dm-snap.c

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/*
 * dm-snapshot.c
 *
 * Copyright (C) 2001-2002 Sistina Software (UK) Limited.
 *
 * This file is released under the GPL.
 */

#include <linux/blkdev.h>
#include <linux/device-mapper.h>
#include <linux/delay.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/kdev_t.h>
#include <linux/list.h>
#include <linux/mempool.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/log2.h>
#include <linux/dm-kcopyd.h>

#include "dm-exception-store.h"

#define DM_MSG_PREFIX "snapshots"

static const char dm_snapshot_merge_target_name[] = "snapshot-merge";

#define dm_target_is_snapshot_merge(ti) \
    ((ti)->type->name == dm_snapshot_merge_target_name)

/*
 * The size of the mempool used to track chunks in use.
 */
#define MIN_IOS 256

#define DM_TRACKED_CHUNK_HASH_SIZE  16
#define DM_TRACKED_CHUNK_HASH(x)    ((unsigned long)(x) & \
                     (DM_TRACKED_CHUNK_HASH_SIZE - 1))

struct dm_exception_table {
    uint32_t hash_mask;
    unsigned hash_shift;
    struct list_head *table;
};

struct dm_snapshot {
    struct rw_semaphore lock;

    struct dm_dev *origin;
    struct dm_dev *cow;

    struct dm_target *ti;

    /* List of snapshots per Origin */
    struct list_head list;

    /*
     * You can't use a snapshot if this is 0 (e.g. if full).
     * A snapshot-merge target never clears this.
     */
    int valid;

    /* Origin writes don't trigger exceptions until this is set */
    int active;

    atomic_t pending_exceptions_count;

    mempool_t *pending_pool;

    struct dm_exception_table pending;
    struct dm_exception_table complete;

    /*
     * pe_lock protects all pending_exception operations and access
     * as well as the snapshot_bios list.
     */
    spinlock_t pe_lock;

    /* Chunks with outstanding reads */
    spinlock_t tracked_chunk_lock;
    mempool_t *tracked_chunk_pool;
    struct hlist_head tracked_chunk_hash[DM_TRACKED_CHUNK_HASH_SIZE];

    /* The on disk metadata handler */
    struct dm_exception_store *store;

    struct dm_kcopyd_client *kcopyd_client;

    /* Wait for events based on state_bits */
    unsigned long state_bits;

    /* Range of chunks currently being merged. */
    chunk_t first_merging_chunk;
    int num_merging_chunks;

    /*
     * The merge operation failed if this flag is set.
     * Failure modes are handled as follows:
     * - I/O error reading the header
     *      => don't load the target; abort.
     * - Header does not have "valid" flag set
     *      => use the origin; forget about the snapshot.
     * - I/O error when reading exceptions
     *      => don't load the target; abort.
     *         (We can't use the intermediate origin state.)
     * - I/O error while merging
     *  => stop merging; set merge_failed; process I/O normally.
     */
    int merge_failed;

    /*
     * Incoming bios that overlap with chunks being merged must wait
     * for them to be committed.
     */
    struct bio_list bios_queued_during_merge;
};

/*
 * state_bits:
 *   RUNNING_MERGE  - Merge operation is in progress.
 *   SHUTDOWN_MERGE - Set to signal that merge needs to be stopped;
 *                    cleared afterwards.
 */
#define RUNNING_MERGE          0
#define SHUTDOWN_MERGE         1

struct dm_dev *dm_snap_origin(struct dm_snapshot *s)
{
    return s->origin;
}
EXPORT_SYMBOL(dm_snap_origin);

struct dm_dev *dm_snap_cow(struct dm_snapshot *s)
{
    return s->cow;
}
EXPORT_SYMBOL(dm_snap_cow);

static sector_t chunk_to_sector(struct dm_exception_store *store,
                chunk_t chunk)
{
    return chunk << store->chunk_shift;
}

static int bdev_equal(struct block_device *lhs, struct block_device *rhs)
{
    /*
     * There is only ever one instance of a particular block
     * device so we can compare pointers safely.
     */
    return lhs == rhs;
}

struct dm_snap_pending_exception {
    struct dm_exception e;

    /*
     * Origin buffers waiting for this to complete are held
     * in a bio list
     */
    struct bio_list origin_bios;
    struct bio_list snapshot_bios;

    /* Pointer back to snapshot context */
    struct dm_snapshot *snap;

    /*
     * 1 indicates the exception has already been sent to
     * kcopyd.
     */
    int started;

    /*
     * For writing a complete chunk, bypassing the copy.
     */
    struct bio *full_bio;
    bio_end_io_t *full_bio_end_io;
    void *full_bio_private;
};

/*
 * Hash table mapping origin volumes to lists of snapshots and
 * a lock to protect it
 */
static struct kmem_cache *exception_cache;
static struct kmem_cache *pending_cache;

struct dm_snap_tracked_chunk {
    struct hlist_node node;
    chunk_t chunk;
};

static struct kmem_cache *tracked_chunk_cache;

static struct dm_snap_tracked_chunk *track_chunk(struct dm_snapshot *s,
                         chunk_t chunk)
{
    struct dm_snap_tracked_chunk *c = mempool_alloc(s->tracked_chunk_pool,
                            GFP_NOIO);
    unsigned long flags;

    c->chunk = chunk;

    spin_lock_irqsave(&s->tracked_chunk_lock, flags);
    hlist_add_head(&c->node,
               &s->tracked_chunk_hash[DM_TRACKED_CHUNK_HASH(chunk)]);
    spin_unlock_irqrestore(&s->tracked_chunk_lock, flags);

    return c;
}

static void stop_tracking_chunk(struct dm_snapshot *s,
                struct dm_snap_tracked_chunk *c)
{
    unsigned long flags;

    spin_lock_irqsave(&s->tracked_chunk_lock, flags);
    hlist_del(&c->node);
    spin_unlock_irqrestore(&s->tracked_chunk_lock, flags);

    mempool_free(c, s->tracked_chunk_pool);
}

static int __chunk_is_tracked(struct dm_snapshot *s, chunk_t chunk)
{
    struct dm_snap_tracked_chunk *c;
    struct hlist_node *hn;
    int found = 0;

    spin_lock_irq(&s->tracked_chunk_lock);

    hlist_for_each_entry(c, hn,
        &s->tracked_chunk_hash[DM_TRACKED_CHUNK_HASH(chunk)], node) {
        if (c->chunk == chunk) {
            found = 1;
            break;
        }
    }

    spin_unlock_irq(&s->tracked_chunk_lock);

    return found;
}

/*
 * This conflicting I/O is extremely improbable in the caller,
 * so msleep(1) is sufficient and there is no need for a wait queue.
 */
static void __check_for_conflicting_io(struct dm_snapshot *s, chunk_t chunk)
{
    while (__chunk_is_tracked(s, chunk))
        msleep(1);
}

