Merge with Linux 2.5.74.

[linux-2.6/linux-mips.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/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/suspend.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-locking.h>
#include <linux/topology.h>

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

#include <linux/swapops.h>

/*
 * The "priority" of VM scanning is how much of the queues we will scan in one
 * go. A value of 12 for DEF_PRIORITY implies that we will scan 1/4096th of the
 * queues ("queue_length >> 12") during an aging round.
 */
#define DEF_PRIORITY 12

/*
 * From 0 .. 100.  Higher means more swappy.
 */
int vm_swappiness = 60;
static long total_memory;

#ifdef ARCH_HAS_PREFETCH
#define prefetch_prev_lru_page(_page, _base, _field)                    \
        do {                                                            \
                if ((_page)->lru.prev != _base) {                       \
                        struct page *prev;                              \
                                                                        \
                        prev = list_entry(_page->lru.prev,              \
                                        struct 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 = list_entry(_page->lru.prev,              \
                                        struct page, lru);              \
                        prefetchw(&prev->_field);                       \
                }                                                       \
        } while (0)
#else
#define prefetchw_prev_lru_page(_page, _base, _field) do { } while (0)
#endif

/*
 * 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 */
};

static LIST_HEAD(shrinker_list);
static DECLARE_MUTEX(shrinker_sem);

/*
 * 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(&shrinker_sem);
                list_add(&shrinker->list, &shrinker_list);
                up(&shrinker_sem);
        }
        return shrinker;
}

/*
 * Remove one
 */
void remove_shrinker(struct shrinker *shrinker)
{
        down(&shrinker_sem);
        list_del(&shrinker->list);
        up(&shrinker_sem);
        kfree(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.
 */
static int shrink_slab(long scanned, unsigned int gfp_mask)
{
        struct shrinker *shrinker;
        long pages;

        if (down_trylock(&shrinker_sem))
                return 0;

        pages = nr_used_zone_pages();
        list_for_each_entry(shrinker, &shrinker_list, list) {
                long long delta;

                delta = scanned * shrinker->seeks;
                delta *= (*shrinker->shrinker)(0, gfp_mask);
                do_div(delta, pages + 1);
                shrinker->nr += delta;
                if (shrinker->nr > SHRINK_BATCH) {
                        long nr_to_scan = shrinker->nr;

                        shrinker->nr = 0;
                        while (nr_to_scan) {
                                long this_scan = nr_to_scan;

                                if (this_scan > 128)
                                        this_scan = 128;
                                (*shrinker->shrinker)(this_scan, gfp_mask);
                                nr_to_scan -= this_scan;
                                cond_resched();
                        }
                }
        }
        up(&shrinker_sem);
        return 0;
}

/* Must be called with page's pte_chain_lock held. */
static inline int page_mapping_inuse(struct page *page)
{
        struct address_space *mapping = page->mapping;

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

        /* XXX: does this happen ? */
        if (!mapping)
                return 0;

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

        /* File is mmap'd by somebody. */
        if (!list_empty(&mapping->i_mmap))
                return 1;
        if (!list_empty(&mapping->i_mmap_shared))
                return 1;

        return 0;
}

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_is_kswapd())
                return 1;
        if (current_is_pdflush())       /* This is unlikely, but why not... */
                return 1;
        if (!bdi_write_congested(bdi))
                return 1;
        if (bdi == current->backing_dev_info)
                return 1;
        return 0;
}

/*
 * shrink_list returns the number of reclaimed pages
 */
static int
shrink_list(struct list_head *page_list, unsigned int gfp_mask,
                int *max_scan, int *nr_mapped)
{
        struct address_space *mapping;
        LIST_HEAD(ret_pages);
        struct pagevec freed_pvec;
        int pgactivate = 0;
        int ret = 0;

        cond_resched();

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

                page = list_entry(page_list->prev, struct page, lru);
                list_del(&page->lru);

                if (TestSetPageLocked(page))
                        goto keep;

