dm-bufio.c

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/*
 * Copyright (C) 2009-2011 Red Hat, Inc.
 *
 * Author: Mikulas Patocka <[email protected]>
 *
 * This file is released under the GPL.
 */

#include "dm-bufio.h"

#include <linux/device-mapper.h>
#include <linux/dm-io.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/shrinker.h>
#include <linux/module.h>

#define DM_MSG_PREFIX "bufio"

/*
 * Memory management policy:
 *  Limit the number of buffers to DM_BUFIO_MEMORY_PERCENT of main memory
 *  or DM_BUFIO_VMALLOC_PERCENT of vmalloc memory (whichever is lower).
 *  Always allocate at least DM_BUFIO_MIN_BUFFERS buffers.
 *  Start background writeback when there are DM_BUFIO_WRITEBACK_PERCENT
 *  dirty buffers.
 */
#define DM_BUFIO_MIN_BUFFERS        8

#define DM_BUFIO_MEMORY_PERCENT     2
#define DM_BUFIO_VMALLOC_PERCENT    25
#define DM_BUFIO_WRITEBACK_PERCENT  75

/*
 * Check buffer ages in this interval (seconds)
 */
#define DM_BUFIO_WORK_TIMER_SECS    10

/*
 * Free buffers when they are older than this (seconds)
 */
#define DM_BUFIO_DEFAULT_AGE_SECS   60

/*
 * The number of bvec entries that are embedded directly in the buffer.
 * If the chunk size is larger, dm-io is used to do the io.
 */
#define DM_BUFIO_INLINE_VECS        16

/*
 * Buffer hash
 */
#define DM_BUFIO_HASH_BITS  20
#define DM_BUFIO_HASH(block) \
    ((((block) >> DM_BUFIO_HASH_BITS) ^ (block)) & \
     ((1 << DM_BUFIO_HASH_BITS) - 1))

/*
 * Don't try to use kmem_cache_alloc for blocks larger than this.
 * For explanation, see alloc_buffer_data below.
 */
#define DM_BUFIO_BLOCK_SIZE_SLAB_LIMIT  (PAGE_SIZE >> 1)
#define DM_BUFIO_BLOCK_SIZE_GFP_LIMIT   (PAGE_SIZE << (MAX_ORDER - 1))

/*
 * dm_buffer->list_mode
 */
#define LIST_CLEAN  0
#define LIST_DIRTY  1
#define LIST_SIZE   2

/*
 * Linking of buffers:
 *  All buffers are linked to cache_hash with their hash_list field.
 *
 *  Clean buffers that are not being written (B_WRITING not set)
 *  are linked to lru[LIST_CLEAN] with their lru_list field.
 *
 *  Dirty and clean buffers that are being written are linked to
 *  lru[LIST_DIRTY] with their lru_list field. When the write
 *  finishes, the buffer cannot be relinked immediately (because we
 *  are in an interrupt context and relinking requires process
 *  context), so some clean-not-writing buffers can be held on
 *  dirty_lru too.  They are later added to lru in the process
 *  context.
 */
struct dm_bufio_client {
    struct mutex lock;

    struct list_head lru[LIST_SIZE];
    unsigned long n_buffers[LIST_SIZE];

    struct block_device *bdev;
    unsigned block_size;
    unsigned char sectors_per_block_bits;
    unsigned char pages_per_block_bits;
    unsigned char blocks_per_page_bits;
    unsigned aux_size;
    void (*alloc_callback)(struct dm_buffer *);
    void (*write_callback)(struct dm_buffer *);

    struct dm_io_client *dm_io;

    struct list_head reserved_buffers;
    unsigned need_reserved_buffers;

    struct hlist_head *cache_hash;
    wait_queue_head_t free_buffer_wait;

    int async_write_error;

    struct list_head client_list;
    struct shrinker shrinker;
};

/*
 * Buffer state bits.
 */
#define B_READING   0
#define B_WRITING   1
#define B_DIRTY     2

/*
 * Describes how the block was allocated:
 * kmem_cache_alloc(), __get_free_pages() or vmalloc().
 * See the comment at alloc_buffer_data.
 */
enum data_mode {
    DATA_MODE_SLAB = 0,
    DATA_MODE_GET_FREE_PAGES = 1,
    DATA_MODE_VMALLOC = 2,
    DATA_MODE_LIMIT = 3
};

struct dm_buffer {
    struct hlist_node hash_list;
    struct list_head lru_list;
    sector_t block;
    void *data;
    enum data_mode data_mode;
    unsigned char list_mode;        /* LIST_* */
    unsigned hold_count;
    int read_error;
    int write_error;
    unsigned long state;
    unsigned long last_accessed;
    struct dm_bufio_client *c;
    struct bio bio;
    struct bio_vec bio_vec[DM_BUFIO_INLINE_VECS];
};

/*----------------------------------------------------------------*/

static struct kmem_cache *dm_bufio_caches[PAGE_SHIFT - SECTOR_SHIFT];
static char *dm_bufio_cache_names[PAGE_SHIFT - SECTOR_SHIFT];

static inline int dm_bufio_cache_index(struct dm_bufio_client *c)
{
    unsigned ret = c->blocks_per_page_bits - 1;

    BUG_ON(ret >= ARRAY_SIZE(dm_bufio_caches));

    return ret;
}

#define DM_BUFIO_CACHE(c)   (dm_bufio_caches[dm_bufio_cache_index(c)])
#define DM_BUFIO_CACHE_NAME(c)  (dm_bufio_cache_names[dm_bufio_cache_index(c)])

#define dm_bufio_in_request()   (!!current->bio_list)

static void dm_bufio_lock(struct dm_bufio_client *c)
{
    mutex_lock_nested(&c->lock, dm_bufio_in_request());
}

static int dm_bufio_trylock(struct dm_bufio_client *c)
{
    return mutex_trylock(&c->lock);
}

static void dm_bufio_unlock(struct dm_bufio_client *c)
{
    mutex_unlock(&c->lock);
}

/*
 * FIXME Move to sched.h?
 */
#ifdef CONFIG_PREEMPT_VOLUNTARY
#  define dm_bufio_cond_resched()       \
do {                        \
    if (unlikely(need_resched()))       \
        _cond_resched();        \
} while (0)
#else
#  define dm_bufio_cond_resched()                do { } while (0)
#endif

/*----------------------------------------------------------------*/

/*
 * Default cache size: available memory divided by the ratio.
 */
static unsigned long dm_bufio_default_cache_size;

/*
 * Total cache size set by the user.
 */
static unsigned long dm_bufio_cache_size;

/*
 * A copy of dm_bufio_cache_size because dm_bufio_cache_size can change
 * at any time.  If it disagrees, the user has changed cache size.
 */
static unsigned long dm_bufio_cache_size_latch;

static DEFINE_SPINLOCK(param_spinlock);

