initial commit with v2.6.9

[linux-2.6.9-moxart.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/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.h>
#include <linux/topology.h>
#include <linux/cpu.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 */

        /* How many pages shrink_cache() should reclaim */
        int nr_to_reclaim;

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

        /* This context's GFP mask */
        unsigned int gfp_mask;

        int may_writepage;
};

/*
 * 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(&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.
 */
static int shrink_slab(unsigned long scanned, unsigned int gfp_mask,
                        unsigned long lru_pages)
{
        struct shrinker *shrinker;

        if (scanned == 0)
                scanned = SWAP_CLUSTER_MAX;

        if (!down_read_trylock(&shrinker_rwsem))
                return 0;

        list_for_each_entry(shrinker, &shrinker_list, list) {
                unsigned long long delta;
                unsigned long total_scan;

                delta = (4 * scanned) / shrinker->seeks;
                delta *= (*shrinker->shrinker)(0, gfp_mask);
                do_div(delta, lru_pages + 1);
                shrinker->nr += delta;
                if (shrinker->nr < 0)
                        shrinker->nr = LONG_MAX;        /* It wrapped! */

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

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

                        shrink_ret = (*shrinker->shrinker)(this_scan, gfp_mask);
                        if (shrink_ret == -1)
                                break;
                        mod_page_state(slabs_scanned, this_scan);
                        total_scan -= this_scan;

                        cond_resched();
                }

                shrinker->nr += total_scan;
        }
        up_read(&shrinker_rwsem);
        return 0;
}

/* 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_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;
}

/*
 * 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)
                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 == WRITEPAGE_ACTIVATE) {
                        ClearPageReclaim(page);
                        return PAGE_ACTIVATE;
                }
                if (!PageWriteback(page)) {
                        /* synchronous write or broken a_ops? */
                        ClearPageReclaim(page);
                }

                return PAGE_SUCCESS;
        }

        return PAGE_CLEAN;
}

/*
 * 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;

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

                if (TestSetPageLocked(page))
                        goto keep;

                BUG_ON(PageActive(page));

                if (PageWriteback(page))
                        goto keep_locked;

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

                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 (!add_to_swap(page))
                                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) {
                        switch (try_to_unmap(page)) {
                        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 (laptop_mode && !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 (!mapping)
                        goto keep_locked;       /* truncate got there first */

                spin_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 (page_count(page) != 2 || PageDirty(page)) {
                        spin_unlock_irq(&mapping->tree_lock);
                        goto keep_locked;
                }

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

                __remove_from_page_cache(page);
                spin_unlock_irq(&mapping->tree_lock);
                __put_page(page);

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;
}

/*
 * 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() adds the number of pages reclaimed to sc->nr_reclaimed
 *
 * For pagecache intensive workloads, the first loop here is the hottest spot
 * in the kernel (apart from the copy_*_user functions).
 */
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 = 0;
                int nr_scan = 0;
                int nr_freed;

                while (nr_scan++ < SWAP_CLUSTER_MAX &&
                                !list_empty(&zone->inactive_list)) {
                        page = lru_to_page(&zone->inactive_list);

                        prefetchw_prev_lru_page(page,
                                                &zone->inactive_list, 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, &zone->inactive_list);
                                continue;
                        }
                        list_add(&page->lru, &page_list);
                        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;
                if (current_is_kswapd())
                        mod_page_state_zone(zone, pgscan_kswapd, nr_scan);
                else
                        mod_page_state_zone(zone, pgscan_direct, nr_scan);
                nr_freed = shrink_list(&page_list, sc);
                if (current_is_kswapd())
                        mod_page_state(kswapd_steal, nr_freed);
                mod_page_state_zone(zone, pgsteal, nr_freed);
                sc->nr_to_reclaim -= nr_freed;

                spin_lock_irq(&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 = 0;
        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;
        long mapped_ratio;
        long distress;
        long swap_tendency;

