slab: clarify and fix calculate_slab_order()

[linux-2.6/mini2440.git] / 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/slab.h>
#include <linux/kernel_stat.h>
#include <linux/swap.h>
#include <linux/pagemap.h>
#include <linux/init.h>
#include <linux/highmem.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/pagevec.h>
#include <linux/backing-dev.h>
#include <linux/rmap.h>
#include <linux/topology.h>
#include <linux/cpu.h>
#include <linux/cpuset.h>
#include <linux/notifier.h>
#include <linux/rwsem.h>

#include <asm/tlbflush.h>
#include <asm/div64.h>

#include <linux/swapops.h>

/* 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;

struct scan_control {
        /* Ask refill_inactive_zone, or shrink_cache to scan this many pages */
        unsigned long nr_to_scan;

        /* Incremented by the number of inactive pages that were scanned */
        unsigned long nr_scanned;

        /* Incremented by the number of pages reclaimed */
        unsigned long nr_reclaimed;

        unsigned long nr_mapped;        /* From page_state */

        /* Ask shrink_caches, or shrink_zone to scan at this priority */
        unsigned int priority;

        /* This context's GFP mask */
        gfp_t gfp_mask;

        int may_writepage;

        /* Can pages be swapped as part of reclaim? */
        int may_swap;

        /* This context's SWAP_CLUSTER_MAX. If freeing memory for
         * suspend, we effectively ignore SWAP_CLUSTER_MAX.
         * In this context, it doesn't matter that we scan the
         * whole list at once. */
        int swap_cluster_max;
};

/*
 * The list of shrinker callbacks used by to apply pressure to
 * ageable caches.
 */
struct shrinker {
        shrinker_t              shrinker;
        struct list_head        list;
        int                     seeks;  /* seeks to recreate an obj */
        long                    nr;     /* objs pending delete */
};

#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;
static long total_memory;

static LIST_HEAD(shrinker_list);
static DECLARE_RWSEM(shrinker_rwsem);

/*
 * Add a shrinker callback to be called from the vm
 */
struct shrinker *set_shrinker(int seeks, shrinker_t theshrinker)
{
        struct shrinker *shrinker;

        shrinker = kmalloc(sizeof(*shrinker), GFP_KERNEL);
        if (shrinker) {
                shrinker->shrinker = theshrinker;
                shrinker->seeks = seeks;
                shrinker->nr = 0;
                down_write(&shrinker_rwsem);
                list_add_tail(&shrinker->list, &shrinker_list);
                up_write(&shrinker_rwsem);
        }
        return shrinker;
}
EXPORT_SYMBOL(set_shrinker);

/*
 * Remove one
 */
void remove_shrinker(struct shrinker *shrinker)
{
        down_write(&shrinker_rwsem);
        list_del(&shrinker->list);
        up_write(&shrinker_rwsem);
        kfree(shrinker);
}
EXPORT_SYMBOL(remove_shrinker);

#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 encounted 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.
 */
int shrink_slab(unsigned long scanned, gfp_t gfp_mask, unsigned long lru_pages)
{
        struct shrinker *shrinker;
        int ret = 0;

        if (scanned == 0)
                scanned = SWAP_CLUSTER_MAX;

        if (!down_read_trylock(&shrinker_rwsem))
                return 1;       /* Assume we'll be able to shrink next time */

        list_for_each_entry(shrinker, &shrinker_list, list) {
                unsigned long long delta;
                unsigned long total_scan;
                unsigned long max_pass = (*shrinker->shrinker)(0, gfp_mask);

                delta = (4 * scanned) / shrinker->seeks;
                delta *= max_pass;
                do_div(delta, lru_pages + 1);
                shrinker->nr += delta;
                if (shrinker->nr < 0) {
                        printk(KERN_ERR "%s: nr=%ld\n",
                                        __FUNCTION__, shrinker->nr);
                        shrinker->nr = max_pass;
                }

                /*
                 * Avoid risking looping forever due to too large nr value:
                 * never try to free more than twice the estimate number of
                 * freeable entries.
                 */
                if (shrinker->nr > max_pass * 2)
                        shrinker->nr = max_pass * 2;

                total_scan = shrinker->nr;
                shrinker->nr = 0;

                while (total_scan >= SHRINK_BATCH) {
                        long this_scan = SHRINK_BATCH;
                        int shrink_ret;
                        int nr_before;

                        nr_before = (*shrinker->shrinker)(0, gfp_mask);
                        shrink_ret = (*shrinker->shrinker)(this_scan, gfp_mask);
                        if (shrink_ret == -1)
                                break;
                        if (shrink_ret < nr_before)
                                ret += nr_before - shrink_ret;
                        mod_page_state(slabs_scanned, this_scan);
                        total_scan -= this_scan;

                        cond_resched();
                }

                shrinker->nr += total_scan;
        }
        up_read(&shrinker_rwsem);
        return ret;
}

/* Called without lock on whether page is mapped, so answer is unstable */
static inline int page_mapping_inuse(struct page *page)
{
        struct address_space *mapping;

        /* Page is in somebody's page tables. */
        if (page_mapped(page))
                return 1;

        /* Be more reluctant to reclaim swapcache than pagecache */
        if (PageSwapCache(page))
                return 1;

        mapping = page_mapping(page);
        if (!mapping)
                return 0;

        /* File is mmap'd by somebody? */
        return mapping_mapped(mapping);
}

static inline int is_page_cache_freeable(struct page *page)
{
        return page_count(page) - !!PagePrivate(page) == 2;
}

static int may_write_to_queue(struct backing_dev_info *bdi)
{
        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) {
                if (error == -ENOSPC)
                        set_bit(AS_ENOSPC, &mapping->flags);
                else
                        set_bit(AS_EIO, &mapping->flags);
        }
        unlock_page(page);
}