/*
 * Buffers are freed after this timeout
 */
static unsigned dm_bufio_max_age = DM_BUFIO_DEFAULT_AGE_SECS;

static unsigned long dm_bufio_peak_allocated;
static unsigned long dm_bufio_allocated_kmem_cache;
static unsigned long dm_bufio_allocated_get_free_pages;
static unsigned long dm_bufio_allocated_vmalloc;
static unsigned long dm_bufio_current_allocated;

/*----------------------------------------------------------------*/

/*
 * Per-client cache: dm_bufio_cache_size / dm_bufio_client_count
 */
static unsigned long dm_bufio_cache_size_per_client;

/*
 * The current number of clients.
 */
static int dm_bufio_client_count;

/*
 * The list of all clients.
 */
static LIST_HEAD(dm_bufio_all_clients);

/*
 * This mutex protects dm_bufio_cache_size_latch,
 * dm_bufio_cache_size_per_client and dm_bufio_client_count
 */
static DEFINE_MUTEX(dm_bufio_clients_lock);

/*----------------------------------------------------------------*/

static void adjust_total_allocated(enum data_mode data_mode, long diff)
{
    static unsigned long * const class_ptr[DATA_MODE_LIMIT] = {
        &dm_bufio_allocated_kmem_cache,
        &dm_bufio_allocated_get_free_pages,
        &dm_bufio_allocated_vmalloc,
    };

    spin_lock(&param_spinlock);

    *class_ptr[data_mode] += diff;

    dm_bufio_current_allocated += diff;

    if (dm_bufio_current_allocated > dm_bufio_peak_allocated)
        dm_bufio_peak_allocated = dm_bufio_current_allocated;

    spin_unlock(&param_spinlock);
}

/*
 * Change the number of clients and recalculate per-client limit.
 */
static void __cache_size_refresh(void)
{
    BUG_ON(!mutex_is_locked(&dm_bufio_clients_lock));
    BUG_ON(dm_bufio_client_count < 0);

    dm_bufio_cache_size_latch = ACCESS_ONCE(dm_bufio_cache_size);

    /*
     * Use default if set to 0 and report the actual cache size used.
     */
    if (!dm_bufio_cache_size_latch) {
        (void)cmpxchg(&dm_bufio_cache_size, 0,
                  dm_bufio_default_cache_size);
        dm_bufio_cache_size_latch = dm_bufio_default_cache_size;
    }

    dm_bufio_cache_size_per_client = dm_bufio_cache_size_latch /
                     (dm_bufio_client_count ? : 1);
}

/*
 * Allocating buffer data.
 *
 * Small buffers are allocated with kmem_cache, to use space optimally.
 *
 * For large buffers, we choose between get_free_pages and vmalloc.
 * Each has advantages and disadvantages.
 *
 * __get_free_pages can randomly fail if the memory is fragmented.
 * __vmalloc won't randomly fail, but vmalloc space is limited (it may be
 * as low as 128M) so using it for caching is not appropriate.
 *
 * If the allocation may fail we use __get_free_pages. Memory fragmentation
 * won't have a fatal effect here, but it just causes flushes of some other
 * buffers and more I/O will be performed. Don't use __get_free_pages if it
 * always fails (i.e. order >= MAX_ORDER).
 *
 * If the allocation shouldn't fail we use __vmalloc. This is only for the
 * initial reserve allocation, so there's no risk of wasting all vmalloc
 * space.
 */
static void *alloc_buffer_data(struct dm_bufio_client *c, gfp_t gfp_mask,
                   enum data_mode *data_mode)
{
    if (c->block_size <= DM_BUFIO_BLOCK_SIZE_SLAB_LIMIT) {
        *data_mode = DATA_MODE_SLAB;
        return kmem_cache_alloc(DM_BUFIO_CACHE(c), gfp_mask);
    }

    if (c->block_size <= DM_BUFIO_BLOCK_SIZE_GFP_LIMIT &&
        gfp_mask & __GFP_NORETRY) {
        *data_mode = DATA_MODE_GET_FREE_PAGES;
        return (void *)__get_free_pages(gfp_mask,
                        c->pages_per_block_bits);
    }

    *data_mode = DATA_MODE_VMALLOC;
    return __vmalloc(c->block_size, gfp_mask, PAGE_KERNEL);
}

/*
 * Free buffer's data.
 */
static void free_buffer_data(struct dm_bufio_client *c,
                 void *data, enum data_mode data_mode)
{
    switch (data_mode) {
    case DATA_MODE_SLAB:
        kmem_cache_free(DM_BUFIO_CACHE(c), data);
        break;

    case DATA_MODE_GET_FREE_PAGES:
        free_pages((unsigned long)data, c->pages_per_block_bits);
        break;

    case DATA_MODE_VMALLOC:
        vfree(data);
        break;

    default:
        DMCRIT("dm_bufio_free_buffer_data: bad data mode: %d",
               data_mode);
        BUG();
    }
}

/*
 * Allocate buffer and its data.
 */
static struct dm_buffer *alloc_buffer(struct dm_bufio_client *c, gfp_t gfp_mask)
{
    struct dm_buffer *b = kmalloc(sizeof(struct dm_buffer) + c->aux_size,
                      gfp_mask);

    if (!b)
        return NULL;

    b->c = c;

    b->data = alloc_buffer_data(c, gfp_mask, &b->data_mode);
    if (!b->data) {
        kfree(b);
        return NULL;
    }

    adjust_total_allocated(b->data_mode, (long)c->block_size);

    return b;
}

/*
 * Free buffer and its data.
 */
static void free_buffer(struct dm_buffer *b)
{
    struct dm_bufio_client *c = b->c;

    adjust_total_allocated(b->data_mode, -(long)c->block_size);

    free_buffer_data(c, b->data, b->data_mode);
    kfree(b);
}

/*
 * Link buffer to the hash list and clean or dirty queue.
 */
static void __link_buffer(struct dm_buffer *b, sector_t block, int dirty)
{
    struct dm_bufio_client *c = b->c;

    c->n_buffers[dirty]++;
    b->block = block;
    b->list_mode = dirty;
    list_add(&b->lru_list, &c->lru[dirty]);
    hlist_add_head(&b->hash_list, &c->cache_hash[DM_BUFIO_HASH(block)]);
    b->last_accessed = jiffies;
}

/*
 * Unlink buffer from the hash list and dirty or clean queue.
 */
static void __unlink_buffer(struct dm_buffer *b)
{
    struct dm_bufio_client *c = b->c;

    BUG_ON(!c->n_buffers[b->list_mode]);

    c->n_buffers[b->list_mode]--;
    hlist_del(&b->hash_list);
    list_del(&b->lru_list);
}

/*
 * Place the buffer to the head of dirty or clean LRU queue.
 */
static void __relink_lru(struct dm_buffer *b, int dirty)
{
    struct dm_bufio_client *c = b->c;

    BUG_ON(!c->n_buffers[b->list_mode]);

    c->n_buffers[b->list_mode]--;
    c->n_buffers[dirty]++;
    b->list_mode = dirty;
    list_move(&b->lru_list, &c->lru[dirty]);
}