/*
 * One of these per registered origin, held in the snapshot_origins hash
 */
struct origin {
    /* The origin device */
    struct block_device *bdev;

    struct list_head hash_list;

    /* List of snapshots for this origin */
    struct list_head snapshots;
};

/*
 * Size of the hash table for origin volumes. If we make this
 * the size of the minors list then it should be nearly perfect
 */
#define ORIGIN_HASH_SIZE 256
#define ORIGIN_MASK      0xFF
static struct list_head *_origins;
static struct rw_semaphore _origins_lock;

static DECLARE_WAIT_QUEUE_HEAD(_pending_exceptions_done);
static DEFINE_SPINLOCK(_pending_exceptions_done_spinlock);
static uint64_t _pending_exceptions_done_count;

static int init_origin_hash(void)
{
    int i;

    _origins = kmalloc(ORIGIN_HASH_SIZE * sizeof(struct list_head),
               GFP_KERNEL);
    if (!_origins) {
        DMERR("unable to allocate memory");
        return -ENOMEM;
    }

    for (i = 0; i < ORIGIN_HASH_SIZE; i++)
        INIT_LIST_HEAD(_origins + i);
    init_rwsem(&_origins_lock);

    return 0;
}

static void exit_origin_hash(void)
{
    kfree(_origins);
}

static unsigned origin_hash(struct block_device *bdev)
{
    return bdev->bd_dev & ORIGIN_MASK;
}

static struct origin *__lookup_origin(struct block_device *origin)
{
    struct list_head *ol;
    struct origin *o;

    ol = &_origins[origin_hash(origin)];
    list_for_each_entry (o, ol, hash_list)
        if (bdev_equal(o->bdev, origin))
            return o;

    return NULL;
}

static void __insert_origin(struct origin *o)
{
    struct list_head *sl = &_origins[origin_hash(o->bdev)];
    list_add_tail(&o->hash_list, sl);
}

/*
 * _origins_lock must be held when calling this function.
 * Returns number of snapshots registered using the supplied cow device, plus:
 * snap_src - a snapshot suitable for use as a source of exception handover
 * snap_dest - a snapshot capable of receiving exception handover.
 * snap_merge - an existing snapshot-merge target linked to the same origin.
 *   There can be at most one snapshot-merge target. The parameter is optional.
 *
 * Possible return values and states of snap_src and snap_dest.
 *   0: NULL, NULL  - first new snapshot
 *   1: snap_src, NULL - normal snapshot
 *   2: snap_src, snap_dest  - waiting for handover
 *   2: snap_src, NULL - handed over, waiting for old to be deleted
 *   1: NULL, snap_dest - source got destroyed without handover
 */
static int __find_snapshots_sharing_cow(struct dm_snapshot *snap,
                    struct dm_snapshot **snap_src,
                    struct dm_snapshot **snap_dest,
                    struct dm_snapshot **snap_merge)
{
    struct dm_snapshot *s;
    struct origin *o;
    int count = 0;
    int active;

    o = __lookup_origin(snap->origin->bdev);
    if (!o)
        goto out;

    list_for_each_entry(s, &o->snapshots, list) {
        if (dm_target_is_snapshot_merge(s->ti) && snap_merge)
            *snap_merge = s;
        if (!bdev_equal(s->cow->bdev, snap->cow->bdev))
            continue;

        down_read(&s->lock);
        active = s->active;
        up_read(&s->lock);

        if (active) {
            if (snap_src)
                *snap_src = s;
        } else if (snap_dest)
            *snap_dest = s;

        count++;
    }

out:
    return count;
}

/*
 * On success, returns 1 if this snapshot is a handover destination,
 * otherwise returns 0.
 */
static int __validate_exception_handover(struct dm_snapshot *snap)
{
    struct dm_snapshot *snap_src = NULL, *snap_dest = NULL;
    struct dm_snapshot *snap_merge = NULL;

    /* Does snapshot need exceptions handed over to it? */
    if ((__find_snapshots_sharing_cow(snap, &snap_src, &snap_dest,
                      &snap_merge) == 2) ||
        snap_dest) {
        snap->ti->error = "Snapshot cow pairing for exception "
                  "table handover failed";
        return -EINVAL;
    }

    /*
     * If no snap_src was found, snap cannot become a handover
     * destination.
     */
    if (!snap_src)
        return 0;

    /*
     * Non-snapshot-merge handover?
     */
    if (!dm_target_is_snapshot_merge(snap->ti))
        return 1;

    /*
     * Do not allow more than one merging snapshot.
     */
    if (snap_merge) {
        snap->ti->error = "A snapshot is already merging.";
        return -EINVAL;
    }

    if (!snap_src->store->type->prepare_merge ||
        !snap_src->store->type->commit_merge) {
        snap->ti->error = "Snapshot exception store does not "
                  "support snapshot-merge.";
        return -EINVAL;
    }

    return 1;
}

static void __insert_snapshot(struct origin *o, struct dm_snapshot *s)
{
    struct dm_snapshot *l;

    /* Sort the list according to chunk size, largest-first smallest-last */
    list_for_each_entry(l, &o->snapshots, list)
        if (l->store->chunk_size < s->store->chunk_size)
            break;
    list_add_tail(&s->list, &l->list);
}

/*
 * Make a note of the snapshot and its origin so we can look it
 * up when the origin has a write on it.
 *
 * Also validate snapshot exception store handovers.
 * On success, returns 1 if this registration is a handover destination,
 * otherwise returns 0.
 */
static int register_snapshot(struct dm_snapshot *snap)
{
    struct origin *o, *new_o = NULL;
    struct block_device *bdev = snap->origin->bdev;
    int r = 0;

    new_o = kmalloc(sizeof(*new_o), GFP_KERNEL);
    if (!new_o)
        return -ENOMEM;

    down_write(&_origins_lock);

    r = __validate_exception_handover(snap);
    if (r < 0) {
        kfree(new_o);
        goto out;
    }

    o = __lookup_origin(bdev);
    if (o)
        kfree(new_o);
    else {
        /* New origin */
        o = new_o;

        /* Initialise the struct */
        INIT_LIST_HEAD(&o->snapshots);
        o->bdev = bdev;