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

                BUG_ON(PageActive(page));
                may_enter_fs = (gfp_mask & __GFP_FS) ||
                                (PageSwapCache(page) && (gfp_mask & __GFP_IO));

                if (PageWriteback(page))
                        goto keep_locked;

                pte_chain_lock(page);
                if (page_referenced(page) && page_mapping_inuse(page)) {
                        /* In active use or really unfreeable.  Activate it. */
                        pte_chain_unlock(page);
                        goto activate_locked;
                }

                mapping = page->mapping;

#ifdef CONFIG_SWAP
                /*
                 * Anonymous process memory without backing store. Try to
                 * allocate it some swap space here.
                 *
                 * XXX: implement swap clustering ?
                 */
                if (page_mapped(page) && !mapping && !PagePrivate(page)) {
                        pte_chain_unlock(page);
                        if (!add_to_swap(page))
                                goto activate_locked;
                        pte_chain_lock(page);
                        mapping = page->mapping;
                }
#endif /* CONFIG_SWAP */

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

                /*
                 * 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 (PageDirty(page)) {
                        if (!is_page_cache_freeable(page))
                                goto keep_locked;
                        if (!mapping)
                                goto keep_locked;
                        if (mapping->a_ops->writepage == NULL)
                                goto activate_locked;
                        if (!may_enter_fs)
                                goto keep_locked;
                        if (!may_write_to_queue(mapping->backing_dev_info))
                                goto keep_locked;
                        spin_lock(&mapping->page_lock);
                        if (test_clear_page_dirty(page)) {
                                int res;
                                struct writeback_control wbc = {
                                        .sync_mode = WB_SYNC_NONE,
                                        .nr_to_write = SWAP_CLUSTER_MAX,
                                        .nonblocking = 1,
                                        .for_reclaim = 1,
                                };

                                list_move(&page->list, &mapping->locked_pages);
                                spin_unlock(&mapping->page_lock);

                                SetPageReclaim(page);
                                res = mapping->a_ops->writepage(page, &wbc);

                                if (res == WRITEPAGE_ACTIVATE) {
                                        ClearPageReclaim(page);
                                        goto activate_locked;
                                }
                                if (!PageWriteback(page)) {
                                        /* synchronous write or broken a_ops? */
                                        ClearPageReclaim(page);
                                }
                                goto keep;
                        }
                        spin_unlock(&mapping->page_lock);
                }

                /*
                 * 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 == 0) it can be freed.
                 * Otherwise, leave the page on the LRU so it is swappable.
                 */
                if (PagePrivate(page)) {
                        if (!try_to_release_page(page, gfp_mask))
                                goto activate_locked;
                        if (!mapping && page_count(page) == 1)
                                goto free_it;
                }

                if (!mapping)
                        goto keep_locked;       /* truncate got there first */

                spin_lock(&mapping->page_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 (page_count(page) != 2 || PageDirty(page)) {
                        spin_unlock(&mapping->page_lock);
                        goto keep_locked;
                }

#ifdef CONFIG_SWAP
                if (PageSwapCache(page)) {
                        swp_entry_t swap = { .val = page->index };
                        __delete_from_swap_cache(page);
                        spin_unlock(&mapping->page_lock);
                        swap_free(swap);
                        __put_page(page);       /* The pagecache ref */
                        goto free_it;
                }
#endif /* CONFIG_SWAP */

                __remove_from_page_cache(page);
                spin_unlock(&mapping->page_lock);
                __put_page(page);

free_it:
                unlock_page(page);
                ret++;
                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(pgsteal, ret);
        if (current_is_kswapd())
                mod_page_state(kswapd_steal, ret);
        mod_page_state(pgactivate, pgactivate);
        return ret;
}

/*
 * zone->lru_lock is heavily contented.  We relieve it by quickly privatising
 * a batch of pages and working on them outside the lock.  Any pages which were
 * not freed will be added back to the LRU.
 *
 * shrink_cache() is passed the number of pages to try to free, and returns
 * the number of pages which were reclaimed.
 *
 * For pagecache intensive workloads, the first loop here is the hottest spot
 * in the kernel (apart from the copy_*_user functions).
 */
static int
shrink_cache(const int nr_pages, struct zone *zone,
                unsigned int gfp_mask, int max_scan, int *nr_mapped)
{
        LIST_HEAD(page_list);
        struct pagevec pvec;
        int nr_to_process;
        int ret = 0;