/*----------------------------------------------------------------
 * Submit I/O on the buffer.
 *
 * Bio interface is faster but it has some problems:
 *  the vector list is limited (increasing this limit increases
 *  memory-consumption per buffer, so it is not viable);
 *
 *  the memory must be direct-mapped, not vmalloced;
 *
 *  the I/O driver can reject requests spuriously if it thinks that
 *  the requests are too big for the device or if they cross a
 *  controller-defined memory boundary.
 *
 * If the buffer is small enough (up to DM_BUFIO_INLINE_VECS pages) and
 * it is not vmalloced, try using the bio interface.
 *
 * If the buffer is big, if it is vmalloced or if the underlying device
 * rejects the bio because it is too large, use dm-io layer to do the I/O.
 * The dm-io layer splits the I/O into multiple requests, avoiding the above
 * shortcomings.
 *--------------------------------------------------------------*/

/*
 * dm-io completion routine. It just calls b->bio.bi_end_io, pretending
 * that the request was handled directly with bio interface.
 */
static void dmio_complete(unsigned long error, void *context)
{
    struct dm_buffer *b = context;

    b->bio.bi_end_io(&b->bio, error ? -EIO : 0);
}

static void use_dmio(struct dm_buffer *b, int rw, sector_t block,
             bio_end_io_t *end_io)
{
    int r;
    struct dm_io_request io_req = {
        .bi_rw = rw,
        .notify.fn = dmio_complete,
        .notify.context = b,
        .client = b->c->dm_io,
    };
    struct dm_io_region region = {
        .bdev = b->c->bdev,
        .sector = block << b->c->sectors_per_block_bits,
        .count = b->c->block_size >> SECTOR_SHIFT,
    };

    if (b->data_mode != DATA_MODE_VMALLOC) {
        io_req.mem.type = DM_IO_KMEM;
        io_req.mem.ptr.addr = b->data;
    } else {
        io_req.mem.type = DM_IO_VMA;
        io_req.mem.ptr.vma = b->data;
    }

    b->bio.bi_end_io = end_io;

    r = dm_io(&io_req, 1, &region, NULL);
    if (r)
        end_io(&b->bio, r);
}

static void use_inline_bio(struct dm_buffer *b, int rw, sector_t block,
               bio_end_io_t *end_io)
{
    char *ptr;
    int len;

    bio_init(&b->bio);
    b->bio.bi_io_vec = b->bio_vec;
    b->bio.bi_max_vecs = DM_BUFIO_INLINE_VECS;
    b->bio.bi_sector = block << b->c->sectors_per_block_bits;
    b->bio.bi_bdev = b->c->bdev;
    b->bio.bi_end_io = end_io;

    /*
     * We assume that if len >= PAGE_SIZE ptr is page-aligned.
     * If len < PAGE_SIZE the buffer doesn't cross page boundary.
     */
    ptr = b->data;
    len = b->c->block_size;

    if (len >= PAGE_SIZE)
        BUG_ON((unsigned long)ptr & (PAGE_SIZE - 1));
    else
        BUG_ON((unsigned long)ptr & (len - 1));

    do {
        if (!bio_add_page(&b->bio, virt_to_page(ptr),
                  len < PAGE_SIZE ? len : PAGE_SIZE,
                  virt_to_phys(ptr) & (PAGE_SIZE - 1))) {
            BUG_ON(b->c->block_size <= PAGE_SIZE);
            use_dmio(b, rw, block, end_io);
            return;
        }

        len -= PAGE_SIZE;
        ptr += PAGE_SIZE;
    } while (len > 0);

    submit_bio(rw, &b->bio);
}

static void submit_io(struct dm_buffer *b, int rw, sector_t block,
              bio_end_io_t *end_io)
{
    if (rw == WRITE && b->c->write_callback)
        b->c->write_callback(b);

    if (b->c->block_size <= DM_BUFIO_INLINE_VECS * PAGE_SIZE &&
        b->data_mode != DATA_MODE_VMALLOC)
        use_inline_bio(b, rw, block, end_io);
    else
        use_dmio(b, rw, block, end_io);
}

/*----------------------------------------------------------------
 * Writing dirty buffers
 *--------------------------------------------------------------*/

/*
 * The endio routine for write.
 *
 * Set the error, clear B_WRITING bit and wake anyone who was waiting on
 * it.
 */
static void write_endio(struct bio *bio, int error)
{
    struct dm_buffer *b = container_of(bio, struct dm_buffer, bio);

    b->write_error = error;
    if (unlikely(error)) {
        struct dm_bufio_client *c = b->c;
        (void)cmpxchg(&c->async_write_error, 0, error);
    }

    BUG_ON(!test_bit(B_WRITING, &b->state));

    smp_mb__before_clear_bit();
    clear_bit(B_WRITING, &b->state);
    smp_mb__after_clear_bit();

    wake_up_bit(&b->state, B_WRITING);
}

/*
 * This function is called when wait_on_bit is actually waiting.
 */
static int do_io_schedule(void *word)
{
    io_schedule();

    return 0;
}

/*
 * Initiate a write on a dirty buffer, but don't wait for it.
 *
 * - If the buffer is not dirty, exit.
 * - If there some previous write going on, wait for it to finish (we can't
 *   have two writes on the same buffer simultaneously).
 * - Submit our write and don't wait on it. We set B_WRITING indicating
 *   that there is a write in progress.
 */
static void __write_dirty_buffer(struct dm_buffer *b)
{
    if (!test_bit(B_DIRTY, &b->state))
        return;

    clear_bit(B_DIRTY, &b->state);
    wait_on_bit_lock(&b->state, B_WRITING,
             do_io_schedule, TASK_UNINTERRUPTIBLE);

    submit_io(b, WRITE, b->block, write_endio);
}

/*
 * Wait until any activity on the buffer finishes.  Possibly write the
 * buffer if it is dirty.  When this function finishes, there is no I/O
 * running on the buffer and the buffer is not dirty.
 */
static void __make_buffer_clean(struct dm_buffer *b)
{
    BUG_ON(b->hold_count);

    if (!b->state)  /* fast case */
        return;

    wait_on_bit(&b->state, B_READING, do_io_schedule, TASK_UNINTERRUPTIBLE);
    __write_dirty_buffer(b);
    wait_on_bit(&b->state, B_WRITING, do_io_schedule, TASK_UNINTERRUPTIBLE);
}