        __insert_origin(o);
    }

    __insert_snapshot(o, snap);

out:
    up_write(&_origins_lock);

    return r;
}

/*
 * Move snapshot to correct place in list according to chunk size.
 */
static void reregister_snapshot(struct dm_snapshot *s)
{
    struct block_device *bdev = s->origin->bdev;

    down_write(&_origins_lock);

    list_del(&s->list);
    __insert_snapshot(__lookup_origin(bdev), s);

    up_write(&_origins_lock);
}

static void unregister_snapshot(struct dm_snapshot *s)
{
    struct origin *o;

    down_write(&_origins_lock);
    o = __lookup_origin(s->origin->bdev);

    list_del(&s->list);
    if (o && list_empty(&o->snapshots)) {
        list_del(&o->hash_list);
        kfree(o);
    }

    up_write(&_origins_lock);
}

/*
 * Implementation of the exception hash tables.
 * The lowest hash_shift bits of the chunk number are ignored, allowing
 * some consecutive chunks to be grouped together.
 */
static int dm_exception_table_init(struct dm_exception_table *et,
                   uint32_t size, unsigned hash_shift)
{
    unsigned int i;

    et->hash_shift = hash_shift;
    et->hash_mask = size - 1;
    et->table = dm_vcalloc(size, sizeof(struct list_head));
    if (!et->table)
        return -ENOMEM;

    for (i = 0; i < size; i++)
        INIT_LIST_HEAD(et->table + i);

    return 0;
}

static void dm_exception_table_exit(struct dm_exception_table *et,
                    struct kmem_cache *mem)
{
    struct list_head *slot;
    struct dm_exception *ex, *next;
    int i, size;

    size = et->hash_mask + 1;
    for (i = 0; i < size; i++) {
        slot = et->table + i;

        list_for_each_entry_safe (ex, next, slot, hash_list)
            kmem_cache_free(mem, ex);
    }

    vfree(et->table);
}

static uint32_t exception_hash(struct dm_exception_table *et, chunk_t chunk)
{
    return (chunk >> et->hash_shift) & et->hash_mask;
}

static void dm_remove_exception(struct dm_exception *e)
{
    list_del(&e->hash_list);
}

/*
 * Return the exception data for a sector, or NULL if not
 * remapped.
 */
static struct dm_exception *dm_lookup_exception(struct dm_exception_table *et,
                        chunk_t chunk)
{
    struct list_head *slot;
    struct dm_exception *e;

    slot = &et->table[exception_hash(et, chunk)];
    list_for_each_entry (e, slot, hash_list)
        if (chunk >= e->old_chunk &&
            chunk <= e->old_chunk + dm_consecutive_chunk_count(e))
            return e;

    return NULL;
}

static struct dm_exception *alloc_completed_exception(void)
{
    struct dm_exception *e;

    e = kmem_cache_alloc(exception_cache, GFP_NOIO);
    if (!e)
        e = kmem_cache_alloc(exception_cache, GFP_ATOMIC);

    return e;
}

static void free_completed_exception(struct dm_exception *e)
{
    kmem_cache_free(exception_cache, e);
}

static struct dm_snap_pending_exception *alloc_pending_exception(struct dm_snapshot *s)
{
    struct dm_snap_pending_exception *pe = mempool_alloc(s->pending_pool,
                                 GFP_NOIO);

    atomic_inc(&s->pending_exceptions_count);
    pe->snap = s;

    return pe;
}

static void free_pending_exception(struct dm_snap_pending_exception *pe)
{
    struct dm_snapshot *s = pe->snap;

    mempool_free(pe, s->pending_pool);
    smp_mb__before_atomic_dec();
    atomic_dec(&s->pending_exceptions_count);
}

static void dm_insert_exception(struct dm_exception_table *eh,
                struct dm_exception *new_e)
{
    struct list_head *l;
    struct dm_exception *e = NULL;

    l = &eh->table[exception_hash(eh, new_e->old_chunk)];

    /* Add immediately if this table doesn't support consecutive chunks */
    if (!eh->hash_shift)
        goto out;

    /* List is ordered by old_chunk */
    list_for_each_entry_reverse(e, l, hash_list) {
        /* Insert after an existing chunk? */
        if (new_e->old_chunk == (e->old_chunk +
                     dm_consecutive_chunk_count(e) + 1) &&
            new_e->new_chunk == (dm_chunk_number(e->new_chunk) +
                     dm_consecutive_chunk_count(e) + 1)) {
            dm_consecutive_chunk_count_inc(e);
            free_completed_exception(new_e);
            return;
        }

        /* Insert before an existing chunk? */
        if (new_e->old_chunk == (e->old_chunk - 1) &&
            new_e->new_chunk == (dm_chunk_number(e->new_chunk) - 1)) {
            dm_consecutive_chunk_count_inc(e);
            e->old_chunk--;
            e->new_chunk--;
            free_completed_exception(new_e);
            return;
        }

        if (new_e->old_chunk > e->old_chunk)
            break;
    }

out:
    list_add(&new_e->hash_list, e ? &e->hash_list : l);
}

/*
 * Callback used by the exception stores to load exceptions when
 * initialising.
 */
static int dm_add_exception(void *context, chunk_t old, chunk_t new)
{
    struct dm_snapshot *s = context;
    struct dm_exception *e;

    e = alloc_completed_exception();
    if (!e)
        return -ENOMEM;

    e->old_chunk = old;

    /* Consecutive_count is implicitly initialised to zero */
    e->new_chunk = new;

    dm_insert_exception(&s->complete, e);

    return 0;
}

/*
 * Return a minimum chunk size of all snapshots that have the specified origin.
 * Return zero if the origin has no snapshots.
 */
static uint32_t __minimum_chunk_size(struct origin *o)
{
    struct dm_snapshot *snap;
    unsigned chunk_size = 0;

    if (o)
        list_for_each_entry(snap, &o->snapshots, list)
            chunk_size = min_not_zero(chunk_size,
                          snap->store->chunk_size);

    return (uint32_t) chunk_size;
}

/*
 * Hard coded magic.
 */
static int calc_max_buckets(void)
{
    /* use a fixed size of 2MB */
    unsigned long mem = 2 * 1024 * 1024;
    mem /= sizeof(struct list_head);

    return mem;
}

/*
 * Allocate room for a suitable hash table.
 */
static int init_hash_tables(struct dm_snapshot *s)
{
    sector_t hash_size, cow_dev_size, origin_dev_size, max_buckets;

    /*
     * Calculate based on the size of the original volume or
     * the COW volume...
     */
    cow_dev_size = get_dev_size(s->cow->bdev);
    origin_dev_size = get_dev_size(s->origin->bdev);
    max_buckets = calc_max_buckets();

    hash_size = min(origin_dev_size, cow_dev_size) >> s->store->chunk_shift;
    hash_size = min(hash_size, max_buckets);

    if (hash_size < 64)
        hash_size = 64;
    hash_size = rounddown_pow_of_two(hash_size);
    if (dm_exception_table_init(&s->complete, hash_size,
                    DM_CHUNK_CONSECUTIVE_BITS))
        return -ENOMEM;

