/*
 * Copyright (C) 2009-2011 Red Hat, Inc.
 *
 * Author: Mikulas Patocka <mpatocka@redhat.com>
 *
 * This file is released under the GPL.
 */

#include <linux/dm-bufio.h>

#include <linux/device-mapper.h>
#include <linux/dm-io.h>
#include <linux/slab.h>
#include <linux/sched/mm.h>
#include <linux/jiffies.h>
#include <linux/vmalloc.h>
#include <linux/shrinker.h>
#include <linux/module.h>
#include <linux/rbtree.h>
#include <linux/stacktrace.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        30

/*
 * Free buffers when they are older than this (seconds)
 */
#define DM_BUFIO_DEFAULT_AGE_SECS       300

/*
 * The nr of bytes of cached data to keep around.
 */
#define DM_BUFIO_DEFAULT_RETAIN_BYTES   (256 * 1024)

/*
 * Align buffer writes to this boundary.
 * Tests show that SSDs have the highest IOPS when using 4k writes.
 */
#define DM_BUFIO_WRITE_ALIGN            4096

/*
 * 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;
        s8 sectors_per_block_bits;
        void (*alloc_callback)(struct dm_buffer *);
        void (*write_callback)(struct dm_buffer *);

        struct kmem_cache *slab_buffer;
        struct kmem_cache *slab_cache;
        struct dm_io_client *dm_io;

        struct list_head reserved_buffers;
        unsigned need_reserved_buffers;

        unsigned minimum_buffers;

        struct rb_root buffer_tree;
        wait_queue_head_t free_buffer_wait;

        sector_t start;

        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 rb_node node;
        struct list_head lru_list;
        sector_t block;
        void *data;
        unsigned char data_mode;                /* DATA_MODE_* */
        unsigned char list_mode;                /* LIST_* */
        blk_status_t read_error;
        blk_status_t write_error;
        unsigned hold_count;
        unsigned long state;
        unsigned long last_accessed;
        unsigned dirty_start;
        unsigned dirty_end;
        unsigned write_start;
        unsigned write_end;
        struct dm_bufio_client *c;
        struct list_head write_list;
        void (*end_io)(struct dm_buffer *, blk_status_t);
#ifdef CONFIG_DM_DEBUG_BLOCK_STACK_TRACING
#define MAX_STACK 10
        struct stack_trace stack_trace;
        unsigned long stack_entries[MAX_STACK];
#endif
};

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

#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);
}

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

/*
 * 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_retain_bytes = DM_BUFIO_DEFAULT_RETAIN_BYTES;

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);

#ifdef CONFIG_DM_DEBUG_BLOCK_STACK_TRACING
static void buffer_record_stack(struct dm_buffer *b)
{
        b->stack_trace.nr_entries = 0;
        b->stack_trace.max_entries = MAX_STACK;
        b->stack_trace.entries = b->stack_entries;
        b->stack_trace.skip = 2;
        save_stack_trace(&b->stack_trace);
}
#endif

/*----------------------------------------------------------------
 * A red/black tree acts as an index for all the buffers.
 *--------------------------------------------------------------*/
static struct dm_buffer *__find(struct dm_bufio_client *c, sector_t block)
{
        struct rb_node *n = c->buffer_tree.rb_node;
        struct dm_buffer *b;

        while (n) {
                b = container_of(n, struct dm_buffer, node);

                if (b->block == block)
                        return b;

                n = (b->block < block) ? n->rb_left : n->rb_right;
        }

        return NULL;
}

static void __insert(struct dm_bufio_client *c, struct dm_buffer *b)
{
        struct rb_node **new = &c->buffer_tree.rb_node, *parent = NULL;
        struct dm_buffer *found;

        while (*new) {
                found = container_of(*new, struct dm_buffer, node);

                if (found->block == b->block) {
                        BUG_ON(found != b);
                        return;
                }

                parent = *new;
                new = (found->block < b->block) ?
                        &((*new)->rb_left) : &((*new)->rb_right);
        }

        rb_link_node(&b->node, parent, new);
        rb_insert_color(&b->node, &c->buffer_tree);
}

static void __remove(struct dm_bufio_client *c, struct dm_buffer *b)
{
        rb_erase(&b->node, &c->buffer_tree);
}