/*
 * pageout is called by shrink_list() for each dirty page. Calls ->writepage().
 */
static pageout_t pageout(struct page *page, struct address_space *mapping)
{
        /*
         * 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_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.
         * See swapfile.c:page_queue_congested().
         */
        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 (PagePrivate(page)) {
                        if (try_to_free_buffers(page)) {
                                ClearPageDirty(page);
                                printk("%s: orphaned page\n", __FUNCTION__);
                                return PAGE_CLEAN;
                        }
                }
                return PAGE_KEEP;
        }
        if (mapping->a_ops->writepage == NULL)
                return PAGE_ACTIVATE;
        if (!may_write_to_queue(mapping->backing_dev_info))
                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,
                        .nonblocking = 1,
                        .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);
                }

                return PAGE_SUCCESS;
        }

        return PAGE_CLEAN;
}

static int remove_mapping(struct address_space *mapping, struct page *page)
{
        if (!mapping)
                return 0;               /* truncate got there first */

        write_lock_irq(&mapping->tree_lock);

        /*
         * The non-racy check for busy page.  It is critical to check
         * PageDirty _after_ making sure that the page is freeable and
         * not in use by anybody.       (pagecache + us == 2)
         */
        if (unlikely(page_count(page) != 2))
                goto cannot_free;
        smp_rmb();
        if (unlikely(PageDirty(page)))
                goto cannot_free;

        if (PageSwapCache(page)) {
                swp_entry_t swap = { .val = page_private(page) };
                __delete_from_swap_cache(page);
                write_unlock_irq(&mapping->tree_lock);
                swap_free(swap);
                __put_page(page);       /* The pagecache ref */
                return 1;
        }

        __remove_from_page_cache(page);
        write_unlock_irq(&mapping->tree_lock);
        __put_page(page);
        return 1;

cannot_free:
        write_unlock_irq(&mapping->tree_lock);
        return 0;
}

/*
 * shrink_list adds the number of reclaimed pages to sc->nr_reclaimed
 */
static int shrink_list(struct list_head *page_list, struct scan_control *sc)
{
        LIST_HEAD(ret_pages);
        struct pagevec freed_pvec;
        int pgactivate = 0;
        int reclaimed = 0;

        cond_resched();

        pagevec_init(&freed_pvec, 1);
        while (!list_empty(page_list)) {
                struct address_space *mapping;
                struct page *page;
                int may_enter_fs;
                int referenced;

                cond_resched();

                page = lru_to_page(page_list);
                list_del(&page->lru);

                if (TestSetPageLocked(page))
                        goto keep;

                BUG_ON(PageActive(page));

                sc->nr_scanned++;

                if (!sc->may_swap && page_mapped(page))
                        goto keep_locked;

                /* Double the slab pressure for mapped and swapcache pages */
                if (page_mapped(page) || PageSwapCache(page))
                        sc->nr_scanned++;

                if (PageWriteback(page))
                        goto keep_locked;

                referenced = page_referenced(page, 1);
                /* In active use or really unfreeable?  Activate it. */
                if (referenced && page_mapping_inuse(page))
                        goto activate_locked;

#ifdef CONFIG_SWAP
                /*
                 * Anonymous process memory has backing store?
                 * Try to allocate it some swap space here.
                 */
                if (PageAnon(page) && !PageSwapCache(page)) {
                        if (!sc->may_swap)
                                goto keep_locked;
                        if (!add_to_swap(page, GFP_ATOMIC))
                                goto activate_locked;
                }
#endif /* CONFIG_SWAP */

                mapping = page_mapping(page);
                may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
                        (PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));

                /*
                 * The page is mapped into the page tables of one or more
                 * processes. Try to unmap it here.
                 */
                if (page_mapped(page) && mapping) {
                        /*
                         * No unmapping if we do not swap
                         */
                        if (!sc->may_swap)
                                goto keep_locked;

                        switch (try_to_unmap(page, 0)) {
                        case SWAP_FAIL:
                                goto activate_locked;
                        case SWAP_AGAIN:
                                goto keep_locked;
                        case SWAP_SUCCESS:
                                ; /* try to free the page below */
                        }
                }

                if (PageDirty(page)) {
                        if (referenced)
                                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)) {
                        case PAGE_KEEP:
                                goto keep_locked;
                        case PAGE_ACTIVATE:
                                goto activate_locked;
                        case PAGE_SUCCESS:
                                if (PageWriteback(page) || PageDirty(page))
                                        goto keep;
                                /*
                                 * A synchronous write - probably a ramdisk.  Go
                                 * ahead and try to reclaim the page.
                                 */
                                if (TestSetPageLocked(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 (PagePrivate(page)) {
                        if (!try_to_release_page(page, sc->gfp_mask))
                                goto activate_locked;
                        if (!mapping && page_count(page) == 1)
                                goto free_it;
                }

                if (!remove_mapping(mapping, page))
                        goto keep_locked;

free_it:
                unlock_page(page);
                reclaimed++;
                if (!pagevec_add(&freed_pvec, page))
                        __pagevec_release_nonlru(&freed_pvec);
                continue;

activate_locked:
                SetPageActive(page);
                pgactivate++;
keep_locked:
                unlock_page(page);
keep:
                list_add(&page->lru, &ret_pages);
                BUG_ON(PageLRU(page));
        }
        list_splice(&ret_pages, page_list);
        if (pagevec_count(&freed_pvec))
                __pagevec_release_nonlru(&freed_pvec);
        mod_page_state(pgactivate, pgactivate);
        sc->nr_reclaimed += reclaimed;
        return reclaimed;
}

#ifdef CONFIG_MIGRATION
static inline void move_to_lru(struct page *page)
{
        list_del(&page->lru);
        if (PageActive(page)) {
                /*
                 * lru_cache_add_active checks that
                 * the PG_active bit is off.
                 */
                ClearPageActive(page);
                lru_cache_add_active(page);
        } else {
                lru_cache_add(page);
        }
        put_page(page);
}