    /*
     * Allocate hash table for in-flight exceptions
     * Make this smaller than the real hash table
     */
    hash_size >>= 3;
    if (hash_size < 64)
        hash_size = 64;

    if (dm_exception_table_init(&s->pending, hash_size, 0)) {
        dm_exception_table_exit(&s->complete, exception_cache);
        return -ENOMEM;
    }

    return 0;
}

static void merge_shutdown(struct dm_snapshot *s)
{
    clear_bit_unlock(RUNNING_MERGE, &s->state_bits);
    smp_mb__after_clear_bit();
    wake_up_bit(&s->state_bits, RUNNING_MERGE);
}

static struct bio *__release_queued_bios_after_merge(struct dm_snapshot *s)
{
    s->first_merging_chunk = 0;
    s->num_merging_chunks = 0;

    return bio_list_get(&s->bios_queued_during_merge);
}

/*
 * Remove one chunk from the index of completed exceptions.
 */
static int __remove_single_exception_chunk(struct dm_snapshot *s,
                       chunk_t old_chunk)
{
    struct dm_exception *e;

    e = dm_lookup_exception(&s->complete, old_chunk);
    if (!e) {
        DMERR("Corruption detected: exception for block %llu is "
              "on disk but not in memory",
              (unsigned long long)old_chunk);
        return -EINVAL;
    }

    /*
     * If this is the only chunk using this exception, remove exception.
     */
    if (!dm_consecutive_chunk_count(e)) {
        dm_remove_exception(e);
        free_completed_exception(e);
        return 0;
    }

    /*
     * The chunk may be either at the beginning or the end of a
     * group of consecutive chunks - never in the middle.  We are
     * removing chunks in the opposite order to that in which they
     * were added, so this should always be true.
     * Decrement the consecutive chunk counter and adjust the
     * starting point if necessary.
     */
    if (old_chunk == e->old_chunk) {
        e->old_chunk++;
        e->new_chunk++;
    } else if (old_chunk != e->old_chunk +
           dm_consecutive_chunk_count(e)) {
        DMERR("Attempt to merge block %llu from the "
              "middle of a chunk range [%llu - %llu]",
              (unsigned long long)old_chunk,
              (unsigned long long)e->old_chunk,
              (unsigned long long)
              e->old_chunk + dm_consecutive_chunk_count(e));
        return -EINVAL;
    }

    dm_consecutive_chunk_count_dec(e);

    return 0;
}

static void flush_bios(struct bio *bio);

static int remove_single_exception_chunk(struct dm_snapshot *s)
{
    struct bio *b = NULL;
    int r;
    chunk_t old_chunk = s->first_merging_chunk + s->num_merging_chunks - 1;

    down_write(&s->lock);

    /*
     * Process chunks (and associated exceptions) in reverse order
     * so that dm_consecutive_chunk_count_dec() accounting works.
     */
    do {
        r = __remove_single_exception_chunk(s, old_chunk);
        if (r)
            goto out;
    } while (old_chunk-- > s->first_merging_chunk);

    b = __release_queued_bios_after_merge(s);

out:
    up_write(&s->lock);
    if (b)
        flush_bios(b);

    return r;
}

static int origin_write_extent(struct dm_snapshot *merging_snap,
                   sector_t sector, unsigned chunk_size);

static void merge_callback(int read_err, unsigned long write_err,
               void *context);

static uint64_t read_pending_exceptions_done_count(void)
{
    uint64_t pending_exceptions_done;

    spin_lock(&_pending_exceptions_done_spinlock);
    pending_exceptions_done = _pending_exceptions_done_count;
    spin_unlock(&_pending_exceptions_done_spinlock);

    return pending_exceptions_done;
}

static void increment_pending_exceptions_done_count(void)
{
    spin_lock(&_pending_exceptions_done_spinlock);
    _pending_exceptions_done_count++;
    spin_unlock(&_pending_exceptions_done_spinlock);

    wake_up_all(&_pending_exceptions_done);
}

static void snapshot_merge_next_chunks(struct dm_snapshot *s)
{
    int i, linear_chunks;
    chunk_t old_chunk, new_chunk;
    struct dm_io_region src, dest;
    sector_t io_size;
    uint64_t previous_count;

    BUG_ON(!test_bit(RUNNING_MERGE, &s->state_bits));
    if (unlikely(test_bit(SHUTDOWN_MERGE, &s->state_bits)))
        goto shut;

    /*
     * valid flag never changes during merge, so no lock required.
     */
    if (!s->valid) {
        DMERR("Snapshot is invalid: can't merge");
        goto shut;
    }

    linear_chunks = s->store->type->prepare_merge(s->store, &old_chunk,
                              &new_chunk);
    if (linear_chunks <= 0) {
        if (linear_chunks < 0) {
            DMERR("Read error in exception store: "
                  "shutting down merge");
            down_write(&s->lock);
            s->merge_failed = 1;
            up_write(&s->lock);
        }
        goto shut;
    }

    /* Adjust old_chunk and new_chunk to reflect start of linear region */
    old_chunk = old_chunk + 1 - linear_chunks;
    new_chunk = new_chunk + 1 - linear_chunks;

    /*
     * Use one (potentially large) I/O to copy all 'linear_chunks'
     * from the exception store to the origin
     */
    io_size = linear_chunks * s->store->chunk_size;

    dest.bdev = s->origin->bdev;
    dest.sector = chunk_to_sector(s->store, old_chunk);
    dest.count = min(io_size, get_dev_size(dest.bdev) - dest.sector);

    src.bdev = s->cow->bdev;
    src.sector = chunk_to_sector(s->store, new_chunk);
    src.count = dest.count;

    /*
     * Reallocate any exceptions needed in other snapshots then
     * wait for the pending exceptions to complete.
     * Each time any pending exception (globally on the system)
     * completes we are woken and repeat the process to find out
     * if we can proceed.  While this may not seem a particularly
     * efficient algorithm, it is not expected to have any
     * significant impact on performance.
     */
    previous_count = read_pending_exceptions_done_count();
    while (origin_write_extent(s, dest.sector, io_size)) {
        wait_event(_pending_exceptions_done,
               (read_pending_exceptions_done_count() !=
                previous_count));
        /* Retry after the wait, until all exceptions are done. */
        previous_count = read_pending_exceptions_done_count();
    }

    down_write(&s->lock);
    s->first_merging_chunk = old_chunk;
    s->num_merging_chunks = linear_chunks;
    up_write(&s->lock);