        lru_add_drain();
        pgmoved = 0;
        spin_lock_irq(&zone->lru_lock);
        while (pgscanned < nr_pages && !list_empty(&zone->active_list)) {
                page = lru_to_page(&zone->active_list);
                prefetchw_prev_lru_page(page, &zone->active_list, flags);
                if (!TestClearPageLRU(page))
                        BUG();
                list_del(&page->lru);
                if (get_page_testone(page)) {
                        /*
                         * It was already free!  release_pages() or put_page()
                         * are about to remove it from the LRU and free it. So
                         * put the refcount back and put the page back on the
                         * LRU
                         */
                        __put_page(page);
                        SetPageLRU(page);
                        list_add(&page->lru, &zone->active_list);
                } else {
                        list_add(&page->lru, &l_hold);
                        pgmoved++;
                }
                pgscanned++;
        }
        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 >> 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;

        while (!list_empty(&l_hold)) {
                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_irq(&zone->lru_lock);
        pagevec_release(&pvec);

        mod_page_state_zone(zone, pgrefill, pgscanned);
        mod_page_state(pgdeactivate, pgdeactivate);
}

/*
 * 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;

        /*
         * 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 >= 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 >= SWAP_CLUSTER_MAX)
                zone->nr_scan_inactive = 0;
        else
                nr_inactive = 0;

        sc->nr_to_reclaim = SWAP_CLUSTER_MAX;

        while (nr_active || nr_inactive) {
                if (nr_active) {
                        sc->nr_to_scan = min(nr_active,
                                        (unsigned long)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)SWAP_CLUSTER_MAX);
                        nr_inactive -= sc->nr_to_scan;
                        shrink_cache(zone, sc);
                        if (sc->nr_to_reclaim <= 0)
                                break;
                }
        }
}

/*
 * 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 (zone->present_pages == 0)
                        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,
                unsigned int gfp_mask, unsigned int order)
{
        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 = 0;

        inc_page_state(allocstall);

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

                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;
                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;
                }
                if (sc.nr_reclaimed >= SWAP_CLUSTER_MAX) {
                        ret = 1;
                        goto out;
                }
                total_scanned += sc.nr_scanned;
                total_reclaimed += sc.nr_reclaimed;

                /*
                 * 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 > SWAP_CLUSTER_MAX + SWAP_CLUSTER_MAX/2) {
                        wakeup_bdflush(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);
        }
        if ((gfp_mask & __GFP_FS) && !(gfp_mask & __GFP_NORETRY))
                out_of_memory(gfp_mask);
out:
        for (i = 0; zones[i] != 0; i++)
                zones[i]->prev_priority = zones[i]->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 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 = 0;
        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;

                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 (zone->present_pages == 0)
                                        continue;

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

                                if (zone->free_pages <= zone->pages_high) {
                                        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;

                        if (zone->present_pages == 0)
                                continue;

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

                        if (nr_pages == 0) {    /* Not software suspend */
                                if (zone->free_pages <= zone->pages_high)
                                        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;
                        shrink_zone(zone, &sc);
                        reclaim_state->reclaimed_slab = 0;
                        shrink_slab(sc.nr_scanned, GFP_KERNEL, lru_pages);
                        sc.nr_reclaimed += reclaim_state->reclaimed_slab;
                        total_reclaimed += sc.nr_reclaimed;
                        if (zone->all_unreclaimable)
                                continue;
                        if (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)
                        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)
{
        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_KSWAPD;

        for ( ; ; ) {
                if (current->flags & PF_FREEZE)
                        refrigerator(PF_FREEZE);
                prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
                schedule();
                finish_wait(&pgdat->kswapd_wait, &wait);

                balance_pgdat(pgdat, 0);
        }
        return 0;
}

/*
 * A zone is low on free memory, so wake its kswapd task to service it.
 */
void wakeup_kswapd(struct zone *zone)
{
        if (zone->present_pages == 0)
                return;
        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_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);
                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)