/*
 * Add isolated pages on the list back to the LRU.
 *
 * returns the number of pages put back.
 */
int putback_lru_pages(struct list_head *l)
{
        struct page *page;
        struct page *page2;
        int count = 0;

        list_for_each_entry_safe(page, page2, l, lru) {
                move_to_lru(page);
                count++;
        }
        return count;
}

/*
 * Non migratable page
 */
int fail_migrate_page(struct page *newpage, struct page *page)
{
        return -EIO;
}
EXPORT_SYMBOL(fail_migrate_page);

/*
 * swapout a single page
 * page is locked upon entry, unlocked on exit
 */
static int swap_page(struct page *page)
{
        struct address_space *mapping = page_mapping(page);

        if (page_mapped(page) && mapping)
                if (try_to_unmap(page, 1) != SWAP_SUCCESS)
                        goto unlock_retry;

        if (PageDirty(page)) {
                /* Page is dirty, try to write it out here */
                switch(pageout(page, mapping)) {
                case PAGE_KEEP:
                case PAGE_ACTIVATE:
                        goto unlock_retry;

                case PAGE_SUCCESS:
                        goto retry;

                case PAGE_CLEAN:
                        ; /* try to free the page below */
                }
        }

        if (PagePrivate(page)) {
                if (!try_to_release_page(page, GFP_KERNEL) ||
                    (!mapping && page_count(page) == 1))
                        goto unlock_retry;
        }

        if (remove_mapping(mapping, page)) {
                /* Success */
                unlock_page(page);
                return 0;
        }

unlock_retry:
        unlock_page(page);

retry:
        return -EAGAIN;
}
EXPORT_SYMBOL(swap_page);

/*
 * Page migration was first developed in the context of the memory hotplug
 * project. The main authors of the migration code are:
 *
 * IWAMOTO Toshihiro <iwamoto@valinux.co.jp>
 * Hirokazu Takahashi <taka@valinux.co.jp>
 * Dave Hansen <haveblue@us.ibm.com>
 * Christoph Lameter <clameter@sgi.com>
 */

/*
 * Remove references for a page and establish the new page with the correct
 * basic settings to be able to stop accesses to the page.
 */
int migrate_page_remove_references(struct page *newpage,
                                struct page *page, int nr_refs)
{
        struct address_space *mapping = page_mapping(page);
        struct page **radix_pointer;

        /*
         * Avoid doing any of the following work if the page count
         * indicates that the page is in use or truncate has removed
         * the page.
         */
        if (!mapping || page_mapcount(page) + nr_refs != page_count(page))
                return 1;

        /*
         * Establish swap ptes for anonymous pages or destroy pte
         * maps for files.
         *
         * In order to reestablish file backed mappings the fault handlers
         * will take the radix tree_lock which may then be used to stop
         * processses from accessing this page until the new page is ready.
         *
         * A process accessing via a swap pte (an anonymous page) will take a
         * page_lock on the old page which will block the process until the
         * migration attempt is complete. At that time the PageSwapCache bit
         * will be examined. If the page was migrated then the PageSwapCache
         * bit will be clear and the operation to retrieve the page will be
         * retried which will find the new page in the radix tree. Then a new
         * direct mapping may be generated based on the radix tree contents.
         *
         * If the page was not migrated then the PageSwapCache bit
         * is still set and the operation may continue.
         */
        try_to_unmap(page, 1);

        /*
         * Give up if we were unable to remove all mappings.
         */
        if (page_mapcount(page))
                return 1;

        write_lock_irq(&mapping->tree_lock);

        radix_pointer = (struct page **)radix_tree_lookup_slot(
                                                &mapping->page_tree,
                                                page_index(page));

        if (!page_mapping(page) || page_count(page) != nr_refs ||
                        *radix_pointer != page) {
                write_unlock_irq(&mapping->tree_lock);
                return 1;
        }

        /*
         * Now we know that no one else is looking at the page.
         *
         * Certain minimal information about a page must be available
         * in order for other subsystems to properly handle the page if they
         * find it through the radix tree update before we are finished
         * copying the page.
         */
        get_page(newpage);
        newpage->index = page->index;
        newpage->mapping = page->mapping;
        if (PageSwapCache(page)) {
                SetPageSwapCache(newpage);
                set_page_private(newpage, page_private(page));
        }

        *radix_pointer = newpage;
        __put_page(page);
        write_unlock_irq(&mapping->tree_lock);

        return 0;
}
EXPORT_SYMBOL(migrate_page_remove_references);

/*
 * Copy the page to its new location
 */
void migrate_page_copy(struct page *newpage, struct page *page)
{
        copy_highpage(newpage, page);

        if (PageError(page))
                SetPageError(newpage);
        if (PageReferenced(page))
                SetPageReferenced(newpage);
        if (PageUptodate(page))
                SetPageUptodate(newpage);
        if (PageActive(page))
                SetPageActive(newpage);
        if (PageChecked(page))
                SetPageChecked(newpage);
        if (PageMappedToDisk(page))
                SetPageMappedToDisk(newpage);

        if (PageDirty(page)) {
                clear_page_dirty_for_io(page);
                set_page_dirty(newpage);
        }

        ClearPageSwapCache(page);
        ClearPageActive(page);
        ClearPagePrivate(page);
        set_page_private(page, 0);
        page->mapping = NULL;

        /*
         * If any waiters have accumulated on the new page then
         * wake them up.
         */
        if (PageWriteback(newpage))
                end_page_writeback(newpage);
}
EXPORT_SYMBOL(migrate_page_copy);

/*
 * Common logic to directly migrate a single page suitable for
 * pages that do not use PagePrivate.
 *
 * Pages are locked upon entry and exit.
 */
int migrate_page(struct page *newpage, struct page *page)
{
        BUG_ON(PageWriteback(page));    /* Writeback must be complete */

        if (migrate_page_remove_references(newpage, page, 2))
                return -EAGAIN;

        migrate_page_copy(newpage, page);