    /* Wait until writes to all 'linear_chunks' drain */
    for (i = 0; i < linear_chunks; i++)
        __check_for_conflicting_io(s, old_chunk + i);

    dm_kcopyd_copy(s->kcopyd_client, &src, 1, &dest, 0, merge_callback, s);
    return;

shut:
    merge_shutdown(s);
}

static void error_bios(struct bio *bio);

static void merge_callback(int read_err, unsigned long write_err, void *context)
{
    struct dm_snapshot *s = context;
    struct bio *b = NULL;

    if (read_err || write_err) {
        if (read_err)
            DMERR("Read error: shutting down merge.");
        else
            DMERR("Write error: shutting down merge.");
        goto shut;
    }

    if (s->store->type->commit_merge(s->store,
                     s->num_merging_chunks) < 0) {
        DMERR("Write error in exception store: shutting down merge");
        goto shut;
    }

    if (remove_single_exception_chunk(s) < 0)
        goto shut;

    snapshot_merge_next_chunks(s);

    return;

shut:
    down_write(&s->lock);
    s->merge_failed = 1;
    b = __release_queued_bios_after_merge(s);
    up_write(&s->lock);
    error_bios(b);

    merge_shutdown(s);
}

static void start_merge(struct dm_snapshot *s)
{
    if (!test_and_set_bit(RUNNING_MERGE, &s->state_bits))
        snapshot_merge_next_chunks(s);
}

static int wait_schedule(void *ptr)
{
    schedule();

    return 0;
}

/*
 * Stop the merging process and wait until it finishes.
 */
static void stop_merge(struct dm_snapshot *s)
{
    set_bit(SHUTDOWN_MERGE, &s->state_bits);
    wait_on_bit(&s->state_bits, RUNNING_MERGE, wait_schedule,
            TASK_UNINTERRUPTIBLE);
    clear_bit(SHUTDOWN_MERGE, &s->state_bits);
}

/*
 * Construct a snapshot mapping: <origin_dev> <COW-dev> <p/n> <chunk-size>
 */
static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
{
    struct dm_snapshot *s;
    int i;
    int r = -EINVAL;
    char *origin_path, *cow_path;
    unsigned args_used, num_flush_requests = 1;
    fmode_t origin_mode = FMODE_READ;

    if (argc != 4) {
        ti->error = "requires exactly 4 arguments";
        r = -EINVAL;
        goto bad;
    }

    if (dm_target_is_snapshot_merge(ti)) {
        num_flush_requests = 2;
        origin_mode = FMODE_WRITE;
    }

    s = kmalloc(sizeof(*s), GFP_KERNEL);
    if (!s) {
        ti->error = "Cannot allocate private snapshot structure";
        r = -ENOMEM;
        goto bad;
    }

    origin_path = argv[0];
    argv++;
    argc--;

    r = dm_get_device(ti, origin_path, origin_mode, &s->origin);
    if (r) {
        ti->error = "Cannot get origin device";
        goto bad_origin;
    }

    cow_path = argv[0];
    argv++;
    argc--;

    r = dm_get_device(ti, cow_path, dm_table_get_mode(ti->table), &s->cow);
    if (r) {
        ti->error = "Cannot get COW device";
        goto bad_cow;
    }

    r = dm_exception_store_create(ti, argc, argv, s, &args_used, &s->store);
    if (r) {
        ti->error = "Couldn't create exception store";
        r = -EINVAL;
        goto bad_store;
    }

    argv += args_used;
    argc -= args_used;

    s->ti = ti;
    s->valid = 1;
    s->active = 0;
    atomic_set(&s->pending_exceptions_count, 0);
    init_rwsem(&s->lock);
    INIT_LIST_HEAD(&s->list);
    spin_lock_init(&s->pe_lock);
    s->state_bits = 0;
    s->merge_failed = 0;
    s->first_merging_chunk = 0;
    s->num_merging_chunks = 0;
    bio_list_init(&s->bios_queued_during_merge);

    /* Allocate hash table for COW data */
    if (init_hash_tables(s)) {
        ti->error = "Unable to allocate hash table space";
        r = -ENOMEM;
        goto bad_hash_tables;
    }

    s->kcopyd_client = dm_kcopyd_client_create();
    if (IS_ERR(s->kcopyd_client)) {
        r = PTR_ERR(s->kcopyd_client);
        ti->error = "Could not create kcopyd client";
        goto bad_kcopyd;
    }

    s->pending_pool = mempool_create_slab_pool(MIN_IOS, pending_cache);
    if (!s->pending_pool) {
        ti->error = "Could not allocate mempool for pending exceptions";
        goto bad_pending_pool;
    }

    s->tracked_chunk_pool = mempool_create_slab_pool(MIN_IOS,
                             tracked_chunk_cache);
    if (!s->tracked_chunk_pool) {
        ti->error = "Could not allocate tracked_chunk mempool for "
                "tracking reads";
        goto bad_tracked_chunk_pool;
    }

    for (i = 0; i < DM_TRACKED_CHUNK_HASH_SIZE; i++)
        INIT_HLIST_HEAD(&s->tracked_chunk_hash[i]);

    spin_lock_init(&s->tracked_chunk_lock);

    ti->private = s;
    ti->num_flush_requests = num_flush_requests;

    /* Add snapshot to the list of snapshots for this origin */
    /* Exceptions aren't triggered till snapshot_resume() is called */
    r = register_snapshot(s);
    if (r == -ENOMEM) {
        ti->error = "Snapshot origin struct allocation failed";
        goto bad_load_and_register;
    } else if (r < 0) {
        /* invalid handover, register_snapshot has set ti->error */
        goto bad_load_and_register;
    }

    /*
     * Metadata must only be loaded into one table at once, so skip this
     * if metadata will be handed over during resume.
     * Chunk size will be set during the handover - set it to zero to
     * ensure it's ignored.
     */
    if (r > 0) {
        s->store->chunk_size = 0;
        return 0;
    }

    r = s->store->type->read_metadata(s->store, dm_add_exception,
                      (void *)s);
    if (r < 0) {
        ti->error = "Failed to read snapshot metadata";
        goto bad_read_metadata;
    } else if (r > 0) {
        s->valid = 0;
        DMWARN("Snapshot is marked invalid.");
    }

    if (!s->store->chunk_size) {
        ti->error = "Chunk size not set";
        goto bad_read_metadata;
    }

    r = dm_set_target_max_io_len(ti, s->store->chunk_size);
    if (r)
        goto bad_read_metadata;

    return 0;

bad_read_metadata:
    unregister_snapshot(s);

bad_load_and_register:
    mempool_destroy(s->tracked_chunk_pool);

bad_tracked_chunk_pool:
    mempool_destroy(s->pending_pool);

bad_pending_pool:
    dm_kcopyd_client_destroy(s->kcopyd_client);

bad_kcopyd:
    dm_exception_table_exit(&s->pending, pending_cache);
    dm_exception_table_exit(&s->complete, exception_cache);

bad_hash_tables:
    dm_exception_store_destroy(s->store);

bad_store:
    dm_put_device(ti, s->cow);

bad_cow:
    dm_put_device(ti, s->origin);

bad_origin:
    kfree(s);

bad:
    return r;
}

static void __free_exceptions(struct dm_snapshot *s)
{
    dm_kcopyd_client_destroy(s->kcopyd_client);
    s->kcopyd_client = NULL;

    dm_exception_table_exit(&s->pending, pending_cache);
    dm_exception_table_exit(&s->complete, exception_cache);
}

static void __handover_exceptions(struct dm_snapshot *snap_src,
                  struct dm_snapshot *snap_dest)
{
    union {
        struct dm_exception_table table_swap;
        struct dm_exception_store *store_swap;
    } u;