        /*
         * Try to ensure that we free `nr_pages' pages in one pass of the loop.
         */
        nr_to_process = nr_pages;
        if (nr_to_process < SWAP_CLUSTER_MAX)
                nr_to_process = SWAP_CLUSTER_MAX;

        pagevec_init(&pvec, 1);

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

                while (nr_scan++ < nr_to_process &&
                                !list_empty(&zone->inactive_list)) {
                        page = list_entry(zone->inactive_list.prev,
                                                struct page, lru);

                        prefetchw_prev_lru_page(page,
                                                &zone->inactive_list, flags);

                        if (!TestClearPageLRU(page))
                                BUG();
                        list_del(&page->lru);
                        if (page_count(page) == 0) {
                                /* It is currently in pagevec_release() */
                                SetPageLRU(page);
                                list_add(&page->lru, &zone->inactive_list);
                                continue;
                        }
                        list_add(&page->lru, &page_list);
                        page_cache_get(page);
                        nr_taken++;
                }
                zone->nr_inactive -= nr_taken;
                zone->pages_scanned += nr_taken;
                spin_unlock_irq(&zone->lru_lock);

                if (nr_taken == 0)
                        goto done;

                max_scan -= nr_scan;
                mod_page_state(pgscan, nr_scan);
                nr_freed = shrink_list(&page_list, gfp_mask,
                                        &max_scan, nr_mapped);
                ret += nr_freed;
                if (nr_freed <= 0 && list_empty(&page_list))
                        goto done;

                spin_lock_irq(&zone->lru_lock);
                /*
                 * Put back any unfreeable pages.
                 */
                while (!list_empty(&page_list)) {
                        page = list_entry(page_list.prev, struct page, lru);
                        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);
        return ret;
}

/*
 * 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, const int nr_pages_in,
                        struct page_state *ps, int priority)
{
        int pgmoved;
        int pgdeactivate = 0;
        int nr_pages = nr_pages_in;
        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;
        long mapped_ratio;
        long distress;
        long swap_tendency;

        lru_add_drain();
        pgmoved = 0;
        spin_lock_irq(&zone->lru_lock);
        while (nr_pages && !list_empty(&zone->active_list)) {
                page = list_entry(zone->active_list.prev, struct page, lru);
                prefetchw_prev_lru_page(page, &zone->active_list, flags);
                if (!TestClearPageLRU(page))
                        BUG();
                list_del(&page->lru);
                if (page_count(page) == 0) {
                        /* It is currently in pagevec_release() */
                        SetPageLRU(page);
                        list_add(&page->lru, &zone->active_list);
                } else {
                        page_cache_get(page);
                        list_add(&page->lru, &l_hold);
                        pgmoved++;
                }
                nr_pages--;
        }
        zone->nr_active -= pgmoved;
        spin_unlock_irq(&zone->lru_lock);

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

        while (!list_empty(&l_hold)) {
                page = list_entry(l_hold.prev, struct page, lru);
                list_del(&page->lru);
                if (page_mapped(page)) {
                        pte_chain_lock(page);
                        if (page_mapped(page) && page_referenced(page)) {
                                pte_chain_unlock(page);
                                list_add(&page->lru, &l_active);
                                continue;
                        }
                        pte_chain_unlock(page);
                        if (!reclaim_mapped) {
                                list_add(&page->lru, &l_active);
                                continue;
                        }
                }
                /*
                 * FIXME: need to consider page_count(page) here if/when we
                 * reap orphaned pages via the LRU (Daniel's locking stuff)
                 */
                if (total_swap_pages == 0 && !page->mapping &&
                                                !PagePrivate(page)) {
                        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 = list_entry(l_inactive.prev, struct page, lru);
                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 = list_entry(l_active.prev, struct page, lru);
                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_irq(&zone->lru_lock);
        pagevec_release(&pvec);

        mod_page_state(pgrefill, nr_pages_in - nr_pages);
        mod_page_state(pgdeactivate, pgdeactivate);
}