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

static void adjust_total_allocated(unsigned char 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 = READ_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,
                               unsigned char *data_mode)
{
        if (unlikely(c->slab_cache != NULL)) {
                *data_mode = DATA_MODE_SLAB;
                return kmem_cache_alloc(c->slab_cache, gfp_mask);
        }

        if (c->block_size <= KMALLOC_MAX_SIZE &&
            gfp_mask & __GFP_NORETRY) {
                *data_mode = DATA_MODE_GET_FREE_PAGES;
                return (void *)__get_free_pages(gfp_mask,
                                                c->sectors_per_block_bits - (PAGE_SHIFT - SECTOR_SHIFT));
        }

        *data_mode = DATA_MODE_VMALLOC;

        /*
         * __vmalloc allocates the data pages and auxiliary structures with
         * gfp_flags that were specified, but pagetables are always allocated
         * with GFP_KERNEL, no matter what was specified as gfp_mask.
         *
         * Consequently, we must set per-process flag PF_MEMALLOC_NOIO so that
         * all allocations done by this process (including pagetables) are done
         * as if GFP_NOIO was specified.
         */
        if (gfp_mask & __GFP_NORETRY) {
                unsigned noio_flag = memalloc_noio_save();
                void *ptr = __vmalloc(c->block_size, gfp_mask, PAGE_KERNEL);

                memalloc_noio_restore(noio_flag);
                return ptr;
        }

        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, unsigned char data_mode)
{
        switch (data_mode) {
        case DATA_MODE_SLAB:
                kmem_cache_free(c->slab_cache, data);
                break;

        case DATA_MODE_GET_FREE_PAGES:
                free_pages((unsigned long)data,
                           c->sectors_per_block_bits - (PAGE_SHIFT - SECTOR_SHIFT));
                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 = kmem_cache_alloc(c->slab_buffer, gfp_mask);

        if (!b)
                return NULL;

        b->c = c;

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

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

#ifdef CONFIG_DM_DEBUG_BLOCK_STACK_TRACING
        memset(&b->stack_trace, 0, sizeof(b->stack_trace));
#endif
        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);
        kmem_cache_free(c->slab_buffer, 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]);
        __insert(b->c, b);
        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]--;
        __remove(b->c, b);
        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]);
        b->last_accessed = jiffies;
}

/*----------------------------------------------------------------
 * 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;
 *
 * 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->end_io(b, unlikely(error != 0) ? BLK_STS_IOERR : 0);
}

static void use_dmio(struct dm_buffer *b, int rw, sector_t sector,
                     unsigned n_sectors, unsigned offset)
{
        int r;
        struct dm_io_request io_req = {
                .bi_op = rw,
                .bi_op_flags = 0,
                .notify.fn = dmio_complete,
                .notify.context = b,
                .client = b->c->dm_io,
        };
        struct dm_io_region region = {
                .bdev = b->c->bdev,
                .sector = sector,
                .count = n_sectors,
        };

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

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

static void bio_complete(struct bio *bio)
{
        struct dm_buffer *b = bio->bi_private;
        blk_status_t status = bio->bi_status;
        bio_put(bio);
        b->end_io(b, status);
}

static void use_bio(struct dm_buffer *b, int rw, sector_t sector,
                    unsigned n_sectors, unsigned offset)
{
        struct bio *bio;
        char *ptr;
        unsigned vec_size, len;

        vec_size = b->c->block_size >> PAGE_SHIFT;
        if (unlikely(b->c->sectors_per_block_bits < PAGE_SHIFT - SECTOR_SHIFT))
                vec_size += 2;

        bio = bio_kmalloc(GFP_NOWAIT | __GFP_NORETRY | __GFP_NOWARN, vec_size);
        if (!bio) {
dmio:
                use_dmio(b, rw, sector, n_sectors, offset);
                return;
        }

