frontswap.c

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/*
 * Frontswap frontend
 *
 * This code provides the generic "frontend" layer to call a matching
 * "backend" driver implementation of frontswap.  See
 * Documentation/vm/frontswap.txt for more information.
 *
 * Copyright (C) 2009-2012 Oracle Corp.  All rights reserved.
 * Author: Dan Magenheimer
 *
 * This work is licensed under the terms of the GNU GPL, version 2.
 */

#include <linux/mman.h>
#include <linux/swap.h>
#include <linux/swapops.h>
#include <linux/security.h>
#include <linux/module.h>
#include <linux/debugfs.h>
#include <linux/frontswap.h>
#include <linux/swapfile.h>

/*
 * frontswap_ops is set by frontswap_register_ops to contain the pointers
 * to the frontswap "backend" implementation functions.
 */
static struct frontswap_ops frontswap_ops __read_mostly;

/*
 * This global enablement flag reduces overhead on systems where frontswap_ops
 * has not been registered, so is preferred to the slower alternative: a
 * function call that checks a non-global.
 */
bool frontswap_enabled __read_mostly;
EXPORT_SYMBOL(frontswap_enabled);

/*
 * If enabled, frontswap_store will return failure even on success.  As
 * a result, the swap subsystem will always write the page to swap, in
 * effect converting frontswap into a writethrough cache.  In this mode,
 * there is no direct reduction in swap writes, but a frontswap backend
 * can unilaterally "reclaim" any pages in use with no data loss, thus
 * providing increases control over maximum memory usage due to frontswap.
 */
static bool frontswap_writethrough_enabled __read_mostly;

/*
 * If enabled, the underlying tmem implementation is capable of doing
 * exclusive gets, so frontswap_load, on a successful tmem_get must
 * mark the page as no longer in frontswap AND mark it dirty.
 */
static bool frontswap_tmem_exclusive_gets_enabled __read_mostly;

#ifdef CONFIG_DEBUG_FS
/*
 * Counters available via /sys/kernel/debug/frontswap (if debugfs is
 * properly configured).  These are for information only so are not protected
 * against increment races.
 */
static u64 frontswap_loads;
static u64 frontswap_succ_stores;
static u64 frontswap_failed_stores;
static u64 frontswap_invalidates;

static inline void inc_frontswap_loads(void) {
    frontswap_loads++;
}
static inline void inc_frontswap_succ_stores(void) {
    frontswap_succ_stores++;
}
static inline void inc_frontswap_failed_stores(void) {
    frontswap_failed_stores++;
}
static inline void inc_frontswap_invalidates(void) {
    frontswap_invalidates++;
}
#else
static inline void inc_frontswap_loads(void) { }
static inline void inc_frontswap_succ_stores(void) { }
static inline void inc_frontswap_failed_stores(void) { }
static inline void inc_frontswap_invalidates(void) { }
#endif
/*
 * Register operations for frontswap, returning previous thus allowing
 * detection of multiple backends and possible nesting.
 */
struct frontswap_ops frontswap_register_ops(struct frontswap_ops *ops)
{
    struct frontswap_ops old = frontswap_ops;

    frontswap_ops = *ops;
    frontswap_enabled = true;
    return old;
}
EXPORT_SYMBOL(frontswap_register_ops);

/*
 * Enable/disable frontswap writethrough (see above).
 */
void frontswap_writethrough(bool enable)
{
    frontswap_writethrough_enabled = enable;
}
EXPORT_SYMBOL(frontswap_writethrough);

/*
 * Enable/disable frontswap exclusive gets (see above).
 */
void frontswap_tmem_exclusive_gets(bool enable)
{
    frontswap_tmem_exclusive_gets_enabled = enable;
}
EXPORT_SYMBOL(frontswap_tmem_exclusive_gets);

/*
 * Called when a swap device is swapon'd.
 */
void __frontswap_init(unsigned type)
{
    struct swap_info_struct *sis = swap_info[type];