/*
 * Try to reclaim `nr_pages' from this zone.  Returns the number of reclaimed
 * pages.  This is a basic per-zone page freer.  Used by both kswapd and
 * direct reclaim.
 */
static int
shrink_zone(struct zone *zone, int max_scan, unsigned int gfp_mask,
        const int nr_pages, int *nr_mapped, struct page_state *ps, int priority)
{
        unsigned long ratio;

        /*
         * Try to keep the active list 2/3 of the size of the cache.  And
         * make sure that refill_inactive is given a decent number of pages.
         *
         * The "ratio+1" here is important.  With pagecache-intensive workloads
         * the inactive list is huge, and `ratio' evaluates to zero all the
         * time.  Which pins the active list memory.  So we add one to `ratio'
         * just to make sure that the kernel will slowly sift through the
         * active list.
         */
        ratio = (unsigned long)nr_pages * zone->nr_active /
                                ((zone->nr_inactive | 1) * 2);
        atomic_add(ratio+1, &zone->refill_counter);
        if (atomic_read(&zone->refill_counter) > SWAP_CLUSTER_MAX) {
                int count;

                /*
                 * Don't try to bring down too many pages in one attempt.
                 * If this fails, the caller will increase `priority' and
                 * we'll try again, with an increased chance of reclaiming
                 * mapped memory.
                 */
                count = atomic_read(&zone->refill_counter);
                if (count > SWAP_CLUSTER_MAX * 4)
                        count = SWAP_CLUSTER_MAX * 4;
                atomic_sub(count, &zone->refill_counter);
                refill_inactive_zone(zone, count, ps, priority);
        }
        return shrink_cache(nr_pages, zone, gfp_mask,
                                max_scan, nr_mapped);
}

/*
 * 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 int
shrink_caches(struct zone *classzone, int priority, int *total_scanned,
                int gfp_mask, int nr_pages, struct page_state *ps)
{
        struct zone *first_classzone;
        struct zone *zone;
        int ret = 0;

        first_classzone = classzone->zone_pgdat->node_zones;
        for (zone = classzone; zone >= first_classzone; zone--) {
                int to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX);
                int nr_mapped = 0;
                int max_scan;

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

                /*
                 * If we cannot reclaim `nr_pages' pages by scanning twice
                 * that many pages then fall back to the next zone.
                 */
                max_scan = zone->nr_inactive >> priority;
                if (max_scan < to_reclaim * 2)
                        max_scan = to_reclaim * 2;
                ret += shrink_zone(zone, max_scan, gfp_mask,
                                to_reclaim, &nr_mapped, ps, priority);
                *total_scanned += max_scan + nr_mapped;
                if (ret >= nr_pages)
                        break;
        }
        return ret;
}
 
/*
 * 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.  So for !__GFP_FS callers, we just perform a
 * small LRU walk and if that didn't work out, fail the allocation back to the
 * caller.  GFP_NOFS allocators need to know how to deal with it.  Kicking
 * bdflush, waiting and retrying will work.
 *
 * This is a fairly lame algorithm - it can result in excessive CPU burning and
 * excessive rotation of the inactive list, which is _supposed_ to be an LRU,
 * yes?
 */
int try_to_free_pages(struct zone *cz,
                unsigned int gfp_mask, unsigned int order)
{
        int priority;
        int ret = 0;
        const int nr_pages = SWAP_CLUSTER_MAX;
        int nr_reclaimed = 0;
        struct reclaim_state *reclaim_state = current->reclaim_state;

        inc_page_state(allocstall);

        for (priority = DEF_PRIORITY; priority >= 0; priority--) {
                int total_scanned = 0;
                struct page_state ps;

                get_page_state(&ps);
                nr_reclaimed += shrink_caches(cz, priority, &total_scanned,
                                                gfp_mask, nr_pages, &ps);
                if (nr_reclaimed >= nr_pages) {
                        ret = 1;
                        goto out;
                }
                if (!(gfp_mask & __GFP_FS))
                        break;          /* Let the caller handle it */
                /*
                 * Try to write back as many pages as we just scanned.  Not
                 * sure if that makes sense, but it's an attempt to avoid
                 * creating IO storms unnecessarily
                 */
                wakeup_bdflush(total_scanned);