        bio->bi_iter.bi_sector = sector;
        bio_set_dev(bio, b->c->bdev);
        bio_set_op_attrs(bio, rw, 0);
        bio->bi_end_io = bio_complete;
        bio->bi_private = b;

        ptr = (char *)b->data + offset;
        len = n_sectors << SECTOR_SHIFT;

        do {
                unsigned this_step = min((unsigned)(PAGE_SIZE - offset_in_page(ptr)), len);
                if (!bio_add_page(bio, virt_to_page(ptr), this_step,
                                  offset_in_page(ptr))) {
                        bio_put(bio);
                        goto dmio;
                }

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

        submit_bio(bio);
}

static void submit_io(struct dm_buffer *b, int rw, void (*end_io)(struct dm_buffer *, blk_status_t))
{
        unsigned n_sectors;
        sector_t sector;
        unsigned offset, end;

        b->end_io = end_io;

        if (likely(b->c->sectors_per_block_bits >= 0))
                sector = b->block << b->c->sectors_per_block_bits;
        else
                sector = b->block * (b->c->block_size >> SECTOR_SHIFT);
        sector += b->c->start;

        if (rw != REQ_OP_WRITE) {
                n_sectors = b->c->block_size >> SECTOR_SHIFT;
                offset = 0;
        } else {
                if (b->c->write_callback)
                        b->c->write_callback(b);
                offset = b->write_start;
                end = b->write_end;
                offset &= -DM_BUFIO_WRITE_ALIGN;
                end += DM_BUFIO_WRITE_ALIGN - 1;
                end &= -DM_BUFIO_WRITE_ALIGN;
                if (unlikely(end > b->c->block_size))
                        end = b->c->block_size;

                sector += offset >> SECTOR_SHIFT;
                n_sectors = (end - offset) >> SECTOR_SHIFT;
        }

        if (b->data_mode != DATA_MODE_VMALLOC)
                use_bio(b, rw, sector, n_sectors, offset);
        else
                use_dmio(b, rw, sector, n_sectors, offset);
}

/*----------------------------------------------------------------
 * 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 dm_buffer *b, blk_status_t status)
{
        b->write_error = status;
        if (unlikely(status)) {
                struct dm_bufio_client *c = b->c;

                (void)cmpxchg(&c->async_write_error, 0,
                                blk_status_to_errno(status));
        }

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

        smp_mb__before_atomic();
        clear_bit(B_WRITING, &b->state);
        smp_mb__after_atomic();

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

/*
 * 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,
                                 struct list_head *write_list)
{
        if (!test_bit(B_DIRTY, &b->state))
                return;

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

        b->write_start = b->dirty_start;
        b->write_end = b->dirty_end;

        if (!write_list)
                submit_io(b, REQ_OP_WRITE, write_endio);
        else
                list_add_tail(&b->write_list, write_list);
}

static void __flush_write_list(struct list_head *write_list)
{
        struct blk_plug plug;
        blk_start_plug(&plug);
        while (!list_empty(write_list)) {
                struct dm_buffer *b =
                        list_entry(write_list->next, struct dm_buffer, write_list);
                list_del(&b->write_list);
                submit_io(b, REQ_OP_WRITE, write_endio);
                cond_resched();
        }
        blk_finish_plug(&plug);
}

/*
 * 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_io(&b->state, B_READING, TASK_UNINTERRUPTIBLE);
        __write_dirty_buffer(b, NULL);
        wait_on_bit_io(&b->state, B_WRITING, 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;
                }
                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;
                }
                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_current_state(TASK_UNINTERRUPTIBLE);
        dm_bufio_unlock(c);

        io_schedule();

        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;
        bool tried_noio_alloc = false;