/*
 * Find some buffer that is not held by anybody, clean it, unlink it and
 * return it.
 */
static struct dm_buffer *__get_unclaimed_buffer(struct dm_bufio_client *c)
{
    struct dm_buffer *b;

    list_for_each_entry_reverse(b, &c->lru[LIST_CLEAN], lru_list) {
        BUG_ON(test_bit(B_WRITING, &b->state));
        BUG_ON(test_bit(B_DIRTY, &b->state));

        if (!b->hold_count) {
            __make_buffer_clean(b);
            __unlink_buffer(b);
            return b;
        }
        dm_bufio_cond_resched();
    }

    list_for_each_entry_reverse(b, &c->lru[LIST_DIRTY], lru_list) {
        BUG_ON(test_bit(B_READING, &b->state));

        if (!b->hold_count) {
            __make_buffer_clean(b);
            __unlink_buffer(b);
            return b;
        }
        dm_bufio_cond_resched();
    }

    return NULL;
}

/*
 * Wait until some other threads free some buffer or release hold count on
 * some buffer.
 *
 * This function is entered with c->lock held, drops it and regains it
 * before exiting.
 */
static void __wait_for_free_buffer(struct dm_bufio_client *c)
{
    DECLARE_WAITQUEUE(wait, current);

    add_wait_queue(&c->free_buffer_wait, &wait);
    set_task_state(current, TASK_UNINTERRUPTIBLE);
    dm_bufio_unlock(c);

    io_schedule();

    set_task_state(current, TASK_RUNNING);
    remove_wait_queue(&c->free_buffer_wait, &wait);

    dm_bufio_lock(c);
}

enum new_flag {
    NF_FRESH = 0,
    NF_READ = 1,
    NF_GET = 2,
    NF_PREFETCH = 3
};

/*
 * Allocate a new buffer. If the allocation is not possible, wait until
 * some other thread frees a buffer.
 *
 * May drop the lock and regain it.
 */
static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client *c, enum new_flag nf)
{
    struct dm_buffer *b;

    /*
     * dm-bufio is resistant to allocation failures (it just keeps
     * one buffer reserved in cases all the allocations fail).
     * So set flags to not try too hard:
     *  GFP_NOIO: don't recurse into the I/O layer
     *  __GFP_NORETRY: don't retry and rather return failure
     *  __GFP_NOMEMALLOC: don't use emergency reserves
     *  __GFP_NOWARN: don't print a warning in case of failure
     *
     * For debugging, if we set the cache size to 1, no new buffers will
     * be allocated.
     */
    while (1) {
        if (dm_bufio_cache_size_latch != 1) {
            b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
            if (b)
                return b;
        }

        if (nf == NF_PREFETCH)
            return NULL;

        if (!list_empty(&c->reserved_buffers)) {
            b = list_entry(c->reserved_buffers.next,
                       struct dm_buffer, lru_list);
            list_del(&b->lru_list);
            c->need_reserved_buffers++;

            return b;
        }

        b = __get_unclaimed_buffer(c);
        if (b)
            return b;

        __wait_for_free_buffer(c);
    }
}

static struct dm_buffer *__alloc_buffer_wait(struct dm_bufio_client *c, enum new_flag nf)
{
    struct dm_buffer *b = __alloc_buffer_wait_no_callback(c, nf);

    if (!b)
        return NULL;

    if (c->alloc_callback)
        c->alloc_callback(b);

    return b;
}

/*
 * Free a buffer and wake other threads waiting for free buffers.
 */
static void __free_buffer_wake(struct dm_buffer *b)
{
    struct dm_bufio_client *c = b->c;

    if (!c->need_reserved_buffers)
        free_buffer(b);
    else {
        list_add(&b->lru_list, &c->reserved_buffers);
        c->need_reserved_buffers--;
    }

    wake_up(&c->free_buffer_wait);
}

static void __write_dirty_buffers_async(struct dm_bufio_client *c, int no_wait)
{
    struct dm_buffer *b, *tmp;

    list_for_each_entry_safe_reverse(b, tmp, &c->lru[LIST_DIRTY], lru_list) {
        BUG_ON(test_bit(B_READING, &b->state));

        if (!test_bit(B_DIRTY, &b->state) &&
            !test_bit(B_WRITING, &b->state)) {
            __relink_lru(b, LIST_CLEAN);
            continue;
        }

        if (no_wait && test_bit(B_WRITING, &b->state))
            return;

        __write_dirty_buffer(b);
        dm_bufio_cond_resched();
    }
}

/*
 * Get writeback threshold and buffer limit for a given client.
 */
static void __get_memory_limit(struct dm_bufio_client *c,
                   unsigned long *threshold_buffers,
                   unsigned long *limit_buffers)
{
    unsigned long buffers;

    if (ACCESS_ONCE(dm_bufio_cache_size) != dm_bufio_cache_size_latch) {
        mutex_lock(&dm_bufio_clients_lock);
        __cache_size_refresh();
        mutex_unlock(&dm_bufio_clients_lock);
    }

    buffers = dm_bufio_cache_size_per_client >>
          (c->sectors_per_block_bits + SECTOR_SHIFT);

    if (buffers < DM_BUFIO_MIN_BUFFERS)
        buffers = DM_BUFIO_MIN_BUFFERS;

    *limit_buffers = buffers;
    *threshold_buffers = buffers * DM_BUFIO_WRITEBACK_PERCENT / 100;
}

/*
 * Check if we're over watermark.
 * If we are over threshold_buffers, start freeing buffers.
 * If we're over "limit_buffers", block until we get under the limit.
 */
static void __check_watermark(struct dm_bufio_client *c)
{
    unsigned long threshold_buffers, limit_buffers;

    __get_memory_limit(c, &threshold_buffers, &limit_buffers);

    while (c->n_buffers[LIST_CLEAN] + c->n_buffers[LIST_DIRTY] >
           limit_buffers) {

        struct dm_buffer *b = __get_unclaimed_buffer(c);

        if (!b)
            return;

        __free_buffer_wake(b);
        dm_bufio_cond_resched();
    }

    if (c->n_buffers[LIST_DIRTY] > threshold_buffers)
        __write_dirty_buffers_async(c, 1);
}

/*
 * Find a buffer in the hash.
 */
static struct dm_buffer *__find(struct dm_bufio_client *c, sector_t block)
{
    struct dm_buffer *b;
    struct hlist_node *hn;

    hlist_for_each_entry(b, hn, &c->cache_hash[DM_BUFIO_HASH(block)],
                 hash_list) {
        dm_bufio_cond_resched();
        if (b->block == block)
            return b;
    }

    return NULL;
}

/*----------------------------------------------------------------
 * Getting a buffer
 *--------------------------------------------------------------*/

static struct dm_buffer *__bufio_new(struct dm_bufio_client *c, sector_t block,
                     enum new_flag nf, int *need_submit)
{
    struct dm_buffer *b, *new_b = NULL;

    *need_submit = 0;

    b = __find(c, block);
    if (b)
        goto found_buffer;

    if (nf == NF_GET)
        return NULL;

    new_b = __alloc_buffer_wait(c, nf);
    if (!new_b)
        return NULL;

    /*
     * We've had a period where the mutex was unlocked, so need to
     * recheck the hash table.
     */
    b = __find(c, block);
    if (b) {
        __free_buffer_wake(new_b);
        goto found_buffer;
    }