    /*
     * Swap all snapshot context information between the two instances.
     */
    u.table_swap = snap_dest->complete;
    snap_dest->complete = snap_src->complete;
    snap_src->complete = u.table_swap;

    u.store_swap = snap_dest->store;
    snap_dest->store = snap_src->store;
    snap_src->store = u.store_swap;

    snap_dest->store->snap = snap_dest;
    snap_src->store->snap = snap_src;

    snap_dest->ti->max_io_len = snap_dest->store->chunk_size;
    snap_dest->valid = snap_src->valid;

    /*
     * Set source invalid to ensure it receives no further I/O.
     */
    snap_src->valid = 0;
}

static void snapshot_dtr(struct dm_target *ti)
{
#ifdef CONFIG_DM_DEBUG
    int i;
#endif
    struct dm_snapshot *s = ti->private;
    struct dm_snapshot *snap_src = NULL, *snap_dest = NULL;

    down_read(&_origins_lock);
    /* Check whether exception handover must be cancelled */
    (void) __find_snapshots_sharing_cow(s, &snap_src, &snap_dest, NULL);
    if (snap_src && snap_dest && (s == snap_src)) {
        down_write(&snap_dest->lock);
        snap_dest->valid = 0;
        up_write(&snap_dest->lock);
        DMERR("Cancelling snapshot handover.");
    }
    up_read(&_origins_lock);

    if (dm_target_is_snapshot_merge(ti))
        stop_merge(s);

    /* Prevent further origin writes from using this snapshot. */
    /* After this returns there can be no new kcopyd jobs. */
    unregister_snapshot(s);

    while (atomic_read(&s->pending_exceptions_count))
        msleep(1);
    /*
     * Ensure instructions in mempool_destroy aren't reordered
     * before atomic_read.
     */
    smp_mb();

#ifdef CONFIG_DM_DEBUG
    for (i = 0; i < DM_TRACKED_CHUNK_HASH_SIZE; i++)
        BUG_ON(!hlist_empty(&s->tracked_chunk_hash[i]));
#endif

    mempool_destroy(s->tracked_chunk_pool);

    __free_exceptions(s);

    mempool_destroy(s->pending_pool);

    dm_exception_store_destroy(s->store);

    dm_put_device(ti, s->cow);

    dm_put_device(ti, s->origin);

    kfree(s);
}

/*
 * Flush a list of buffers.
 */
static void flush_bios(struct bio *bio)
{
    struct bio *n;

    while (bio) {
        n = bio->bi_next;
        bio->bi_next = NULL;
        generic_make_request(bio);
        bio = n;
    }
}

static int do_origin(struct dm_dev *origin, struct bio *bio);

/*
 * Flush a list of buffers.
 */
static void retry_origin_bios(struct dm_snapshot *s, struct bio *bio)
{
    struct bio *n;
    int r;

    while (bio) {
        n = bio->bi_next;
        bio->bi_next = NULL;
        r = do_origin(s->origin, bio);
        if (r == DM_MAPIO_REMAPPED)
            generic_make_request(bio);
        bio = n;
    }
}

/*
 * Error a list of buffers.
 */
static void error_bios(struct bio *bio)
{
    struct bio *n;

    while (bio) {
        n = bio->bi_next;
        bio->bi_next = NULL;
        bio_io_error(bio);
        bio = n;
    }
}

static void __invalidate_snapshot(struct dm_snapshot *s, int err)
{
    if (!s->valid)
        return;

    if (err == -EIO)
        DMERR("Invalidating snapshot: Error reading/writing.");
    else if (err == -ENOMEM)
        DMERR("Invalidating snapshot: Unable to allocate exception.");

    if (s->store->type->drop_snapshot)
        s->store->type->drop_snapshot(s->store);

    s->valid = 0;

    dm_table_event(s->ti->table);
}

static void pending_complete(struct dm_snap_pending_exception *pe, int success)
{
    struct dm_exception *e;
    struct dm_snapshot *s = pe->snap;
    struct bio *origin_bios = NULL;
    struct bio *snapshot_bios = NULL;
    struct bio *full_bio = NULL;
    int error = 0;

    if (!success) {
        /* Read/write error - snapshot is unusable */
        down_write(&s->lock);
        __invalidate_snapshot(s, -EIO);
        error = 1;
        goto out;
    }

    e = alloc_completed_exception();
    if (!e) {
        down_write(&s->lock);
        __invalidate_snapshot(s, -ENOMEM);
        error = 1;
        goto out;
    }
    *e = pe->e;

    down_write(&s->lock);
    if (!s->valid) {
        free_completed_exception(e);
        error = 1;
        goto out;
    }

    /* Check for conflicting reads */
    __check_for_conflicting_io(s, pe->e.old_chunk);

    /*
     * Add a proper exception, and remove the
     * in-flight exception from the list.
     */
    dm_insert_exception(&s->complete, e);

out:
    dm_remove_exception(&pe->e);
    snapshot_bios = bio_list_get(&pe->snapshot_bios);
    origin_bios = bio_list_get(&pe->origin_bios);
    full_bio = pe->full_bio;
    if (full_bio) {
        full_bio->bi_end_io = pe->full_bio_end_io;
        full_bio->bi_private = pe->full_bio_private;
    }
    free_pending_exception(pe);

    increment_pending_exceptions_done_count();

    up_write(&s->lock);

    /* Submit any pending write bios */
    if (error) {
        if (full_bio)
            bio_io_error(full_bio);
        error_bios(snapshot_bios);
    } else {
        if (full_bio)
            bio_endio(full_bio, 0);
        flush_bios(snapshot_bios);
    }

    retry_origin_bios(s, origin_bios);
}

static void commit_callback(void *context, int success)
{
    struct dm_snap_pending_exception *pe = context;

    pending_complete(pe, success);
}

/*
 * Called when the copy I/O has finished.  kcopyd actually runs
 * this code so don't block.
 */
static void copy_callback(int read_err, unsigned long write_err, void *context)
{
    struct dm_snap_pending_exception *pe = context;
    struct dm_snapshot *s = pe->snap;

    if (read_err || write_err)
        pending_complete(pe, 0);

    else
        /* Update the metadata if we are persistent */
        s->store->type->commit_exception(s->store, &pe->e,
                         commit_callback, pe);
}