        /*
         * 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_NOWAIT: don't wait; if we need to sleep we'll release our
         *                  mutex and wait ourselves.
         *      __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_NOWAIT | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
                        if (b)
                                return b;
                }

                if (nf == NF_PREFETCH)
                        return NULL;

                if (dm_bufio_cache_size_latch != 1 && !tried_noio_alloc) {
                        dm_bufio_unlock(c);
                        b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
                        dm_bufio_lock(c);
                        if (b)
                                return b;
                        tried_noio_alloc = true;
                }

                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 list_head *write_list)
{
        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, write_list);
                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 (unlikely(READ_ONCE(dm_bufio_cache_size) != dm_bufio_cache_size_latch)) {
                if (mutex_trylock(&dm_bufio_clients_lock)) {
                        __cache_size_refresh();
                        mutex_unlock(&dm_bufio_clients_lock);
                }
        }

        buffers = dm_bufio_cache_size_per_client;
        if (likely(c->sectors_per_block_bits >= 0))
                buffers >>= c->sectors_per_block_bits + SECTOR_SHIFT;
        else
                buffers /= c->block_size;

        if (buffers < c->minimum_buffers)
                buffers = c->minimum_buffers;

        *limit_buffers = buffers;
        *threshold_buffers = mult_frac(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,
                              struct list_head *write_list)
{
        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);
                cond_resched();
        }

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

/*----------------------------------------------------------------
 * 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 list_head *write_list)
{
        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, write_list);

        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 dm_buffer *b, blk_status_t status)
{
        b->read_error = status;

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

        smp_mb__before_atomic();
        clear_bit(B_READING, &b->state);
        smp_mb__after_atomic();

        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;

        LIST_HEAD(write_list);

        dm_bufio_lock(c);
        b = __bufio_new(c, block, nf, &need_submit, &write_list);
#ifdef CONFIG_DM_DEBUG_BLOCK_STACK_TRACING
        if (b && b->hold_count == 1)
                buffer_record_stack(b);
#endif
        dm_bufio_unlock(c);

        __flush_write_list(&write_list);

        if (!b)
                return NULL;

        if (need_submit)
                submit_io(b, REQ_OP_READ, read_endio);

        wait_on_bit_io(&b->state, B_READING, TASK_UNINTERRUPTIBLE);

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

        LIST_HEAD(write_list);

        BUG_ON(dm_bufio_in_request());

        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,
                                &write_list);
                if (unlikely(!list_empty(&write_list))) {
                        dm_bufio_unlock(c);
                        blk_finish_plug(&plug);
                        __flush_write_list(&write_list);
                        blk_start_plug(&plug);
                        dm_bufio_lock(c);
                }
                if (unlikely(b != NULL)) {
                        dm_bufio_unlock(c);

                        if (need_submit)
                                submit_io(b, REQ_OP_READ, read_endio);
                        dm_bufio_release(b);

                        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_partial_buffer_dirty(struct dm_buffer *b,
                                        unsigned start, unsigned end)
{
        struct dm_bufio_client *c = b->c;

        BUG_ON(start >= end);
        BUG_ON(end > b->c->block_size);

        dm_bufio_lock(c);

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

        if (!test_and_set_bit(B_DIRTY, &b->state)) {
                b->dirty_start = start;
                b->dirty_end = end;
                __relink_lru(b, LIST_DIRTY);
        } else {
                if (start < b->dirty_start)
                        b->dirty_start = start;
                if (end > b->dirty_end)
                        b->dirty_end = end;
        }

        dm_bufio_unlock(c);
}
EXPORT_SYMBOL_GPL(dm_bufio_mark_partial_buffer_dirty);

void dm_bufio_mark_buffer_dirty(struct dm_buffer *b)
{
        dm_bufio_mark_partial_buffer_dirty(b, 0, b->c->block_size);
}
EXPORT_SYMBOL_GPL(dm_bufio_mark_buffer_dirty);

void dm_bufio_write_dirty_buffers_async(struct dm_bufio_client *c)
{
        LIST_HEAD(write_list);

        BUG_ON(dm_bufio_in_request());

        dm_bufio_lock(c);
        __write_dirty_buffers_async(c, 0, &write_list);
        dm_bufio_unlock(c);
        __flush_write_list(&write_list);
}
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;