    __check_watermark(c);

    b = new_b;
    b->hold_count = 1;
    b->read_error = 0;
    b->write_error = 0;
    __link_buffer(b, block, LIST_CLEAN);

    if (nf == NF_FRESH) {
        b->state = 0;
        return b;
    }

    b->state = 1 << B_READING;
    *need_submit = 1;

    return b;

found_buffer:
    if (nf == NF_PREFETCH)
        return NULL;
    /*
     * Note: it is essential that we don't wait for the buffer to be
     * read if dm_bufio_get function is used. Both dm_bufio_get and
     * dm_bufio_prefetch can be used in the driver request routine.
     * If the user called both dm_bufio_prefetch and dm_bufio_get on
     * the same buffer, it would deadlock if we waited.
     */
    if (nf == NF_GET && unlikely(test_bit(B_READING, &b->state)))
        return NULL;

    b->hold_count++;
    __relink_lru(b, test_bit(B_DIRTY, &b->state) ||
             test_bit(B_WRITING, &b->state));
    return b;
}

/*
 * The endio routine for reading: set the error, clear the bit and wake up
 * anyone waiting on the buffer.
 */
static void read_endio(struct bio *bio, int error)
{
    struct dm_buffer *b = container_of(bio, struct dm_buffer, bio);

    b->read_error = error;

    BUG_ON(!test_bit(B_READING, &b->state));

    smp_mb__before_clear_bit();
    clear_bit(B_READING, &b->state);
    smp_mb__after_clear_bit();

    wake_up_bit(&b->state, B_READING);
}

/*
 * A common routine for dm_bufio_new and dm_bufio_read.  Operation of these
 * functions is similar except that dm_bufio_new doesn't read the
 * buffer from the disk (assuming that the caller overwrites all the data
 * and uses dm_bufio_mark_buffer_dirty to write new data back).
 */
static void *new_read(struct dm_bufio_client *c, sector_t block,
              enum new_flag nf, struct dm_buffer **bp)
{
    int need_submit;
    struct dm_buffer *b;

    dm_bufio_lock(c);
    b = __bufio_new(c, block, nf, &need_submit);
    dm_bufio_unlock(c);

    if (!b)
        return b;

    if (need_submit)
        submit_io(b, READ, b->block, read_endio);

    wait_on_bit(&b->state, B_READING, do_io_schedule, TASK_UNINTERRUPTIBLE);

    if (b->read_error) {
        int error = b->read_error;

        dm_bufio_release(b);

        return ERR_PTR(error);
    }

    *bp = b;

    return b->data;
}

void *dm_bufio_get(struct dm_bufio_client *c, sector_t block,
           struct dm_buffer **bp)
{
    return new_read(c, block, NF_GET, bp);
}
EXPORT_SYMBOL_GPL(dm_bufio_get);

void *dm_bufio_read(struct dm_bufio_client *c, sector_t block,
            struct dm_buffer **bp)
{
    BUG_ON(dm_bufio_in_request());

    return new_read(c, block, NF_READ, bp);
}
EXPORT_SYMBOL_GPL(dm_bufio_read);

void *dm_bufio_new(struct dm_bufio_client *c, sector_t block,
           struct dm_buffer **bp)
{
    BUG_ON(dm_bufio_in_request());

    return new_read(c, block, NF_FRESH, bp);
}
EXPORT_SYMBOL_GPL(dm_bufio_new);

void dm_bufio_prefetch(struct dm_bufio_client *c,
               sector_t block, unsigned n_blocks)
{
    struct blk_plug plug;

    blk_start_plug(&plug);
    dm_bufio_lock(c);

    for (; n_blocks--; block++) {
        int need_submit;
        struct dm_buffer *b;
        b = __bufio_new(c, block, NF_PREFETCH, &need_submit);
        if (unlikely(b != NULL)) {
            dm_bufio_unlock(c);

            if (need_submit)
                submit_io(b, READ, b->block, read_endio);
            dm_bufio_release(b);

            dm_bufio_cond_resched();

            if (!n_blocks)
                goto flush_plug;
            dm_bufio_lock(c);
        }

    }

    dm_bufio_unlock(c);

flush_plug:
    blk_finish_plug(&plug);
}
EXPORT_SYMBOL_GPL(dm_bufio_prefetch);

void dm_bufio_release(struct dm_buffer *b)
{
    struct dm_bufio_client *c = b->c;

    dm_bufio_lock(c);

    BUG_ON(!b->hold_count);

    b->hold_count--;
    if (!b->hold_count) {
        wake_up(&c->free_buffer_wait);

        /*
         * If there were errors on the buffer, and the buffer is not
         * to be written, free the buffer. There is no point in caching
         * invalid buffer.
         */
        if ((b->read_error || b->write_error) &&
            !test_bit(B_READING, &b->state) &&
            !test_bit(B_WRITING, &b->state) &&
            !test_bit(B_DIRTY, &b->state)) {
            __unlink_buffer(b);
            __free_buffer_wake(b);
        }
    }

    dm_bufio_unlock(c);
}
EXPORT_SYMBOL_GPL(dm_bufio_release);

void dm_bufio_mark_buffer_dirty(struct dm_buffer *b)
{
    struct dm_bufio_client *c = b->c;

    dm_bufio_lock(c);

    BUG_ON(test_bit(B_READING, &b->state));

    if (!test_and_set_bit(B_DIRTY, &b->state))
        __relink_lru(b, LIST_DIRTY);

    dm_bufio_unlock(c);
}
EXPORT_SYMBOL_GPL(dm_bufio_mark_buffer_dirty);

void dm_bufio_write_dirty_buffers_async(struct dm_bufio_client *c)
{
    BUG_ON(dm_bufio_in_request());

    dm_bufio_lock(c);
    __write_dirty_buffers_async(c, 0);
    dm_bufio_unlock(c);
}
EXPORT_SYMBOL_GPL(dm_bufio_write_dirty_buffers_async);

/*
 * For performance, it is essential that the buffers are written asynchronously
 * and simultaneously (so that the block layer can merge the writes) and then
 * waited upon.
 *
 * Finally, we flush hardware disk cache.
 */
int dm_bufio_write_dirty_buffers(struct dm_bufio_client *c)
{
    int a, f;
    unsigned long buffers_processed = 0;
    struct dm_buffer *b, *tmp;

    dm_bufio_lock(c);
    __write_dirty_buffers_async(c, 0);

again:
    list_for_each_entry_safe_reverse(b, tmp, &c->lru[LIST_DIRTY], lru_list) {
        int dropped_lock = 0;

        if (buffers_processed < c->n_buffers[LIST_DIRTY])
            buffers_processed++;