/*
 * Dispatches the copy operation to kcopyd.
 */
static void start_copy(struct dm_snap_pending_exception *pe)
{
    struct dm_snapshot *s = pe->snap;
    struct dm_io_region src, dest;
    struct block_device *bdev = s->origin->bdev;
    sector_t dev_size;

    dev_size = get_dev_size(bdev);

    src.bdev = bdev;
    src.sector = chunk_to_sector(s->store, pe->e.old_chunk);
    src.count = min((sector_t)s->store->chunk_size, dev_size - src.sector);

    dest.bdev = s->cow->bdev;
    dest.sector = chunk_to_sector(s->store, pe->e.new_chunk);
    dest.count = src.count;

    /* Hand over to kcopyd */
    dm_kcopyd_copy(s->kcopyd_client, &src, 1, &dest, 0, copy_callback, pe);
}

static void full_bio_end_io(struct bio *bio, int error)
{
    void *callback_data = bio->bi_private;

    dm_kcopyd_do_callback(callback_data, 0, error ? 1 : 0);
}

static void start_full_bio(struct dm_snap_pending_exception *pe,
               struct bio *bio)
{
    struct dm_snapshot *s = pe->snap;
    void *callback_data;

    pe->full_bio = bio;
    pe->full_bio_end_io = bio->bi_end_io;
    pe->full_bio_private = bio->bi_private;

    callback_data = dm_kcopyd_prepare_callback(s->kcopyd_client,
                           copy_callback, pe);

    bio->bi_end_io = full_bio_end_io;
    bio->bi_private = callback_data;

    generic_make_request(bio);
}

static struct dm_snap_pending_exception *
__lookup_pending_exception(struct dm_snapshot *s, chunk_t chunk)
{
    struct dm_exception *e = dm_lookup_exception(&s->pending, chunk);

    if (!e)
        return NULL;

    return container_of(e, struct dm_snap_pending_exception, e);
}

/*
 * Looks to see if this snapshot already has a pending exception
 * for this chunk, otherwise it allocates a new one and inserts
 * it into the pending table.
 *
 * NOTE: a write lock must be held on snap->lock before calling
 * this.
 */
static struct dm_snap_pending_exception *
__find_pending_exception(struct dm_snapshot *s,
             struct dm_snap_pending_exception *pe, chunk_t chunk)
{
    struct dm_snap_pending_exception *pe2;

    pe2 = __lookup_pending_exception(s, chunk);
    if (pe2) {
        free_pending_exception(pe);
        return pe2;
    }

    pe->e.old_chunk = chunk;
    bio_list_init(&pe->origin_bios);
    bio_list_init(&pe->snapshot_bios);
    pe->started = 0;
    pe->full_bio = NULL;

    if (s->store->type->prepare_exception(s->store, &pe->e)) {
        free_pending_exception(pe);
        return NULL;
    }

    dm_insert_exception(&s->pending, &pe->e);

    return pe;
}

static void remap_exception(struct dm_snapshot *s, struct dm_exception *e,
                struct bio *bio, chunk_t chunk)
{
    bio->bi_bdev = s->cow->bdev;
    bio->bi_sector = chunk_to_sector(s->store,
                     dm_chunk_number(e->new_chunk) +
                     (chunk - e->old_chunk)) +
                     (bio->bi_sector &
                      s->store->chunk_mask);
}

static int snapshot_map(struct dm_target *ti, struct bio *bio,
            union map_info *map_context)
{
    struct dm_exception *e;
    struct dm_snapshot *s = ti->private;
    int r = DM_MAPIO_REMAPPED;
    chunk_t chunk;
    struct dm_snap_pending_exception *pe = NULL;

    if (bio->bi_rw & REQ_FLUSH) {
        bio->bi_bdev = s->cow->bdev;
        return DM_MAPIO_REMAPPED;
    }

    chunk = sector_to_chunk(s->store, bio->bi_sector);

    /* Full snapshots are not usable */
    /* To get here the table must be live so s->active is always set. */
    if (!s->valid)
        return -EIO;

    /* FIXME: should only take write lock if we need
     * to copy an exception */
    down_write(&s->lock);

    if (!s->valid) {
        r = -EIO;
        goto out_unlock;
    }

    /* If the block is already remapped - use that, else remap it */
    e = dm_lookup_exception(&s->complete, chunk);
    if (e) {
        remap_exception(s, e, bio, chunk);
        goto out_unlock;
    }

    /*
     * Write to snapshot - higher level takes care of RW/RO
     * flags so we should only get this if we are
     * writeable.
     */
    if (bio_rw(bio) == WRITE) {
        pe = __lookup_pending_exception(s, chunk);
        if (!pe) {
            up_write(&s->lock);
            pe = alloc_pending_exception(s);
            down_write(&s->lock);

            if (!s->valid) {
                free_pending_exception(pe);
                r = -EIO;
                goto out_unlock;
            }

            e = dm_lookup_exception(&s->complete, chunk);
            if (e) {
                free_pending_exception(pe);
                remap_exception(s, e, bio, chunk);
                goto out_unlock;
            }

            pe = __find_pending_exception(s, pe, chunk);
            if (!pe) {
                __invalidate_snapshot(s, -ENOMEM);
                r = -EIO;
                goto out_unlock;
            }
        }

        remap_exception(s, &pe->e, bio, chunk);

        r = DM_MAPIO_SUBMITTED;

        if (!pe->started &&
            bio->bi_size == (s->store->chunk_size << SECTOR_SHIFT)) {
            pe->started = 1;
            up_write(&s->lock);
            start_full_bio(pe, bio);
            goto out;
        }

        bio_list_add(&pe->snapshot_bios, bio);

        if (!pe->started) {
            /* this is protected by snap->lock */
            pe->started = 1;
            up_write(&s->lock);
            start_copy(pe);
            goto out;
        }
    } else {
        bio->bi_bdev = s->origin->bdev;
        map_context->ptr = track_chunk(s, chunk);
    }

out_unlock:
    up_write(&s->lock);
out:
    return r;
}

/*
 * A snapshot-merge target behaves like a combination of a snapshot
 * target and a snapshot-origin target.  It only generates new
 * exceptions in other snapshots and not in the one that is being
 * merged.
 *
 * For each chunk, if there is an existing exception, it is used to
 * redirect I/O to the cow device.  Otherwise I/O is sent to the origin,
 * which in turn might generate exceptions in other snapshots.
 * If merging is currently taking place on the chunk in question, the
 * I/O is deferred by adding it to s->bios_queued_during_merge.
 */
static int snapshot_merge_map(struct dm_target *ti, struct bio *bio,
                  union map_info *map_context)
{
    struct dm_exception *e;
    struct dm_snapshot *s = ti->private;
    int r = DM_MAPIO_REMAPPED;
    chunk_t chunk;

    if (bio->bi_rw & REQ_FLUSH) {
        if (!map_context->target_request_nr)
            bio->bi_bdev = s->origin->bdev;
        else
            bio->bi_bdev = s->cow->bdev;
        map_context->ptr = NULL;
        return DM_MAPIO_REMAPPED;
    }

    chunk = sector_to_chunk(s->store, bio->bi_sector);

    down_write(&s->lock);