        LIST_HEAD(write_list);

        dm_bufio_lock(c);
        __write_dirty_buffers_async(c, 0, &write_list);
        dm_bufio_unlock(c);
        __flush_write_list(&write_list);
        dm_bufio_lock(c);

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_io(&b->state, B_WRITING,
                                               TASK_UNINTERRUPTIBLE);
                                dm_bufio_lock(c);
                                b->hold_count--;
                        } else
                                wait_on_bit_io(&b->state, B_WRITING,
                                               TASK_UNINTERRUPTIBLE);
                }

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

                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_op = REQ_OP_WRITE,
                .bi_op_flags = REQ_PREFLUSH | REQ_SYNC,
                .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, NULL);
        if (b->hold_count == 1) {
                wait_on_bit_io(&b->state, B_WRITING,
                               TASK_UNINTERRUPTIBLE);
                set_bit(B_DIRTY, &b->state);
                b->dirty_start = 0;
                b->dirty_end = c->block_size;
                __unlink_buffer(b);
                __link_buffer(b, new_block, LIST_DIRTY);
        } else {
                sector_t old_block;
                wait_on_bit_lock_io(&b->state, B_WRITING,
                                    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, REQ_OP_WRITE, write_endio);
                wait_on_bit_io(&b->state, B_WRITING,
                               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);

/*
 * Free the given buffer.
 *
 * This is just a hint, if the buffer is in use or dirty, this function
 * does nothing.
 */
void dm_bufio_forget(struct dm_bufio_client *c, sector_t block)
{
        struct dm_buffer *b;

        dm_bufio_lock(c);

        b = __find(c, block);
        if (b && likely(!b->hold_count) && likely(!b->state)) {
                __unlink_buffer(b);
                __free_buffer_wake(b);
        }

        dm_bufio_unlock(c);
}
EXPORT_SYMBOL_GPL(dm_bufio_forget);

void dm_bufio_set_minimum_buffers(struct dm_bufio_client *c, unsigned n)
{
        c->minimum_buffers = n;
}
EXPORT_SYMBOL_GPL(dm_bufio_set_minimum_buffers);

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)
{
        sector_t s = i_size_read(c->bdev->bd_inode) >> SECTOR_SHIFT;
        if (likely(c->sectors_per_block_bits >= 0))
                s >>= c->sectors_per_block_bits;
        else
                sector_div(s, c->block_size >> SECTOR_SHIFT);
        return s;
}
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;
        bool warned = false;

        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) {
                        WARN_ON(!warned);
                        warned = true;
                        DMERR("leaked buffer %llx, hold count %u, list %d",
                              (unsigned long long)b->block, b->hold_count, i);
#ifdef CONFIG_DM_DEBUG_BLOCK_STACK_TRACING
                        print_stack_trace(&b->stack_trace, 1);
                        b->hold_count = 0; /* mark unclaimed to avoid BUG_ON below */
#endif
                }

#ifdef CONFIG_DM_DEBUG_BLOCK_STACK_TRACING
        while ((b = __get_unclaimed_buffer(c)))
                __free_buffer_wake(b);
#endif

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

        dm_bufio_unlock(c);
}

/*
 * We may not be able to evict this buffer if IO pending or the client
 * is still using it.  Caller is expected to know buffer is too old.
 *
 * And if GFP_NOFS is used, 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 bool __try_evict_buffer(struct dm_buffer *b, gfp_t gfp)
{
        if (!(gfp & __GFP_FS)) {
                if (test_bit(B_READING, &b->state) ||
                    test_bit(B_WRITING, &b->state) ||
                    test_bit(B_DIRTY, &b->state))
                        return false;
        }

        if (b->hold_count)
                return false;