                /* Take a nap, wait for some writeback to complete */
                blk_congestion_wait(WRITE, HZ/10);
                if (cz - cz->zone_pgdat->node_zones < ZONE_HIGHMEM) {
                        shrink_slab(total_scanned, gfp_mask);
                        if (reclaim_state) {
                                nr_reclaimed += reclaim_state->reclaimed_slab;
                                reclaim_state->reclaimed_slab = 0;
                        }
                }
        }
        if ((gfp_mask & __GFP_FS) && !(gfp_mask & __GFP_NORETRY))
                out_of_memory();
out:
        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.
 */
static int balance_pgdat(pg_data_t *pgdat, int nr_pages, struct page_state *ps)
{
        int to_free = nr_pages;
        int priority;
        int i;
        struct reclaim_state *reclaim_state = current->reclaim_state;

        inc_page_state(pageoutrun);

        for (priority = DEF_PRIORITY; priority; priority--) {
                int all_zones_ok = 1;

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

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

                        if (nr_pages && to_free > 0) {  /* Software suspend */
                                to_reclaim = min(to_free, SWAP_CLUSTER_MAX*8);
                        } else {                        /* Zone balancing */
                                to_reclaim = zone->pages_high-zone->free_pages;
                                if (to_reclaim <= 0)
                                        continue;
                        }
                        all_zones_ok = 0;
                        max_scan = zone->nr_inactive >> priority;
                        if (max_scan < to_reclaim * 2)
                                max_scan = to_reclaim * 2;
                        if (max_scan < SWAP_CLUSTER_MAX)
                                max_scan = SWAP_CLUSTER_MAX;
                        to_free -= shrink_zone(zone, max_scan, GFP_KERNEL,
                                        to_reclaim, &nr_mapped, ps, priority);
                        if (i < ZONE_HIGHMEM) {
                                reclaim_state->reclaimed_slab = 0;
                                shrink_slab(max_scan + nr_mapped, GFP_KERNEL);
                                to_free += reclaim_state->reclaimed_slab;
                        }
                        if (zone->all_unreclaimable)
                                continue;
                        if (zone->pages_scanned > zone->present_pages * 2)
                                zone->all_unreclaimable = 1;
                }
                if (all_zones_ok)
                        break;
                blk_congestion_wait(WRITE, HZ/10);
        }
        return nr_pages - to_free;
}

/*
 * 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.
 */
int kswapd(void *p)
{
        pg_data_t *pgdat = (pg_data_t*)p;
        struct task_struct *tsk = current;
        DEFINE_WAIT(wait);
        struct reclaim_state reclaim_state = {
                .reclaimed_slab = 0,
        };
        unsigned long cpumask;

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

        for ( ; ; ) {
                struct page_state ps;

                if (current->flags & PF_FREEZE)
                        refrigerator(PF_IOTHREAD);
                prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
                schedule();
                finish_wait(&pgdat->kswapd_wait, &wait);
                get_page_state(&ps);
                balance_pgdat(pgdat, 0, &ps);
        }
}

/*
 * A zone is low on free memory, so wake its kswapd task to service it.
 */
void wakeup_kswapd(struct zone *zone)
{
        if (zone->free_pages > zone->pages_low)
                return;
        if (!waitqueue_active(&zone->zone_pgdat->kswapd_wait))
                return;
        wake_up_interruptible(&zone->zone_pgdat->kswapd_wait);
}

#ifdef CONFIG_SOFTWARE_SUSPEND
/*
 * 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;
                struct page_state ps;

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

static int __init kswapd_init(void)
{
        pg_data_t *pgdat;
        swap_setup();
        for_each_pgdat(pgdat)
                kernel_thread(kswapd, pgdat, CLONE_KERNEL);
        total_memory = nr_free_pagecache_pages();
        return 0;
}

module_init(kswapd_init)