        /*
         * Remove auxiliary swap entries and replace
         * them with real ptes.
         *
         * Note that a real pte entry will allow processes that are not
         * waiting on the page lock to use the new page via the page tables
         * before the new page is unlocked.
         */
        remove_from_swap(newpage);
        return 0;
}
EXPORT_SYMBOL(migrate_page);

/*
 * migrate_pages
 *
 * Two lists are passed to this function. The first list
 * contains the pages isolated from the LRU to be migrated.
 * The second list contains new pages that the pages isolated
 * can be moved to. If the second list is NULL then all
 * pages are swapped out.
 *
 * The function returns after 10 attempts or if no pages
 * are movable anymore because to has become empty
 * or no retryable pages exist anymore.
 *
 * Return: Number of pages not migrated when "to" ran empty.
 */
int migrate_pages(struct list_head *from, struct list_head *to,
                  struct list_head *moved, struct list_head *failed)
{
        int retry;
        int nr_failed = 0;
        int pass = 0;
        struct page *page;
        struct page *page2;
        int swapwrite = current->flags & PF_SWAPWRITE;
        int rc;

        if (!swapwrite)
                current->flags |= PF_SWAPWRITE;

redo:
        retry = 0;

        list_for_each_entry_safe(page, page2, from, lru) {
                struct page *newpage = NULL;
                struct address_space *mapping;

                cond_resched();

                rc = 0;
                if (page_count(page) == 1)
                        /* page was freed from under us. So we are done. */
                        goto next;

                if (to && list_empty(to))
                        break;

                /*
                 * Skip locked pages during the first two passes to give the
                 * functions holding the lock time to release the page. Later we
                 * use lock_page() to have a higher chance of acquiring the
                 * lock.
                 */
                rc = -EAGAIN;
                if (pass > 2)
                        lock_page(page);
                else
                        if (TestSetPageLocked(page))
                                goto next;

                /*
                 * Only wait on writeback if we have already done a pass where
                 * we we may have triggered writeouts for lots of pages.
                 */
                if (pass > 0) {
                        wait_on_page_writeback(page);
                } else {
                        if (PageWriteback(page))
                                goto unlock_page;
                }

                /*
                 * Anonymous pages must have swap cache references otherwise
                 * the information contained in the page maps cannot be
                 * preserved.
                 */
                if (PageAnon(page) && !PageSwapCache(page)) {
                        if (!add_to_swap(page, GFP_KERNEL)) {
                                rc = -ENOMEM;
                                goto unlock_page;
                        }
                }

                if (!to) {
                        rc = swap_page(page);
                        goto next;
                }

                newpage = lru_to_page(to);
                lock_page(newpage);

                /*
                 * Pages are properly locked and writeback is complete.
                 * Try to migrate the page.
                 */
                mapping = page_mapping(page);
                if (!mapping)
                        goto unlock_both;

                if (mapping->a_ops->migratepage) {
                        /*
                         * Most pages have a mapping and most filesystems
                         * should provide a migration function. Anonymous
                         * pages are part of swap space which also has its
                         * own migration function. This is the most common
                         * path for page migration.
                         */
                        rc = mapping->a_ops->migratepage(newpage, page);
                        goto unlock_both;
                }

                /*
                 * Default handling if a filesystem does not provide
                 * a migration function. We can only migrate clean
                 * pages so try to write out any dirty pages first.
                 */
                if (PageDirty(page)) {
                        switch (pageout(page, mapping)) {
                        case PAGE_KEEP:
                        case PAGE_ACTIVATE:
                                goto unlock_both;

                        case PAGE_SUCCESS:
                                unlock_page(newpage);
                                goto next;

                        case PAGE_CLEAN:
                                ; /* try to migrate the page below */
                        }
                }

                /*
                 * Buffers are managed in a filesystem specific way.
                 * We must have no buffers or drop them.
                 */
                if (!page_has_buffers(page) ||
                    try_to_release_page(page, GFP_KERNEL)) {
                        rc = migrate_page(newpage, page);
                        goto unlock_both;
                }

                /*
                 * On early passes with mapped pages simply
                 * retry. There may be a lock held for some
                 * buffers that may go away. Later
                 * swap them out.
                 */
                if (pass > 4) {
                        /*
                         * Persistently unable to drop buffers..... As a
                         * measure of last resort we fall back to
                         * swap_page().
                         */
                        unlock_page(newpage);
                        newpage = NULL;
                        rc = swap_page(page);
                        goto next;
                }

unlock_both:
                unlock_page(newpage);

unlock_page:
                unlock_page(page);

next:
                if (rc == -EAGAIN) {
                        retry++;
                } else if (rc) {
                        /* Permanent failure */
                        list_move(&page->lru, failed);
                        nr_failed++;
                } else {
                        if (newpage) {
                                /* Successful migration. Return page to LRU */
                                move_to_lru(newpage);
                        }
                        list_move(&page->lru, moved);
                }
        }
        if (retry && pass++ < 10)
                goto redo;

        if (!swapwrite)
                current->flags &= ~PF_SWAPWRITE;

        return nr_failed + retry;
}

/*
 * Isolate one page from the LRU lists and put it on the
 * indicated list with elevated refcount.
 *
 * Result:
 *  0 = page not on LRU list
 *  1 = page removed from LRU list and added to the specified list.
 */
int isolate_lru_page(struct page *page)
{
        int ret = 0;

        if (PageLRU(page)) {
                struct zone *zone = page_zone(page);
                spin_lock_irq(&zone->lru_lock);
                if (TestClearPageLRU(page)) {
                        ret = 1;
                        get_page(page);
                        if (PageActive(page))
                                del_page_from_active_list(zone, page);
                        else
                                del_page_from_inactive_list(zone, page);
                }
                spin_unlock_irq(&zone->lru_lock);
        }

        return ret;
}
#endif

/*
 * 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.
 * @src:        The LRU list to pull pages off.
 * @dst:        The temp list to put pages on to.
 * @scanned:    The number of pages that were scanned.
 *
 * returns how many pages were moved onto *@dst.
 */
static int isolate_lru_pages(int nr_to_scan, struct list_head *src,
                             struct list_head *dst, int *scanned)
{
        int nr_taken = 0;
        struct page *page;
        int scan = 0;

        while (scan++ < nr_to_scan && !list_empty(src)) {
                page = lru_to_page(src);
                prefetchw_prev_lru_page(page, src, flags);

                if (!TestClearPageLRU(page))
                        BUG();
                list_del(&page->lru);
                if (get_page_testone(page)) {
                        /*
                         * It is being freed elsewhere
                         */
                        __put_page(page);
                        SetPageLRU(page);
                        list_add(&page->lru, src);
                        continue;
                } else {
                        list_add(&page->lru, dst);
                        nr_taken++;
                }
        }