        BUG_ON(test_bit(B_READING, &b->state));

        if (test_bit(B_WRITING, &b->state)) {
            if (buffers_processed < c->n_buffers[LIST_DIRTY]) {
                dropped_lock = 1;
                b->hold_count++;
                dm_bufio_unlock(c);
                wait_on_bit(&b->state, B_WRITING,
                        do_io_schedule,
                        TASK_UNINTERRUPTIBLE);
                dm_bufio_lock(c);
                b->hold_count--;
            } else
                wait_on_bit(&b->state, B_WRITING,
                        do_io_schedule,
                        TASK_UNINTERRUPTIBLE);
        }

        if (!test_bit(B_DIRTY, &b->state) &&
            !test_bit(B_WRITING, &b->state))
            __relink_lru(b, LIST_CLEAN);

        dm_bufio_cond_resched();

        /*
         * If we dropped the lock, the list is no longer consistent,
         * so we must restart the search.
         *
         * In the most common case, the buffer just processed is
         * relinked to the clean list, so we won't loop scanning the
         * same buffer again and again.
         *
         * This may livelock if there is another thread simultaneously
         * dirtying buffers, so we count the number of buffers walked
         * and if it exceeds the total number of buffers, it means that
         * someone is doing some writes simultaneously with us.  In
         * this case, stop, dropping the lock.
         */
        if (dropped_lock)
            goto again;
    }
    wake_up(&c->free_buffer_wait);
    dm_bufio_unlock(c);

    a = xchg(&c->async_write_error, 0);
    f = dm_bufio_issue_flush(c);
    if (a)
        return a;

    return f;
}
EXPORT_SYMBOL_GPL(dm_bufio_write_dirty_buffers);

/*
 * Use dm-io to send and empty barrier flush the device.
 */
int dm_bufio_issue_flush(struct dm_bufio_client *c)
{
    struct dm_io_request io_req = {
        .bi_rw = REQ_FLUSH,
        .mem.type = DM_IO_KMEM,
        .mem.ptr.addr = NULL,
        .client = c->dm_io,
    };
    struct dm_io_region io_reg = {
        .bdev = c->bdev,
        .sector = 0,
        .count = 0,
    };

    BUG_ON(dm_bufio_in_request());

    return dm_io(&io_req, 1, &io_reg, NULL);
}
EXPORT_SYMBOL_GPL(dm_bufio_issue_flush);

/*
 * We first delete any other buffer that may be at that new location.
 *
 * Then, we write the buffer to the original location if it was dirty.
 *
 * Then, if we are the only one who is holding the buffer, relink the buffer
 * in the hash queue for the new location.
 *
 * If there was someone else holding the buffer, we write it to the new
 * location but not relink it, because that other user needs to have the buffer
 * at the same place.
 */
void dm_bufio_release_move(struct dm_buffer *b, sector_t new_block)
{
    struct dm_bufio_client *c = b->c;
    struct dm_buffer *new;

    BUG_ON(dm_bufio_in_request());

    dm_bufio_lock(c);

retry:
    new = __find(c, new_block);
    if (new) {
        if (new->hold_count) {
            __wait_for_free_buffer(c);
            goto retry;
        }

        /*
         * FIXME: Is there any point waiting for a write that's going
         * to be overwritten in a bit?
         */
        __make_buffer_clean(new);
        __unlink_buffer(new);
        __free_buffer_wake(new);
    }

    BUG_ON(!b->hold_count);
    BUG_ON(test_bit(B_READING, &b->state));

    __write_dirty_buffer(b);
    if (b->hold_count == 1) {
        wait_on_bit(&b->state, B_WRITING,
                do_io_schedule, TASK_UNINTERRUPTIBLE);
        set_bit(B_DIRTY, &b->state);
        __unlink_buffer(b);
        __link_buffer(b, new_block, LIST_DIRTY);
    } else {
        sector_t old_block;
        wait_on_bit_lock(&b->state, B_WRITING,
                 do_io_schedule, TASK_UNINTERRUPTIBLE);
        /*
         * Relink buffer to "new_block" so that write_callback
         * sees "new_block" as a block number.
         * After the write, link the buffer back to old_block.
         * All this must be done in bufio lock, so that block number
         * change isn't visible to other threads.
         */
        old_block = b->block;
        __unlink_buffer(b);
        __link_buffer(b, new_block, b->list_mode);
        submit_io(b, WRITE, new_block, write_endio);
        wait_on_bit(&b->state, B_WRITING,
                do_io_schedule, TASK_UNINTERRUPTIBLE);
        __unlink_buffer(b);
        __link_buffer(b, old_block, b->list_mode);
    }

    dm_bufio_unlock(c);
    dm_bufio_release(b);
}
EXPORT_SYMBOL_GPL(dm_bufio_release_move);

unsigned dm_bufio_get_block_size(struct dm_bufio_client *c)
{
    return c->block_size;
}
EXPORT_SYMBOL_GPL(dm_bufio_get_block_size);

sector_t dm_bufio_get_device_size(struct dm_bufio_client *c)
{
    return i_size_read(c->bdev->bd_inode) >>
               (SECTOR_SHIFT + c->sectors_per_block_bits);
}
EXPORT_SYMBOL_GPL(dm_bufio_get_device_size);

sector_t dm_bufio_get_block_number(struct dm_buffer *b)
{
    return b->block;
}
EXPORT_SYMBOL_GPL(dm_bufio_get_block_number);

void *dm_bufio_get_block_data(struct dm_buffer *b)
{
    return b->data;
}
EXPORT_SYMBOL_GPL(dm_bufio_get_block_data);

void *dm_bufio_get_aux_data(struct dm_buffer *b)
{
    return b + 1;
}
EXPORT_SYMBOL_GPL(dm_bufio_get_aux_data);

struct dm_bufio_client *dm_bufio_get_client(struct dm_buffer *b)
{
    return b->c;
}
EXPORT_SYMBOL_GPL(dm_bufio_get_client);

static void drop_buffers(struct dm_bufio_client *c)
{
    struct dm_buffer *b;
    int i;

    BUG_ON(dm_bufio_in_request());

    /*
     * An optimization so that the buffers are not written one-by-one.
     */
    dm_bufio_write_dirty_buffers_async(c);

    dm_bufio_lock(c);

    while ((b = __get_unclaimed_buffer(c)))
        __free_buffer_wake(b);

    for (i = 0; i < LIST_SIZE; i++)
        list_for_each_entry(b, &c->lru[i], lru_list)
            DMERR("leaked buffer %llx, hold count %u, list %d",
                  (unsigned long long)b->block, b->hold_count, i);

    for (i = 0; i < LIST_SIZE; i++)
        BUG_ON(!list_empty(&c->lru[i]));

    dm_bufio_unlock(c);
}

/*
 * Test if the buffer is unused and too old, and commit it.
 * At if noio is set, we must not do any I/O because we hold
 * dm_bufio_clients_lock and we would risk deadlock if the I/O gets rerouted to
 * different bufio client.
 */
static int __cleanup_old_buffer(struct dm_buffer *b, gfp_t gfp,
                unsigned long max_jiffies)
{
    if (jiffies - b->last_accessed < max_jiffies)
        return 1;

    if (!(gfp & __GFP_IO)) {
        if (test_bit(B_READING, &b->state) ||
            test_bit(B_WRITING, &b->state) ||
            test_bit(B_DIRTY, &b->state))
            return 1;
    }

    if (b->hold_count)
        return 1;