    BUG_ON(sis == NULL);
    if (sis->frontswap_map == NULL)
        return;
    frontswap_ops.init(type);
}
EXPORT_SYMBOL(__frontswap_init);

static inline void __frontswap_clear(struct swap_info_struct *sis, pgoff_t offset)
{
    frontswap_clear(sis, offset);
    atomic_dec(&sis->frontswap_pages);
}

/*
 * "Store" data from a page to frontswap and associate it with the page's
 * swaptype and offset.  Page must be locked and in the swap cache.
 * If frontswap already contains a page with matching swaptype and
 * offset, the frontswap implementation may either overwrite the data and
 * return success or invalidate the page from frontswap and return failure.
 */
int __frontswap_store(struct page *page)
{
    int ret = -1, dup = 0;
    swp_entry_t entry = { .val = page_private(page), };
    int type = swp_type(entry);
    struct swap_info_struct *sis = swap_info[type];
    pgoff_t offset = swp_offset(entry);

    BUG_ON(!PageLocked(page));
    BUG_ON(sis == NULL);
    if (frontswap_test(sis, offset))
        dup = 1;
    ret = frontswap_ops.store(type, offset, page);
    if (ret == 0) {
        frontswap_set(sis, offset);
        inc_frontswap_succ_stores();
        if (!dup)
            atomic_inc(&sis->frontswap_pages);
    } else {
        /*
          failed dup always results in automatic invalidate of
          the (older) page from frontswap
         */
        inc_frontswap_failed_stores();
        if (dup)
            __frontswap_clear(sis, offset);
    }
    if (frontswap_writethrough_enabled)
        /* report failure so swap also writes to swap device */
        ret = -1;
    return ret;
}
EXPORT_SYMBOL(__frontswap_store);

/*
 * "Get" data from frontswap associated with swaptype and offset that were
 * specified when the data was put to frontswap and use it to fill the
 * specified page with data. Page must be locked and in the swap cache.
 */
int __frontswap_load(struct page *page)
{
    int ret = -1;
    swp_entry_t entry = { .val = page_private(page), };
    int type = swp_type(entry);
    struct swap_info_struct *sis = swap_info[type];
    pgoff_t offset = swp_offset(entry);

    BUG_ON(!PageLocked(page));
    BUG_ON(sis == NULL);
    if (frontswap_test(sis, offset))
        ret = frontswap_ops.load(type, offset, page);
    if (ret == 0) {
        inc_frontswap_loads();
        if (frontswap_tmem_exclusive_gets_enabled) {
            SetPageDirty(page);
            frontswap_clear(sis, offset);
        }
    }
    return ret;
}
EXPORT_SYMBOL(__frontswap_load);

/*
 * Invalidate any data from frontswap associated with the specified swaptype
 * and offset so that a subsequent "get" will fail.
 */
void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
{
    struct swap_info_struct *sis = swap_info[type];

    BUG_ON(sis == NULL);
    if (frontswap_test(sis, offset)) {
        frontswap_ops.invalidate_page(type, offset);
        __frontswap_clear(sis, offset);
        inc_frontswap_invalidates();
    }
}
EXPORT_SYMBOL(__frontswap_invalidate_page);

/*
 * Invalidate all data from frontswap associated with all offsets for the
 * specified swaptype.
 */
void __frontswap_invalidate_area(unsigned type)
{
    struct swap_info_struct *sis = swap_info[type];

    BUG_ON(sis == NULL);
    if (sis->frontswap_map == NULL)
        return;
    frontswap_ops.invalidate_area(type);
    atomic_set(&sis->frontswap_pages, 0);
    memset(sis->frontswap_map, 0, sis->max / sizeof(long));
}
EXPORT_SYMBOL(__frontswap_invalidate_area);

static unsigned long __frontswap_curr_pages(void)
{
    int type;
    unsigned long totalpages = 0;
    struct swap_info_struct *si = NULL;

    assert_spin_locked(&swap_lock);
    for (type = swap_list.head; type >= 0; type = si->next) {
        si = swap_info[type];
        totalpages += atomic_read(&si->frontswap_pages);
    }
    return totalpages;
}