        *scanned = scan;
        return nr_taken;
}

/*
 * shrink_cache() adds the number of pages reclaimed to sc->nr_reclaimed
 */
static void shrink_cache(struct zone *zone, struct scan_control *sc)
{
        LIST_HEAD(page_list);
        struct pagevec pvec;
        int max_scan = sc->nr_to_scan;

        pagevec_init(&pvec, 1);

        lru_add_drain();
        spin_lock_irq(&zone->lru_lock);
        while (max_scan > 0) {
                struct page *page;
                int nr_taken;
                int nr_scan;
                int nr_freed;

                nr_taken = isolate_lru_pages(sc->swap_cluster_max,
                                             &zone->inactive_list,
                                             &page_list, &nr_scan);
                zone->nr_inactive -= nr_taken;
                zone->pages_scanned += nr_scan;
                spin_unlock_irq(&zone->lru_lock);

                if (nr_taken == 0)
                        goto done;

                max_scan -= nr_scan;
                nr_freed = shrink_list(&page_list, sc);

                local_irq_disable();
                if (current_is_kswapd()) {
                        __mod_page_state_zone(zone, pgscan_kswapd, nr_scan);
                        __mod_page_state(kswapd_steal, nr_freed);
                } else
                        __mod_page_state_zone(zone, pgscan_direct, nr_scan);
                __mod_page_state_zone(zone, pgsteal, nr_freed);

                spin_lock(&zone->lru_lock);
                /*
                 * Put back any unfreeable pages.
                 */
                while (!list_empty(&page_list)) {
                        page = lru_to_page(&page_list);
                        if (TestSetPageLRU(page))
                                BUG();
                        list_del(&page->lru);
                        if (PageActive(page))
                                add_page_to_active_list(zone, page);
                        else
                                add_page_to_inactive_list(zone, page);
                        if (!pagevec_add(&pvec, page)) {
                                spin_unlock_irq(&zone->lru_lock);
                                __pagevec_release(&pvec);
                                spin_lock_irq(&zone->lru_lock);
                        }
                }
        }
        spin_unlock_irq(&zone->lru_lock);
done:
        pagevec_release(&pvec);
}

/*
 * 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
refill_inactive_zone(struct zone *zone, struct scan_control *sc)
{
        int pgmoved;
        int pgdeactivate = 0;
        int pgscanned;
        int nr_pages = sc->nr_to_scan;
        LIST_HEAD(l_hold);      /* The pages which were snipped off */
        LIST_HEAD(l_inactive);  /* Pages to go onto the inactive_list */
        LIST_HEAD(l_active);    /* Pages to go onto the active_list */
        struct page *page;
        struct pagevec pvec;
        int reclaim_mapped = 0;

        if (unlikely(sc->may_swap)) {
                long mapped_ratio;
                long distress;
                long swap_tendency;

                /*
                 * `distress' is a measure of how much trouble we're having
                 * reclaiming pages.  0 -> no problems.  100 -> great trouble.
                 */
                distress = 100 >> zone->prev_priority;

                /*
                 * The point of this algorithm is to decide when to start
                 * reclaiming mapped memory instead of just pagecache.  Work out
                 * how much memory
                 * is mapped.
                 */
                mapped_ratio = (sc->nr_mapped * 100) / total_memory;

                /*
                 * Now decide how much we really want to unmap some pages.  The
                 * mapped ratio is downgraded - just because there's a lot of
                 * mapped memory doesn't necessarily mean that page reclaim
                 * isn't succeeding.
                 *
                 * The distress ratio is important - we don't want to start
                 * going oom.
                 *
                 * A 100% value of vm_swappiness overrides this algorithm
                 * altogether.
                 */
                swap_tendency = mapped_ratio / 2 + distress + vm_swappiness;

                /*
                 * Now use this metric to decide whether to start moving mapped
                 * memory onto the inactive list.
                 */
                if (swap_tendency >= 100)
                        reclaim_mapped = 1;
        }

        lru_add_drain();
        spin_lock_irq(&zone->lru_lock);
        pgmoved = isolate_lru_pages(nr_pages, &zone->active_list,
                                    &l_hold, &pgscanned);
        zone->pages_scanned += pgscanned;
        zone->nr_active -= pgmoved;
        spin_unlock_irq(&zone->lru_lock);

        while (!list_empty(&l_hold)) {
                cond_resched();
                page = lru_to_page(&l_hold);
                list_del(&page->lru);
                if (page_mapped(page)) {
                        if (!reclaim_mapped ||
                            (total_swap_pages == 0 && PageAnon(page)) ||
                            page_referenced(page, 0)) {
                                list_add(&page->lru, &l_active);
                                continue;
                        }
                }
                list_add(&page->lru, &l_inactive);
        }