    __make_buffer_clean(b);
    __unlink_buffer(b);
    __free_buffer_wake(b);

    return 0;
}

static void __scan(struct dm_bufio_client *c, unsigned long nr_to_scan,
           struct shrink_control *sc)
{
    int l;
    struct dm_buffer *b, *tmp;

    for (l = 0; l < LIST_SIZE; l++) {
        list_for_each_entry_safe_reverse(b, tmp, &c->lru[l], lru_list)
            if (!__cleanup_old_buffer(b, sc->gfp_mask, 0) &&
                !--nr_to_scan)
                return;
        dm_bufio_cond_resched();
    }
}

static int shrink(struct shrinker *shrinker, struct shrink_control *sc)
{
    struct dm_bufio_client *c =
        container_of(shrinker, struct dm_bufio_client, shrinker);
    unsigned long r;
    unsigned long nr_to_scan = sc->nr_to_scan;

    if (sc->gfp_mask & __GFP_IO)
        dm_bufio_lock(c);
    else if (!dm_bufio_trylock(c))
        return !nr_to_scan ? 0 : -1;

    if (nr_to_scan)
        __scan(c, nr_to_scan, sc);

    r = c->n_buffers[LIST_CLEAN] + c->n_buffers[LIST_DIRTY];
    if (r > INT_MAX)
        r = INT_MAX;

    dm_bufio_unlock(c);

    return r;
}

/*
 * Create the buffering interface
 */
struct dm_bufio_client *dm_bufio_client_create(struct block_device *bdev, unsigned block_size,
                           unsigned reserved_buffers, unsigned aux_size,
                           void (*alloc_callback)(struct dm_buffer *),
                           void (*write_callback)(struct dm_buffer *))
{
    int r;
    struct dm_bufio_client *c;
    unsigned i;

    BUG_ON(block_size < 1 << SECTOR_SHIFT ||
           (block_size & (block_size - 1)));

    c = kmalloc(sizeof(*c), GFP_KERNEL);
    if (!c) {
        r = -ENOMEM;
        goto bad_client;
    }
    c->cache_hash = vmalloc(sizeof(struct hlist_head) << DM_BUFIO_HASH_BITS);
    if (!c->cache_hash) {
        r = -ENOMEM;
        goto bad_hash;
    }

    c->bdev = bdev;
    c->block_size = block_size;
    c->sectors_per_block_bits = ffs(block_size) - 1 - SECTOR_SHIFT;
    c->pages_per_block_bits = (ffs(block_size) - 1 >= PAGE_SHIFT) ?
                  ffs(block_size) - 1 - PAGE_SHIFT : 0;
    c->blocks_per_page_bits = (ffs(block_size) - 1 < PAGE_SHIFT ?
                  PAGE_SHIFT - (ffs(block_size) - 1) : 0);

    c->aux_size = aux_size;
    c->alloc_callback = alloc_callback;
    c->write_callback = write_callback;

    for (i = 0; i < LIST_SIZE; i++) {
        INIT_LIST_HEAD(&c->lru[i]);
        c->n_buffers[i] = 0;
    }

    for (i = 0; i < 1 << DM_BUFIO_HASH_BITS; i++)
        INIT_HLIST_HEAD(&c->cache_hash[i]);

    mutex_init(&c->lock);
    INIT_LIST_HEAD(&c->reserved_buffers);
    c->need_reserved_buffers = reserved_buffers;

    init_waitqueue_head(&c->free_buffer_wait);
    c->async_write_error = 0;

    c->dm_io = dm_io_client_create();
    if (IS_ERR(c->dm_io)) {
        r = PTR_ERR(c->dm_io);
        goto bad_dm_io;
    }

    mutex_lock(&dm_bufio_clients_lock);
    if (c->blocks_per_page_bits) {
        if (!DM_BUFIO_CACHE_NAME(c)) {
            DM_BUFIO_CACHE_NAME(c) = kasprintf(GFP_KERNEL, "dm_bufio_cache-%u", c->block_size);
            if (!DM_BUFIO_CACHE_NAME(c)) {
                r = -ENOMEM;
                mutex_unlock(&dm_bufio_clients_lock);
                goto bad_cache;
            }
        }

        if (!DM_BUFIO_CACHE(c)) {
            DM_BUFIO_CACHE(c) = kmem_cache_create(DM_BUFIO_CACHE_NAME(c),
                                  c->block_size,
                                  c->block_size, 0, NULL);
            if (!DM_BUFIO_CACHE(c)) {
                r = -ENOMEM;
                mutex_unlock(&dm_bufio_clients_lock);
                goto bad_cache;
            }
        }
    }
    mutex_unlock(&dm_bufio_clients_lock);

    while (c->need_reserved_buffers) {
        struct dm_buffer *b = alloc_buffer(c, GFP_KERNEL);

        if (!b) {
            r = -ENOMEM;
            goto bad_buffer;
        }
        __free_buffer_wake(b);
    }

    mutex_lock(&dm_bufio_clients_lock);
    dm_bufio_client_count++;
    list_add(&c->client_list, &dm_bufio_all_clients);
    __cache_size_refresh();
    mutex_unlock(&dm_bufio_clients_lock);

    c->shrinker.shrink = shrink;
    c->shrinker.seeks = 1;
    c->shrinker.batch = 0;
    register_shrinker(&c->shrinker);

    return c;

bad_buffer:
bad_cache:
    while (!list_empty(&c->reserved_buffers)) {
        struct dm_buffer *b = list_entry(c->reserved_buffers.next,
                         struct dm_buffer, lru_list);
        list_del(&b->lru_list);
        free_buffer(b);
    }
    dm_io_client_destroy(c->dm_io);
bad_dm_io:
    vfree(c->cache_hash);
bad_hash:
    kfree(c);
bad_client:
    return ERR_PTR(r);
}
EXPORT_SYMBOL_GPL(dm_bufio_client_create);

/*
 * Free the buffering interface.
 * It is required that there are no references on any buffers.
 */
void dm_bufio_client_destroy(struct dm_bufio_client *c)
{
    unsigned i;

    drop_buffers(c);

    unregister_shrinker(&c->shrinker);

    mutex_lock(&dm_bufio_clients_lock);

    list_del(&c->client_list);
    dm_bufio_client_count--;
    __cache_size_refresh();

    mutex_unlock(&dm_bufio_clients_lock);

    for (i = 0; i < 1 << DM_BUFIO_HASH_BITS; i++)
        BUG_ON(!hlist_empty(&c->cache_hash[i]));