static int __frontswap_unuse_pages(unsigned long total, unsigned long *unused,
                    int *swapid)
{
    int ret = -EINVAL;
    struct swap_info_struct *si = NULL;
    int si_frontswap_pages;
    unsigned long total_pages_to_unuse = total;
    unsigned long pages = 0, pages_to_unuse = 0;
    int type;

    assert_spin_locked(&swap_lock);
    for (type = swap_list.head; type >= 0; type = si->next) {
        si = swap_info[type];
        si_frontswap_pages = atomic_read(&si->frontswap_pages);
        if (total_pages_to_unuse < si_frontswap_pages) {
            pages = pages_to_unuse = total_pages_to_unuse;
        } else {
            pages = si_frontswap_pages;
            pages_to_unuse = 0; /* unuse all */
        }
        /* ensure there is enough RAM to fetch pages from frontswap */
        if (security_vm_enough_memory_mm(current->mm, pages)) {
            ret = -ENOMEM;
            continue;
        }
        vm_unacct_memory(pages);
        *unused = pages_to_unuse;
        *swapid = type;
        ret = 0;
        break;
    }

    return ret;
}

/*
 * Used to check if it's necessory and feasible to unuse pages.
 * Return 1 when nothing to do, 0 when need to shink pages,
 * error code when there is an error.
 */
static int __frontswap_shrink(unsigned long target_pages,
                unsigned long *pages_to_unuse,
                int *type)
{
    unsigned long total_pages = 0, total_pages_to_unuse;

    assert_spin_locked(&swap_lock);

    total_pages = __frontswap_curr_pages();
    if (total_pages <= target_pages) {
        /* Nothing to do */
        *pages_to_unuse = 0;
        return 1;
    }
    total_pages_to_unuse = total_pages - target_pages;
    return __frontswap_unuse_pages(total_pages_to_unuse, pages_to_unuse, type);
}

/*
 * Frontswap, like a true swap device, may unnecessarily retain pages
 * under certain circumstances; "shrink" frontswap is essentially a
 * "partial swapoff" and works by calling try_to_unuse to attempt to
 * unuse enough frontswap pages to attempt to -- subject to memory
 * constraints -- reduce the number of pages in frontswap to the
 * number given in the parameter target_pages.
 */
void frontswap_shrink(unsigned long target_pages)
{
    unsigned long pages_to_unuse = 0;
    int uninitialized_var(type), ret;

    /*
     * we don't want to hold swap_lock while doing a very
     * lengthy try_to_unuse, but swap_list may change
     * so restart scan from swap_list.head each time
     */
    spin_lock(&swap_lock);
    ret = __frontswap_shrink(target_pages, &pages_to_unuse, &type);
    spin_unlock(&swap_lock);
    if (ret == 0)
        try_to_unuse(type, true, pages_to_unuse);
    return;
}
EXPORT_SYMBOL(frontswap_shrink);

/*
 * Count and return the number of frontswap pages across all
 * swap devices.  This is exported so that backend drivers can
 * determine current usage without reading debugfs.
 */
unsigned long frontswap_curr_pages(void)
{
    unsigned long totalpages = 0;

    spin_lock(&swap_lock);
    totalpages = __frontswap_curr_pages();
    spin_unlock(&swap_lock);

    return totalpages;
}
EXPORT_SYMBOL(frontswap_curr_pages);

static int __init init_frontswap(void)
{
#ifdef CONFIG_DEBUG_FS
    struct dentry *root = debugfs_create_dir("frontswap", NULL);
    if (root == NULL)
        return -ENXIO;
    debugfs_create_u64("loads", S_IRUGO, root, &frontswap_loads);
    debugfs_create_u64("succ_stores", S_IRUGO, root, &frontswap_succ_stores);
    debugfs_create_u64("failed_stores", S_IRUGO, root,
                &frontswap_failed_stores);
    debugfs_create_u64("invalidates", S_IRUGO,
                root, &frontswap_invalidates);
#endif
    return 0;
}

module_init(init_frontswap);