        pagevec_init(&pvec, 1);
        pgmoved = 0;
        spin_lock_irq(&zone->lru_lock);
        while (!list_empty(&l_inactive)) {
                page = lru_to_page(&l_inactive);
                prefetchw_prev_lru_page(page, &l_inactive, flags);
                if (TestSetPageLRU(page))
                        BUG();
                if (!TestClearPageActive(page))
                        BUG();
                list_move(&page->lru, &zone->inactive_list);
                pgmoved++;
                if (!pagevec_add(&pvec, page)) {
                        zone->nr_inactive += pgmoved;
                        spin_unlock_irq(&zone->lru_lock);
                        pgdeactivate += pgmoved;
                        pgmoved = 0;
                        if (buffer_heads_over_limit)
                                pagevec_strip(&pvec);
                        __pagevec_release(&pvec);
                        spin_lock_irq(&zone->lru_lock);
                }
        }
        zone->nr_inactive += pgmoved;
        pgdeactivate += pgmoved;
        if (buffer_heads_over_limit) {
                spin_unlock_irq(&zone->lru_lock);
                pagevec_strip(&pvec);
                spin_lock_irq(&zone->lru_lock);
        }

        pgmoved = 0;
        while (!list_empty(&l_active)) {
                page = lru_to_page(&l_active);
                prefetchw_prev_lru_page(page, &l_active, flags);
                if (TestSetPageLRU(page))
                        BUG();
                BUG_ON(!PageActive(page));
                list_move(&page->lru, &zone->active_list);
                pgmoved++;
                if (!pagevec_add(&pvec, page)) {
                        zone->nr_active += pgmoved;
                        pgmoved = 0;
                        spin_unlock_irq(&zone->lru_lock);
                        __pagevec_release(&pvec);
                        spin_lock_irq(&zone->lru_lock);
                }
        }
        zone->nr_active += pgmoved;
        spin_unlock(&zone->lru_lock);

        __mod_page_state_zone(zone, pgrefill, pgscanned);
        __mod_page_state(pgdeactivate, pgdeactivate);
        local_irq_enable();

        pagevec_release(&pvec);
}

/*
 * This is a basic per-zone page freer.  Used by both kswapd and direct reclaim.
 */
static void
shrink_zone(struct zone *zone, struct scan_control *sc)
{
        unsigned long nr_active;
        unsigned long nr_inactive;

        atomic_inc(&zone->reclaim_in_progress);

        /*
         * Add one to `nr_to_scan' just to make sure that the kernel will
         * slowly sift through the active list.
         */
        zone->nr_scan_active += (zone->nr_active >> sc->priority) + 1;
        nr_active = zone->nr_scan_active;
        if (nr_active >= sc->swap_cluster_max)
                zone->nr_scan_active = 0;
        else
                nr_active = 0;

        zone->nr_scan_inactive += (zone->nr_inactive >> sc->priority) + 1;
        nr_inactive = zone->nr_scan_inactive;
        if (nr_inactive >= sc->swap_cluster_max)
                zone->nr_scan_inactive = 0;
        else
                nr_inactive = 0;

        while (nr_active || nr_inactive) {
                if (nr_active) {
                        sc->nr_to_scan = min(nr_active,
                                        (unsigned long)sc->swap_cluster_max);
                        nr_active -= sc->nr_to_scan;
                        refill_inactive_zone(zone, sc);
                }

                if (nr_inactive) {
                        sc->nr_to_scan = min(nr_inactive,
                                        (unsigned long)sc->swap_cluster_max);
                        nr_inactive -= sc->nr_to_scan;
                        shrink_cache(zone, sc);
                }
        }

        throttle_vm_writeout();

        atomic_dec(&zone->reclaim_in_progress);
}

/*
 * 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 pages_high.  Because:
 * a) The caller may be trying to free *extra* pages to satisfy a higher-order
 *    allocation or
 * b) The zones may be over pages_high but they must go *over* pages_high to
 *    satisfy the `incremental min' zone defense algorithm.
 *
 * Returns the number of reclaimed pages.
 *
 * If a zone is deemed to be full of pinned pages then just give it a light
 * scan then give up on it.
 */
static void
shrink_caches(struct zone **zones, struct scan_control *sc)
{
        int i;

        for (i = 0; zones[i] != NULL; i++) {
                struct zone *zone = zones[i];

                if (!populated_zone(zone))
                        continue;

                if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
                        continue;

                zone->temp_priority = sc->priority;
                if (zone->prev_priority > sc->priority)
                        zone->prev_priority = sc->priority;

                if (zone->all_unreclaimable && sc->priority != DEF_PRIORITY)
                        continue;       /* Let kswapd poll it */

                shrink_zone(zone, sc);
        }
}
 
/*
 * 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 pdflush 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.
 */
int try_to_free_pages(struct zone **zones, gfp_t gfp_mask)
{
        int priority;
        int ret = 0;
        int total_scanned = 0, total_reclaimed = 0;
        struct reclaim_state *reclaim_state = current->reclaim_state;
        struct scan_control sc;
        unsigned long lru_pages = 0;
        int i;

        sc.gfp_mask = gfp_mask;
        sc.may_writepage = !laptop_mode;
        sc.may_swap = 1;

        inc_page_state(allocstall);

        for (i = 0; zones[i] != NULL; i++) {
                struct zone *zone = zones[i];

                if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
                        continue;

                zone->temp_priority = DEF_PRIORITY;
                lru_pages += zone->nr_active + zone->nr_inactive;
        }

        for (priority = DEF_PRIORITY; priority >= 0; priority--) {
                sc.nr_mapped = read_page_state(nr_mapped);
                sc.nr_scanned = 0;
                sc.nr_reclaimed = 0;
                sc.priority = priority;
                sc.swap_cluster_max = SWAP_CLUSTER_MAX;
                if (!priority)
                        disable_swap_token();
                shrink_caches(zones, &sc);
                shrink_slab(sc.nr_scanned, gfp_mask, lru_pages);
                if (reclaim_state) {
                        sc.nr_reclaimed += reclaim_state->reclaimed_slab;
                        reclaim_state->reclaimed_slab = 0;
                }
                total_scanned += sc.nr_scanned;
                total_reclaimed += sc.nr_reclaimed;
                if (total_reclaimed >= sc.swap_cluster_max) {
                        ret = 1;
                        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.
                 */
                if (total_scanned > sc.swap_cluster_max + sc.swap_cluster_max/2) {
                        wakeup_pdflush(laptop_mode ? 0 : total_scanned);
                        sc.may_writepage = 1;
                }

