FreeBSD/Linux Kernel Cross Reference
sys/mm/vmscan.c

     /*
      *  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;
  }
  
  /*
   * A direct reclaimer may isolate SWAP_CLUSTER_MAX pages from the LRU list and
   * then get resheduled. When there are massive number of tasks doing page
   * allocation, such sleeping direct reclaimers may keep piling up on each CPU,
   * the LRU list will go small and be scanned faster than necessary, leading to
   * unnecessary swapping, thrashing and OOM.
   */
  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);
          }
  
          /*
           * GFP_NOIO/GFP_NOFS callers are allowed to isolate more pages, so they
           * won't get blocked by normal direct-reclaimers, forming a circular
           * deadlock.
           */
          if ((sc->gfp_mask & GFP_IOFS) == GFP_IOFS)
                  inactive >>= 3;
  
          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 (unlikely(file + free <= high_wmark_pages(zone))) {
                          /*
                           * If we have very few page cache pages, force-scan
                           * anon pages.
                           */
                          fraction[0] = 1;
                          fraction[1] = 0;
                          denominator = 1;
                          goto out;
                  } else if (!inactive_file_is_low_global(zone)) {
                          /*
                           * There is enough inactive page cache, do not
                           * reclaim anything from the working set right now.
                           */
                          fraction[0] = 0;
                          fraction[1] = 1;
                          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 (IS_ENABLED(CONFIG_COMPACTION) && 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 (IS_ENABLED(CONFIG_COMPACTION)) {
                                  /*
                                   * 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 (IS_ENABLED(CONFIG_COMPACTION) && order &&
              !compaction_suitable(zone, order))
                  return false;
  
          return true;
  }
  
  /*
   * pgdat_balanced() is used when checking if a node is balanced.
   *
   * For order-0, all zones must be 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, int order, int classzone_idx)
  {
          unsigned long present_pages = 0;
          unsigned long balanced_pages = 0;
          int i;
  
          /* Check the watermark levels */
          for (i = 0; i <= classzone_idx; i++) {
                  struct zone *zone = pgdat->node_zones + i;
  
                  if (!populated_zone(zone))
                          continue;
  
                  present_pages += zone->present_pages;
  
                  /*
                   * A special case here:
                   *
                   * balance_pgdat() skips over all_unreclaimable after
                   * DEF_PRIORITY. Effectively, it considers them balanced so
                   * they must be considered balanced here as well!
                   */
                  if (zone->all_unreclaimable) {
                          balanced_pages += zone->present_pages;
                          continue;
                  }
  
                  if (zone_balanced(zone, order, 0, i))
                          balanced_pages += zone->present_pages;
                  else if (!order)
                          return false;
          }
  
          if (order)
                  return balanced_pages >= (present_pages >> 2);
          else
                  return true;
  }
  
  /*
   * 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)
  {
          /* 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;
          }
  
          return pgdat_balanced(pgdat, order, classzone_idx);
  }
  
  /*
   * 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)
  {
          struct zone *unbalanced_zone;
          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;
  
                  unbalanced_zone = NULL;
  
                  /*
                   * 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 (IS_ENABLED(CONFIG_COMPACTION) && 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)) {
                                  unbalanced_zone = zone;
                                  /*
                                   * 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 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 (pgdat_balanced(pgdat, order, *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 if (unbalanced_zone)
                                  wait_iff_congested(unbalanced_zone, 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:
  
          if (!pgdat_balanced(pgdat, order, *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 ( ; ; ) {
                  bool 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 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_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_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, "\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