swap_state.c

Go to the documentation of this file.

/*
 *  linux/mm/swap_state.c
 *
 *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
 *  Swap reorganised 29.12.95, Stephen Tweedie
 *
 *  Rewritten to use page cache, (C) 1998 Stephen Tweedie
 */
#include <linux/mm.h>
#include <linux/gfp.h>
#include <linux/kernel_stat.h>
#include <linux/swap.h>
#include <linux/swapops.h>
#include <linux/init.h>
#include <linux/pagemap.h>
#include <linux/backing-dev.h>
#include <linux/blkdev.h>
#include <linux/pagevec.h>
#include <linux/migrate.h>
#include <linux/page_cgroup.h>

#include <asm/pgtable.h>

/*
 * swapper_space is a fiction, retained to simplify the path through
 * vmscan's shrink_page_list.
 */
static const struct address_space_operations swap_aops = {
    .writepage  = swap_writepage,
    .set_page_dirty = swap_set_page_dirty,
    .migratepage    = migrate_page,
};

static struct backing_dev_info swap_backing_dev_info = {
    .name       = "swap",
    .capabilities   = BDI_CAP_NO_ACCT_AND_WRITEBACK | BDI_CAP_SWAP_BACKED,
};

struct address_space swapper_space = {
    .page_tree  = RADIX_TREE_INIT(GFP_ATOMIC|__GFP_NOWARN),
    .tree_lock  = __SPIN_LOCK_UNLOCKED(swapper_space.tree_lock),
    .a_ops      = &swap_aops,
    .i_mmap_nonlinear = LIST_HEAD_INIT(swapper_space.i_mmap_nonlinear),
    .backing_dev_info = &swap_backing_dev_info,
};

#define INC_CACHE_INFO(x)   do { swap_cache_info.x++; } while (0)

static struct {
    unsigned long add_total;
    unsigned long del_total;
    unsigned long find_success;
    unsigned long find_total;
} swap_cache_info;

void show_swap_cache_info(void)
{
    printk("%lu pages in swap cache\n", total_swapcache_pages);
    printk("Swap cache stats: add %lu, delete %lu, find %lu/%lu\n",
        swap_cache_info.add_total, swap_cache_info.del_total,
        swap_cache_info.find_success, swap_cache_info.find_total);
    printk("Free swap  = %ldkB\n", nr_swap_pages << (PAGE_SHIFT - 10));
    printk("Total swap = %lukB\n", total_swap_pages << (PAGE_SHIFT - 10));
}

/*
 * __add_to_swap_cache resembles add_to_page_cache_locked on swapper_space,
 * but sets SwapCache flag and private instead of mapping and index.
 */
static int __add_to_swap_cache(struct page *page, swp_entry_t entry)
{
    int error;

    VM_BUG_ON(!PageLocked(page));
    VM_BUG_ON(PageSwapCache(page));
    VM_BUG_ON(!PageSwapBacked(page));

    page_cache_get(page);
    SetPageSwapCache(page);
    set_page_private(page, entry.val);

    spin_lock_irq(&swapper_space.tree_lock);
    error = radix_tree_insert(&swapper_space.page_tree, entry.val, page);
    if (likely(!error)) {
        total_swapcache_pages++;
        __inc_zone_page_state(page, NR_FILE_PAGES);
        INC_CACHE_INFO(add_total);
    }
    spin_unlock_irq(&swapper_space.tree_lock);

    if (unlikely(error)) {
        /*
         * Only the context which have set SWAP_HAS_CACHE flag
         * would call add_to_swap_cache().
         * So add_to_swap_cache() doesn't returns -EEXIST.
         */
        VM_BUG_ON(error == -EEXIST);
        set_page_private(page, 0UL);
        ClearPageSwapCache(page);
        page_cache_release(page);
    }

    return error;
}


int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
{
    int error;

    error = radix_tree_preload(gfp_mask);
    if (!error) {
        error = __add_to_swap_cache(page, entry);
        radix_tree_preload_end();
    }
    return error;
}

/*
 * This must be called only on pages that have
 * been verified to be in the swap cache.
 */
void __delete_from_swap_cache(struct page *page)
{
    VM_BUG_ON(!PageLocked(page));
    VM_BUG_ON(!PageSwapCache(page));
    VM_BUG_ON(PageWriteback(page));

    radix_tree_delete(&swapper_space.page_tree, page_private(page));
    set_page_private(page, 0);
    ClearPageSwapCache(page);
    total_swapcache_pages--;
    __dec_zone_page_state(page, NR_FILE_PAGES);
    INC_CACHE_INFO(del_total);
}

int add_to_swap(struct page *page)
{
    swp_entry_t entry;
    int err;

    VM_BUG_ON(!PageLocked(page));
    VM_BUG_ON(!PageUptodate(page));

    entry = get_swap_page();
    if (!entry.val)
        return 0;

    if (unlikely(PageTransHuge(page)))
        if (unlikely(split_huge_page(page))) {
            swapcache_free(entry, NULL);
            return 0;
        }

    /*
     * Radix-tree node allocations from PF_MEMALLOC contexts could
     * completely exhaust the page allocator. __GFP_NOMEMALLOC
     * stops emergency reserves from being allocated.
     *
     * TODO: this could cause a theoretical memory reclaim
     * deadlock in the swap out path.
     */
    /*
     * Add it to the swap cache and mark it dirty
     */
    err = add_to_swap_cache(page, entry,
            __GFP_HIGH|__GFP_NOMEMALLOC|__GFP_NOWARN);

    if (!err) { /* Success */
        SetPageDirty(page);
        return 1;
    } else {    /* -ENOMEM radix-tree allocation failure */
        /*
         * add_to_swap_cache() doesn't return -EEXIST, so we can safely
         * clear SWAP_HAS_CACHE flag.
         */
        swapcache_free(entry, NULL);
        return 0;
    }
}