                /* Take a nap, wait for some writeback to complete */
                if (sc.nr_scanned && priority < DEF_PRIORITY - 2)
                        blk_congestion_wait(WRITE, HZ/10);
        }
out:
        for (i = 0; zones[i] != 0; i++) {
                struct zone *zone = zones[i];

                if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
                        continue;

                zone->prev_priority = zone->temp_priority;
        }
        return ret;
}

/*
 * For kswapd, balance_pgdat() will work across all this node's zones until
 * they are all at pages_high.
 *
 * If `nr_pages' is non-zero then it is the number of pages which are to be
 * reclaimed, regardless of the zone occupancies.  This is a software suspend
 * special.
 *
 * Returns the number of pages which were actually freed.
 *
 * 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 > pages_high, but once a zone is found to have
 * free_pages <= pages_high, 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 int balance_pgdat(pg_data_t *pgdat, int nr_pages, int order)
{
        int to_free = nr_pages;
        int all_zones_ok;
        int priority;
        int i;
        int total_scanned, total_reclaimed;
        struct reclaim_state *reclaim_state = current->reclaim_state;
        struct scan_control sc;

loop_again:
        total_scanned = 0;
        total_reclaimed = 0;
        sc.gfp_mask = GFP_KERNEL;
        sc.may_writepage = !laptop_mode;
        sc.may_swap = 1;
        sc.nr_mapped = read_page_state(nr_mapped);

        inc_page_state(pageoutrun);

        for (i = 0; i < pgdat->nr_zones; i++) {
                struct zone *zone = pgdat->node_zones + i;

                zone->temp_priority = DEF_PRIORITY;
        }

        for (priority = DEF_PRIORITY; priority >= 0; priority--) {
                int end_zone = 0;       /* Inclusive.  0 = ZONE_DMA */
                unsigned long lru_pages = 0;

                /* The swap token gets in the way of swapout... */
                if (!priority)
                        disable_swap_token();

                all_zones_ok = 1;

                if (nr_pages == 0) {
                        /*
                         * Scan in the highmem->dma direction for the highest
                         * zone which needs scanning
                         */
                        for (i = pgdat->nr_zones - 1; i >= 0; i--) {
                                struct zone *zone = pgdat->node_zones + i;

                                if (!populated_zone(zone))
                                        continue;

                                if (zone->all_unreclaimable &&
                                                priority != DEF_PRIORITY)
                                        continue;

                                if (!zone_watermark_ok(zone, order,
                                                zone->pages_high, 0, 0)) {
                                        end_zone = i;
                                        goto scan;
                                }
                        }
                        goto out;
                } else {
                        end_zone = pgdat->nr_zones - 1;
                }
scan:
                for (i = 0; i <= end_zone; i++) {
                        struct zone *zone = pgdat->node_zones + i;

                        lru_pages += zone->nr_active + zone->nr_inactive;
                }

                /*
                 * 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;

                        if (!populated_zone(zone))
                                continue;

                        if (zone->all_unreclaimable && priority != DEF_PRIORITY)
                                continue;

                        if (nr_pages == 0) {    /* Not software suspend */
                                if (!zone_watermark_ok(zone, order,
                                                zone->pages_high, end_zone, 0))
                                        all_zones_ok = 0;
                        }
                        zone->temp_priority = priority;
                        if (zone->prev_priority > priority)
                                zone->prev_priority = priority;
                        sc.nr_scanned = 0;
                        sc.nr_reclaimed = 0;
                        sc.priority = priority;
                        sc.swap_cluster_max = nr_pages? nr_pages : SWAP_CLUSTER_MAX;
                        shrink_zone(zone, &sc);
                        reclaim_state->reclaimed_slab = 0;
                        nr_slab = shrink_slab(sc.nr_scanned, GFP_KERNEL,
                                                lru_pages);
                        sc.nr_reclaimed += reclaim_state->reclaimed_slab;
                        total_reclaimed += sc.nr_reclaimed;
                        total_scanned += sc.nr_scanned;
                        if (zone->all_unreclaimable)
                                continue;
                        if (nr_slab == 0 && zone->pages_scanned >=
                                    (zone->nr_active + zone->nr_inactive) * 4)
                                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 > total_reclaimed+total_reclaimed/2)
                                sc.may_writepage = 1;
                }
                if (nr_pages && to_free > total_reclaimed)
                        continue;       /* swsusp: need to do more work */
                if (all_zones_ok)
                        break;          /* kswapd: all done */
                /*
                 * OK, kswapd is getting into trouble.  Take a nap, then take
                 * another pass across the zones.
                 */
                if (total_scanned && priority < DEF_PRIORITY - 2)
                        blk_congestion_wait(WRITE, 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 ((total_reclaimed >= SWAP_CLUSTER_MAX) && (!nr_pages))
                        break;
        }
out:
        for (i = 0; i < pgdat->nr_zones; i++) {
                struct zone *zone = pgdat->node_zones + i;

                zone->prev_priority = zone->temp_priority;
        }
        if (!all_zones_ok) {
                cond_resched();
                goto loop_again;
        }

        return total_reclaimed;
}

/*
 * 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;
        pg_data_t *pgdat = (pg_data_t*)p;
        struct task_struct *tsk = current;
        DEFINE_WAIT(wait);
        struct reclaim_state reclaim_state = {
                .reclaimed_slab = 0,
        };
        cpumask_t cpumask;

        daemonize("kswapd%d", pgdat->node_id);
        cpumask = node_to_cpumask(pgdat->node_id);
        if (!cpus_empty(cpumask))
                set_cpus_allowed(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;

        order = 0;
        for ( ; ; ) {
                unsigned long new_order;

                try_to_freeze();

                prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
                new_order = pgdat->kswapd_max_order;
                pgdat->kswapd_max_order = 0;
                if (order < new_order) {
                        /*
                         * Don't sleep if someone wants a larger 'order'
                         * allocation
                         */
                        order = new_order;
                } else {
                        schedule();
                        order = pgdat->kswapd_max_order;
                }
                finish_wait(&pgdat->kswapd_wait, &wait);

                balance_pgdat(pgdat, 0, order);
        }
        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)
{
        pg_data_t *pgdat;

        if (!populated_zone(zone))
                return;

        pgdat = zone->zone_pgdat;
        if (zone_watermark_ok(zone, order, zone->pages_low, 0, 0))
                return;
        if (pgdat->kswapd_max_order < order)
                pgdat->kswapd_max_order = order;
        if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
                return;
        if (!waitqueue_active(&pgdat->kswapd_wait))
                return;
        wake_up_interruptible(&pgdat->kswapd_wait);
}