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

        return true;
}

static unsigned long get_retain_buffers(struct dm_bufio_client *c)
{
        unsigned long retain_bytes = READ_ONCE(dm_bufio_retain_bytes);
        if (likely(c->sectors_per_block_bits >= 0))
                retain_bytes >>= c->sectors_per_block_bits + SECTOR_SHIFT;
        else
                retain_bytes /= c->block_size;
        return retain_bytes;
}

static unsigned long __scan(struct dm_bufio_client *c, unsigned long nr_to_scan,
                            gfp_t gfp_mask)
{
        int l;
        struct dm_buffer *b, *tmp;
        unsigned long freed = 0;
        unsigned long count = c->n_buffers[LIST_CLEAN] +
                              c->n_buffers[LIST_DIRTY];
        unsigned long retain_target = get_retain_buffers(c);

        for (l = 0; l < LIST_SIZE; l++) {
                list_for_each_entry_safe_reverse(b, tmp, &c->lru[l], lru_list) {
                        if (__try_evict_buffer(b, gfp_mask))
                                freed++;
                        if (!--nr_to_scan || ((count - freed) <= retain_target))
                                return freed;
                        cond_resched();
                }
        }
        return freed;
}

static unsigned long
dm_bufio_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
{
        struct dm_bufio_client *c;
        unsigned long freed;

        c = container_of(shrink, struct dm_bufio_client, shrinker);
        if (sc->gfp_mask & __GFP_FS)
                dm_bufio_lock(c);
        else if (!dm_bufio_trylock(c))
                return SHRINK_STOP;

        freed  = __scan(c, sc->nr_to_scan, sc->gfp_mask);
        dm_bufio_unlock(c);
        return freed;
}

static unsigned long
dm_bufio_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
{
        struct dm_bufio_client *c = container_of(shrink, struct dm_bufio_client, shrinker);
        unsigned long count = READ_ONCE(c->n_buffers[LIST_CLEAN]) +
                              READ_ONCE(c->n_buffers[LIST_DIRTY]);
        unsigned long retain_target = get_retain_buffers(c);

        return (count < retain_target) ? 0 : (count - retain_target);
}

/*
 * 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;
        char slab_name[27];

        if (!block_size || block_size & ((1 << SECTOR_SHIFT) - 1)) {
                DMERR("%s: block size not specified or is not multiple of 512b", __func__);
                r = -EINVAL;
                goto bad_client;
        }

        c = kzalloc(sizeof(*c), GFP_KERNEL);
        if (!c) {
                r = -ENOMEM;
                goto bad_client;
        }
        c->buffer_tree = RB_ROOT;

        c->bdev = bdev;
        c->block_size = block_size;
        if (is_power_of_2(block_size))
                c->sectors_per_block_bits = __ffs(block_size) - SECTOR_SHIFT;
        else
                c->sectors_per_block_bits = -1;

        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;
        }

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

        dm_bufio_set_minimum_buffers(c, DM_BUFIO_MIN_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;
        }

        if (block_size <= KMALLOC_MAX_SIZE &&
            (block_size < PAGE_SIZE || !is_power_of_2(block_size))) {
                unsigned align = min(1U << __ffs(block_size), (unsigned)PAGE_SIZE);
                snprintf(slab_name, sizeof slab_name, "dm_bufio_cache-%u", block_size);
                c->slab_cache = kmem_cache_create(slab_name, block_size, align,
                                                  SLAB_RECLAIM_ACCOUNT, NULL);
                if (!c->slab_cache) {
                        r = -ENOMEM;
                        goto bad;
                }
        }
        if (aux_size)
                snprintf(slab_name, sizeof slab_name, "dm_bufio_buffer-%u", aux_size);
        else
                snprintf(slab_name, sizeof slab_name, "dm_bufio_buffer");
        c->slab_buffer = kmem_cache_create(slab_name, sizeof(struct dm_buffer) + aux_size,
                                           0, SLAB_RECLAIM_ACCOUNT, NULL);
        if (!c->slab_buffer) {
                r = -ENOMEM;
                goto bad;
        }

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

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

        c->shrinker.count_objects = dm_bufio_shrink_count;
        c->shrinker.scan_objects = dm_bufio_shrink_scan;
        c->shrinker.seeks = 1;
        c->shrinker.batch = 0;
        r = register_shrinker(&c->shrinker);
        if (r)
                goto bad;