/*
 * This must be called only on pages that have
 * been verified to be in the swap cache and locked.
 * It will never put the page into the free list,
 * the caller has a reference on the page.
 */
void delete_from_swap_cache(struct page *page)
{
    swp_entry_t entry;

    entry.val = page_private(page);

    spin_lock_irq(&swapper_space.tree_lock);
    __delete_from_swap_cache(page);
    spin_unlock_irq(&swapper_space.tree_lock);

    swapcache_free(entry, page);
    page_cache_release(page);
}

/* 
 * If we are the only user, then try to free up the swap cache. 
 * 
 * Its ok to check for PageSwapCache without the page lock
 * here because we are going to recheck again inside
 * try_to_free_swap() _with_ the lock.
 *                  - Marcelo
 */
static inline void free_swap_cache(struct page *page)
{
    if (PageSwapCache(page) && !page_mapped(page) && trylock_page(page)) {
        try_to_free_swap(page);
        unlock_page(page);
    }
}

/* 
 * Perform a free_page(), also freeing any swap cache associated with
 * this page if it is the last user of the page.
 */
void free_page_and_swap_cache(struct page *page)
{
    free_swap_cache(page);
    page_cache_release(page);
}

/*
 * Passed an array of pages, drop them all from swapcache and then release
 * them.  They are removed from the LRU and freed if this is their last use.
 */
void free_pages_and_swap_cache(struct page **pages, int nr)
{
    struct page **pagep = pages;

    lru_add_drain();
    while (nr) {
        int todo = min(nr, PAGEVEC_SIZE);
        int i;

        for (i = 0; i < todo; i++)
            free_swap_cache(pagep[i]);
        release_pages(pagep, todo, 0);
        pagep += todo;
        nr -= todo;
    }
}

/*
 * Lookup a swap entry in the swap cache. A found page will be returned
 * unlocked and with its refcount incremented - we rely on the kernel
 * lock getting page table operations atomic even if we drop the page
 * lock before returning.
 */
struct page * lookup_swap_cache(swp_entry_t entry)
{
    struct page *page;

    page = find_get_page(&swapper_space, entry.val);

    if (page)
        INC_CACHE_INFO(find_success);

    INC_CACHE_INFO(find_total);
    return page;
}

/* 
 * Locate a page of swap in physical memory, reserving swap cache space
 * and reading the disk if it is not already cached.
 * A failure return means that either the page allocation failed or that
 * the swap entry is no longer in use.
 */
struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
            struct vm_area_struct *vma, unsigned long addr)
{
    struct page *found_page, *new_page = NULL;
    int err;

    do {
        /*
         * First check the swap cache.  Since this is normally
         * called after lookup_swap_cache() failed, re-calling
         * that would confuse statistics.
         */
        found_page = find_get_page(&swapper_space, entry.val);
        if (found_page)
            break;

        /*
         * Get a new page to read into from swap.
         */
        if (!new_page) {
            new_page = alloc_page_vma(gfp_mask, vma, addr);
            if (!new_page)
                break;      /* Out of memory */
        }

        /*
         * call radix_tree_preload() while we can wait.
         */
        err = radix_tree_preload(gfp_mask & GFP_KERNEL);
        if (err)
            break;

        /*
         * Swap entry may have been freed since our caller observed it.
         */
        err = swapcache_prepare(entry);
        if (err == -EEXIST) {   /* seems racy */
            radix_tree_preload_end();
            continue;
        }
        if (err) {      /* swp entry is obsolete ? */
            radix_tree_preload_end();
            break;
        }

        /* May fail (-ENOMEM) if radix-tree node allocation failed. */
        __set_page_locked(new_page);
        SetPageSwapBacked(new_page);
        err = __add_to_swap_cache(new_page, entry);
        if (likely(!err)) {
            radix_tree_preload_end();
            /*
             * Initiate read into locked page and return.
             */
            lru_cache_add_anon(new_page);
            swap_readpage(new_page);
            return new_page;
        }
        radix_tree_preload_end();
        ClearPageSwapBacked(new_page);
        __clear_page_locked(new_page);
        /*
         * add_to_swap_cache() doesn't return -EEXIST, so we can safely
         * clear SWAP_HAS_CACHE flag.
         */
        swapcache_free(entry, NULL);
    } while (err != -ENOMEM);

    if (new_page)
        page_cache_release(new_page);
    return found_page;
}

struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
            struct vm_area_struct *vma, unsigned long addr)
{
    struct page *page;
    unsigned long offset = swp_offset(entry);
    unsigned long start_offset, end_offset;
    unsigned long mask = (1UL << page_cluster) - 1;
    struct blk_plug plug;

    /* Read a page_cluster sized and aligned cluster around offset. */
    start_offset = offset & ~mask;
    end_offset = offset | mask;
    if (!start_offset)  /* First page is swap header. */
        start_offset++;

    blk_start_plug(&plug);
    for (offset = start_offset; offset <= end_offset ; offset++) {
        /* Ok, do the async read-ahead now */
        page = read_swap_cache_async(swp_entry(swp_type(entry), offset),
                        gfp_mask, vma, addr);
        if (!page)
            continue;
        page_cache_release(page);
    }
    blk_finish_plug(&plug);

    lru_add_drain();    /* Push any new pages onto the LRU now */
    return read_swap_cache_async(entry, gfp_mask, vma, addr);
}