#ifdef CONFIG_PM
/*
 * Try to free `nr_pages' of memory, system-wide.  Returns the number of freed
 * pages.
 */
int shrink_all_memory(int nr_pages)
{
        pg_data_t *pgdat;
        int nr_to_free = nr_pages;
        int ret = 0;
        struct reclaim_state reclaim_state = {
                .reclaimed_slab = 0,
        };

        current->reclaim_state = &reclaim_state;
        for_each_pgdat(pgdat) {
                int freed;
                freed = balance_pgdat(pgdat, nr_to_free, 0);
                ret += freed;
                nr_to_free -= freed;
                if (nr_to_free <= 0)
                        break;
        }
        current->reclaim_state = NULL;
        return ret;
}
#endif

#ifdef CONFIG_HOTPLUG_CPU
/* It's optimal to keep kswapds on the same CPUs as their memory, but
   not required for correctness.  So if the last cpu in a node goes
   away, we get changed to run anywhere: as the first one comes back,
   restore their cpu bindings. */
static int __devinit cpu_callback(struct notifier_block *nfb,
                                  unsigned long action,
                                  void *hcpu)
{
        pg_data_t *pgdat;
        cpumask_t mask;

        if (action == CPU_ONLINE) {
                for_each_pgdat(pgdat) {
                        mask = node_to_cpumask(pgdat->node_id);
                        if (any_online_cpu(mask) != NR_CPUS)
                                /* One of our CPUs online: restore mask */
                                set_cpus_allowed(pgdat->kswapd, mask);
                }
        }
        return NOTIFY_OK;
}
#endif /* CONFIG_HOTPLUG_CPU */

static int __init kswapd_init(void)
{
        pg_data_t *pgdat;
        swap_setup();
        for_each_pgdat(pgdat)
                pgdat->kswapd
                = find_task_by_pid(kernel_thread(kswapd, pgdat, CLONE_KERNEL));
        total_memory = nr_free_pagecache_pages();
        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.
 *
 * In the future we may add flags to the mode. However, the page allocator
 * should only have to check that zone_reclaim_mode != 0 before calling
 * zone_reclaim().
 */
int zone_reclaim_mode __read_mostly;

#define RECLAIM_OFF 0
#define RECLAIM_ZONE (1<<0)     /* Run shrink_cache on the zone */
#define RECLAIM_WRITE (1<<1)    /* Writeout pages during reclaim */
#define RECLAIM_SWAP (1<<2)     /* Swap pages out during reclaim */
#define RECLAIM_SLAB (1<<3)     /* Do a global slab shrink if the zone is out of memory */

/*
 * Mininum time between zone reclaim scans
 */
int zone_reclaim_interval __read_mostly = 30*HZ;

/*
 * 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

/*
 * Try to free up some pages from this zone through reclaim.
 */
int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
{
        int nr_pages;
        struct task_struct *p = current;
        struct reclaim_state reclaim_state;
        struct scan_control sc;
        cpumask_t mask;
        int node_id;

        if (time_before(jiffies,
                zone->last_unsuccessful_zone_reclaim + zone_reclaim_interval))
                        return 0;

        if (!(gfp_mask & __GFP_WAIT) ||
                zone->all_unreclaimable ||
                atomic_read(&zone->reclaim_in_progress) > 0)
                        return 0;

        node_id = zone->zone_pgdat->node_id;
        mask = node_to_cpumask(node_id);
        if (!cpus_empty(mask) && node_id != numa_node_id())
                return 0;

        sc.may_writepage = !!(zone_reclaim_mode & RECLAIM_WRITE);
        sc.may_swap = !!(zone_reclaim_mode & RECLAIM_SWAP);
        sc.nr_scanned = 0;
        sc.nr_reclaimed = 0;
        sc.priority = ZONE_RECLAIM_PRIORITY + 1;
        sc.nr_mapped = read_page_state(nr_mapped);
        sc.gfp_mask = gfp_mask;

        disable_swap_token();

        nr_pages = 1 << order;
        if (nr_pages > SWAP_CLUSTER_MAX)
                sc.swap_cluster_max = nr_pages;
        else
                sc.swap_cluster_max = SWAP_CLUSTER_MAX;

        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;
        reclaim_state.reclaimed_slab = 0;
        p->reclaim_state = &reclaim_state;

        /*
         * Free memory by calling shrink zone with increasing priorities
         * until we have enough memory freed.
         */
        do {
                sc.priority--;
                shrink_zone(zone, &sc);

        } while (sc.nr_reclaimed < nr_pages && sc.priority > 0);

        if (sc.nr_reclaimed < nr_pages && (zone_reclaim_mode & RECLAIM_SLAB)) {
                /*
                 * shrink_slab does not currently allow us to determine
                 * how many pages were freed in the zone. So we just
                 * shake the slab and then go offnode for a single allocation.
                 *
                 * shrink_slab will free memory on all zones and may take
                 * a long time.
                 */
                shrink_slab(sc.nr_scanned, gfp_mask, order);
        }

        p->reclaim_state = NULL;
        current->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE);

        if (sc.nr_reclaimed == 0)
                zone->last_unsuccessful_zone_reclaim = jiffies;

        return sc.nr_reclaimed >= nr_pages;
}
#endif