,
  					   struct device_attribute *attr,
  					const char *buf, size_t count)
  {
  	warn_scan_unevictable_pages();
  	return 1;
  }
  
  
  static DEVICE_ATTR(scan_unevictable_pages, S_IRUGO | S_IWUSR,
  			read_scan_unevictable_node,
  			write_scan_unevictable_node);
  
  int scan_unevictable_register_node(struct node *node)
  {
  	return device_create_file(&node->dev, &dev_attr_scan_unevictable_pages);
  }
  
  void scan_unevictable_unregister_node(struct node *node)
  {
  	device_remove_file(&node->dev, &dev_attr_scan_unevictable_pages);
  }
  #endif
  