/*
 *  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/kernel_stat.h>
#include <linux/swap.h>
#include <linux/swapctl.h>
#include <linux/init.h>
#include <linux/pagemap.h>
#include <linux/smp_lock.h>

#include <asm/pgtable.h>

/*
 * We may have stale swap cache pages in memory: notice
 * them here and get rid of the unnecessary final write.
 */
static int swap_writepage(struct page *page)
{
        if (remove_exclusive_swap_page(page)) {
                UnlockPage(page);
                return 0;
        }
        rw_swap_page(WRITE, page);
        return 0;
}

static struct address_space_operations swap_aops = {
        writepage: swap_writepage,
        sync_page: block_sync_page,
};

struct address_space swapper_space = {
        LIST_HEAD_INIT(swapper_space.clean_pages),
        LIST_HEAD_INIT(swapper_space.dirty_pages),
        LIST_HEAD_INIT(swapper_space.locked_pages),
        0,                              /* nrpages      */
        &swap_aops,
};

#ifdef SWAP_CACHE_INFO
#define INC_CACHE_INFO(x)       (swap_cache_info.x++)

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

void show_swap_cache_info(void)
{
        printk("Swap cache: add %lu, delete %lu, find %lu/%lu, race %lu+%lu\n",
                swap_cache_info.add_total, swap_cache_info.del_total,
                swap_cache_info.find_success, swap_cache_info.find_total,
                swap_cache_info.noent_race, swap_cache_info.exist_race);
}
#else
#define INC_CACHE_INFO(x)       do { } while (0)
#endif

int add_to_swap_cache(struct page *page, swp_entry_t entry)
{
        if (page->mapping)
                BUG();
        if (!swap_duplicate(entry)) {
                INC_CACHE_INFO(noent_race);
                return -ENOENT;
        }
        if (add_to_page_cache_unique(page, &swapper_space, entry.val,
                        page_hash(&swapper_space, entry.val)) != 0) {
                swap_free(entry);
                INC_CACHE_INFO(exist_race);
                return -EEXIST;
        }
        if (!PageLocked(page))
                BUG();
        if (!PageSwapCache(page))
                BUG();
        INC_CACHE_INFO(add_total);
        return 0;
}

/*
 * 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)
{
        if (!PageLocked(page))
                BUG();
        if (!PageSwapCache(page))
                BUG();
        ClearPageDirty(page);
        __remove_inode_page(page);
        INC_CACHE_INFO(del_total);
}

/*
 * 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;

        if (!PageLocked(page))
                BUG();

        if (unlikely(!block_flushpage(page, 0)))
                BUG();  /* an anonymous page cannot have page->buffers set */

        entry.val = page->index;

        spin_lock(&pagecache_lock);
        __delete_from_swap_cache(page);
        spin_unlock(&pagecache_lock);

        swap_free(entry);
        page_cache_release(page);
}

/* 
 * Perform a free_page(), also freeing any swap cache associated with
 * this page if it is the last user of the page. Can not do a lock_page,
 * as we are holding the page_table_lock spinlock.
 */
void free_page_and_swap_cache(struct page *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 
         * exclusive_swap_page() _with_ the lock. 
         *                                      - Marcelo
         */
        if (PageSwapCache(page) && !TryLockPage(page)) {
                remove_exclusive_swap_page(page);
                UnlockPage(page);
        }
        page_cache_release(page);
}

/*
 * 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 *found;

        found = find_get_page(&swapper_space, entry.val);
        /*
         * Unsafe to assert PageSwapCache and mapping on page found:
         * if SMP nothing prevents swapoff from deleting this page from
         * the swap cache at this moment.  find_lock_page would prevent
         * that, but no need to change: we _have_ got the right page.
         */
        INC_CACHE_INFO(find_total);
        if (found)
                INC_CACHE_INFO(find_success);
        return found;
}

/* 
 * 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)
{
        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: use find_get_page()
                 * directly.
                 */
                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(GFP_HIGHUSER);
                        if (!new_page)
                                break;          /* Out of memory */
                }

                /*
                 * Associate the page with swap entry in the swap cache.
                 * May fail (-ENOENT) if swap entry has been freed since
                 * our caller observed it.  May fail (-EEXIST) if there
                 * is already a page associated with this entry in the
                 * swap cache: added by a racing read_swap_cache_async,
                 * or by try_to_swap_out (or shmem_writepage) re-using
                 * the just freed swap entry for an existing page.
                 */
                err = add_to_swap_cache(new_page, entry);
                if (!err) {
                        /*
                         * Initiate read into locked page and return.
                         */
                        rw_swap_page(READ, new_page);
                        return new_page;
                }
        } while (err != -ENOENT);

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