        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);

        return c;

bad:
        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);
        }
        kmem_cache_destroy(c->slab_cache);
        kmem_cache_destroy(c->slab_buffer);
        dm_io_client_destroy(c->dm_io);
bad_dm_io:
        mutex_destroy(&c->lock);
        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);

        BUG_ON(!RB_EMPTY_ROOT(&c->buffer_tree));
        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]);

        kmem_cache_destroy(c->slab_cache);
        kmem_cache_destroy(c->slab_buffer);
        dm_io_client_destroy(c->dm_io);
        mutex_destroy(&c->lock);
        kfree(c);
}
EXPORT_SYMBOL_GPL(dm_bufio_client_destroy);

void dm_bufio_set_sector_offset(struct dm_bufio_client *c, sector_t start)
{
        c->start = start;
}
EXPORT_SYMBOL_GPL(dm_bufio_set_sector_offset);

static unsigned get_max_age_hz(void)
{
        unsigned max_age = READ_ONCE(dm_bufio_max_age);

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

        return max_age * HZ;
}

static bool older_than(struct dm_buffer *b, unsigned long age_hz)
{
        return time_after_eq(jiffies, b->last_accessed + age_hz);
}

static void __evict_old_buffers(struct dm_bufio_client *c, unsigned long age_hz)
{
        struct dm_buffer *b, *tmp;
        unsigned long retain_target = get_retain_buffers(c);
        unsigned long count;
        LIST_HEAD(write_list);

        dm_bufio_lock(c);

        __check_watermark(c, &write_list);
        if (unlikely(!list_empty(&write_list))) {
                dm_bufio_unlock(c);
                __flush_write_list(&write_list);
                dm_bufio_lock(c);
        }

        count = c->n_buffers[LIST_CLEAN] + c->n_buffers[LIST_DIRTY];
        list_for_each_entry_safe_reverse(b, tmp, &c->lru[LIST_CLEAN], lru_list) {
                if (count <= retain_target)
                        break;

                if (!older_than(b, age_hz))
                        break;

                if (__try_evict_buffer(b, 0))
                        count--;

                cond_resched();
        }

        dm_bufio_unlock(c);
}

static void cleanup_old_buffers(void)
{
        unsigned long max_age_hz = get_max_age_hz();
        struct dm_bufio_client *c;

        mutex_lock(&dm_bufio_clients_lock);

        __cache_size_refresh();

        list_for_each_entry(c, &dm_bufio_all_clients, client_list)
                __evict_old_buffers(c, max_age_hz);

        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;

        dm_bufio_allocated_kmem_cache = 0;
        dm_bufio_allocated_get_free_pages = 0;
        dm_bufio_allocated_vmalloc = 0;
        dm_bufio_current_allocated = 0;

        mem = (__u64)mult_frac(totalram_pages - totalhigh_pages,
                               DM_BUFIO_MEMORY_PERCENT, 100) << PAGE_SHIFT;

        if (mem > ULONG_MAX)
                mem = ULONG_MAX;

#ifdef CONFIG_MMU
        if (mem > mult_frac(VMALLOC_TOTAL, DM_BUFIO_VMALLOC_PERCENT, 100))
                mem = mult_frac(VMALLOC_TOTAL, 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 = alloc_workqueue("dm_bufio_cache", WQ_MEM_RECLAIM, 0);
        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;

        cancel_delayed_work_sync(&dm_bufio_work);
        destroy_workqueue(dm_bufio_wq);

        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;
        }

        BUG_ON(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(retain_bytes, dm_bufio_retain_bytes, ulong, S_IRUGO | S_IWUSR);
MODULE_PARM_DESC(retain_bytes, "Try to keep at least this many bytes cached in memory");

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 <dm-devel@redhat.com>");
MODULE_DESCRIPTION(DM_NAME " buffered I/O library");
MODULE_LICENSE("GPL");