    BUG_ON(c->need_reserved_buffers);

    while (!list_empty(&c->reserved_buffers)) {
        struct dm_buffer *b = list_entry(c->reserved_buffers.next,
                         struct dm_buffer, lru_list);
        list_del(&b->lru_list);
        free_buffer(b);
    }

    for (i = 0; i < LIST_SIZE; i++)
        if (c->n_buffers[i])
            DMERR("leaked buffer count %d: %ld", i, c->n_buffers[i]);

    for (i = 0; i < LIST_SIZE; i++)
        BUG_ON(c->n_buffers[i]);

    dm_io_client_destroy(c->dm_io);
    vfree(c->cache_hash);
    kfree(c);
}
EXPORT_SYMBOL_GPL(dm_bufio_client_destroy);

static void cleanup_old_buffers(void)
{
    unsigned long max_age = ACCESS_ONCE(dm_bufio_max_age);
    struct dm_bufio_client *c;

    if (max_age > ULONG_MAX / HZ)
        max_age = ULONG_MAX / HZ;

    mutex_lock(&dm_bufio_clients_lock);
    list_for_each_entry(c, &dm_bufio_all_clients, client_list) {
        if (!dm_bufio_trylock(c))
            continue;

        while (!list_empty(&c->lru[LIST_CLEAN])) {
            struct dm_buffer *b;
            b = list_entry(c->lru[LIST_CLEAN].prev,
                       struct dm_buffer, lru_list);
            if (__cleanup_old_buffer(b, 0, max_age * HZ))
                break;
            dm_bufio_cond_resched();
        }

        dm_bufio_unlock(c);
        dm_bufio_cond_resched();
    }
    mutex_unlock(&dm_bufio_clients_lock);
}

static struct workqueue_struct *dm_bufio_wq;
static struct delayed_work dm_bufio_work;

static void work_fn(struct work_struct *w)
{
    cleanup_old_buffers();

    queue_delayed_work(dm_bufio_wq, &dm_bufio_work,
               DM_BUFIO_WORK_TIMER_SECS * HZ);
}

/*----------------------------------------------------------------
 * Module setup
 *--------------------------------------------------------------*/

/*
 * This is called only once for the whole dm_bufio module.
 * It initializes memory limit.
 */
static int __init dm_bufio_init(void)
{
    __u64 mem;

    memset(&dm_bufio_caches, 0, sizeof dm_bufio_caches);
    memset(&dm_bufio_cache_names, 0, sizeof dm_bufio_cache_names);

    mem = (__u64)((totalram_pages - totalhigh_pages) *
              DM_BUFIO_MEMORY_PERCENT / 100) << PAGE_SHIFT;

    if (mem > ULONG_MAX)
        mem = ULONG_MAX;

#ifdef CONFIG_MMU
    /*
     * Get the size of vmalloc space the same way as VMALLOC_TOTAL
     * in fs/proc/internal.h
     */
    if (mem > (VMALLOC_END - VMALLOC_START) * DM_BUFIO_VMALLOC_PERCENT / 100)
        mem = (VMALLOC_END - VMALLOC_START) * DM_BUFIO_VMALLOC_PERCENT / 100;
#endif

    dm_bufio_default_cache_size = mem;

    mutex_lock(&dm_bufio_clients_lock);
    __cache_size_refresh();
    mutex_unlock(&dm_bufio_clients_lock);

    dm_bufio_wq = create_singlethread_workqueue("dm_bufio_cache");
    if (!dm_bufio_wq)
        return -ENOMEM;

    INIT_DELAYED_WORK(&dm_bufio_work, work_fn);
    queue_delayed_work(dm_bufio_wq, &dm_bufio_work,
               DM_BUFIO_WORK_TIMER_SECS * HZ);

    return 0;
}

/*
 * This is called once when unloading the dm_bufio module.
 */
static void __exit dm_bufio_exit(void)
{
    int bug = 0;
    int i;

    cancel_delayed_work_sync(&dm_bufio_work);
    destroy_workqueue(dm_bufio_wq);

    for (i = 0; i < ARRAY_SIZE(dm_bufio_caches); i++) {
        struct kmem_cache *kc = dm_bufio_caches[i];

        if (kc)
            kmem_cache_destroy(kc);
    }

    for (i = 0; i < ARRAY_SIZE(dm_bufio_cache_names); i++)
        kfree(dm_bufio_cache_names[i]);

    if (dm_bufio_client_count) {
        DMCRIT("%s: dm_bufio_client_count leaked: %d",
            __func__, dm_bufio_client_count);
        bug = 1;
    }

    if (dm_bufio_current_allocated) {
        DMCRIT("%s: dm_bufio_current_allocated leaked: %lu",
            __func__, dm_bufio_current_allocated);
        bug = 1;
    }

    if (dm_bufio_allocated_get_free_pages) {
        DMCRIT("%s: dm_bufio_allocated_get_free_pages leaked: %lu",
               __func__, dm_bufio_allocated_get_free_pages);
        bug = 1;
    }

    if (dm_bufio_allocated_vmalloc) {
        DMCRIT("%s: dm_bufio_vmalloc leaked: %lu",
               __func__, dm_bufio_allocated_vmalloc);
        bug = 1;
    }

    if (bug)
        BUG();
}

module_init(dm_bufio_init)
module_exit(dm_bufio_exit)

module_param_named(max_cache_size_bytes, dm_bufio_cache_size, ulong, S_IRUGO | S_IWUSR);
MODULE_PARM_DESC(max_cache_size_bytes, "Size of metadata cache");

module_param_named(max_age_seconds, dm_bufio_max_age, uint, S_IRUGO | S_IWUSR);
MODULE_PARM_DESC(max_age_seconds, "Max age of a buffer in seconds");

module_param_named(peak_allocated_bytes, dm_bufio_peak_allocated, ulong, S_IRUGO | S_IWUSR);
MODULE_PARM_DESC(peak_allocated_bytes, "Tracks the maximum allocated memory");

module_param_named(allocated_kmem_cache_bytes, dm_bufio_allocated_kmem_cache, ulong, S_IRUGO);
MODULE_PARM_DESC(allocated_kmem_cache_bytes, "Memory allocated with kmem_cache_alloc");

module_param_named(allocated_get_free_pages_bytes, dm_bufio_allocated_get_free_pages, ulong, S_IRUGO);
MODULE_PARM_DESC(allocated_get_free_pages_bytes, "Memory allocated with get_free_pages");

module_param_named(allocated_vmalloc_bytes, dm_bufio_allocated_vmalloc, ulong, S_IRUGO);
MODULE_PARM_DESC(allocated_vmalloc_bytes, "Memory allocated with vmalloc");

module_param_named(current_allocated_bytes, dm_bufio_current_allocated, ulong, S_IRUGO);
MODULE_PARM_DESC(current_allocated_bytes, "Memory currently used by the cache");

MODULE_AUTHOR("Mikulas Patocka <[email protected]>");
MODULE_DESCRIPTION(DM_NAME " buffered I/O library");
MODULE_LICENSE("GPL");