    /* Full merging snapshots are redirected to the origin */
    if (!s->valid)
        goto redirect_to_origin;

    /* If the block is already remapped - use that */
    e = dm_lookup_exception(&s->complete, chunk);
    if (e) {
        /* Queue writes overlapping with chunks being merged */
        if (bio_rw(bio) == WRITE &&
            chunk >= s->first_merging_chunk &&
            chunk < (s->first_merging_chunk +
                 s->num_merging_chunks)) {
            bio->bi_bdev = s->origin->bdev;
            bio_list_add(&s->bios_queued_during_merge, bio);
            r = DM_MAPIO_SUBMITTED;
            goto out_unlock;
        }

        remap_exception(s, e, bio, chunk);

        if (bio_rw(bio) == WRITE)
            map_context->ptr = track_chunk(s, chunk);
        goto out_unlock;
    }

redirect_to_origin:
    bio->bi_bdev = s->origin->bdev;

    if (bio_rw(bio) == WRITE) {
        up_write(&s->lock);
        return do_origin(s->origin, bio);
    }

out_unlock:
    up_write(&s->lock);

    return r;
}

static int snapshot_end_io(struct dm_target *ti, struct bio *bio,
               int error, union map_info *map_context)
{
    struct dm_snapshot *s = ti->private;
    struct dm_snap_tracked_chunk *c = map_context->ptr;

    if (c)
        stop_tracking_chunk(s, c);

    return 0;
}

static void snapshot_merge_presuspend(struct dm_target *ti)
{
    struct dm_snapshot *s = ti->private;

    stop_merge(s);
}

static int snapshot_preresume(struct dm_target *ti)
{
    int r = 0;
    struct dm_snapshot *s = ti->private;
    struct dm_snapshot *snap_src = NULL, *snap_dest = NULL;

    down_read(&_origins_lock);
    (void) __find_snapshots_sharing_cow(s, &snap_src, &snap_dest, NULL);
    if (snap_src && snap_dest) {
        down_read(&snap_src->lock);
        if (s == snap_src) {
            DMERR("Unable to resume snapshot source until "
                  "handover completes.");
            r = -EINVAL;
        } else if (!dm_suspended(snap_src->ti)) {
            DMERR("Unable to perform snapshot handover until "
                  "source is suspended.");
            r = -EINVAL;
        }
        up_read(&snap_src->lock);
    }
    up_read(&_origins_lock);

    return r;
}

static void snapshot_resume(struct dm_target *ti)
{
    struct dm_snapshot *s = ti->private;
    struct dm_snapshot *snap_src = NULL, *snap_dest = NULL;

    down_read(&_origins_lock);
    (void) __find_snapshots_sharing_cow(s, &snap_src, &snap_dest, NULL);
    if (snap_src && snap_dest) {
        down_write(&snap_src->lock);
        down_write_nested(&snap_dest->lock, SINGLE_DEPTH_NESTING);
        __handover_exceptions(snap_src, snap_dest);
        up_write(&snap_dest->lock);
        up_write(&snap_src->lock);
    }
    up_read(&_origins_lock);

    /* Now we have correct chunk size, reregister */
    reregister_snapshot(s);

    down_write(&s->lock);
    s->active = 1;
    up_write(&s->lock);
}

static uint32_t get_origin_minimum_chunksize(struct block_device *bdev)
{
    uint32_t min_chunksize;

    down_read(&_origins_lock);
    min_chunksize = __minimum_chunk_size(__lookup_origin(bdev));
    up_read(&_origins_lock);

    return min_chunksize;
}

static void snapshot_merge_resume(struct dm_target *ti)
{
    struct dm_snapshot *s = ti->private;

    /*
     * Handover exceptions from existing snapshot.
     */
    snapshot_resume(ti);

    /*
     * snapshot-merge acts as an origin, so set ti->max_io_len
     */
    ti->max_io_len = get_origin_minimum_chunksize(s->origin->bdev);

    start_merge(s);
}

static int snapshot_status(struct dm_target *ti, status_type_t type,
               unsigned status_flags, char *result, unsigned maxlen)
{
    unsigned sz = 0;
    struct dm_snapshot *snap = ti->private;

    switch (type) {
    case STATUSTYPE_INFO:

        down_write(&snap->lock);

        if (!snap->valid)
            DMEMIT("Invalid");
        else if (snap->merge_failed)
            DMEMIT("Merge failed");
        else {
            if (snap->store->type->usage) {
                sector_t total_sectors, sectors_allocated,
                     metadata_sectors;
                snap->store->type->usage(snap->store,
                             &total_sectors,
                             &sectors_allocated,
                             &metadata_sectors);
                DMEMIT("%llu/%llu %llu",
                       (unsigned long long)sectors_allocated,
                       (unsigned long long)total_sectors,
                       (unsigned long long)metadata_sectors);
            }
            else
                DMEMIT("Unknown");
        }

        up_write(&snap->lock);

        break;

    case STATUSTYPE_TABLE:
        /*
         * kdevname returns a static pointer so we need
         * to make private copies if the output is to
         * make sense.
         */
        DMEMIT("%s %s", snap->origin->name, snap->cow->name);
        snap->store->type->status(snap->store, type, result + sz,
                      maxlen - sz);
        break;
    }

    return 0;
}

static int snapshot_iterate_devices(struct dm_target *ti,
                    iterate_devices_callout_fn fn, void *data)
{
    struct dm_snapshot *snap = ti->private;
    int r;

    r = fn(ti, snap->origin, 0, ti->len, data);

    if (!r)
        r = fn(ti, snap->cow, 0, get_dev_size(snap->cow->bdev), data);

    return r;
}


/*-----------------------------------------------------------------
 * Origin methods
 *---------------------------------------------------------------*/

/*
 * If no exceptions need creating, DM_MAPIO_REMAPPED is returned and any
 * supplied bio was ignored.  The caller may submit it immediately.
 * (No remapping actually occurs as the origin is always a direct linear
 * map.)
 *
 * If further exceptions are required, DM_MAPIO_SUBMITTED is returned
 * and any supplied bio is added to a list to be submitted once all
 * the necessary exceptions exist.
 */
static int __origin_write(struct list_head *snapshots, sector_t sector,
              struct bio *bio)
{
    int r = DM_MAPIO_REMAPPED;
    struct dm_snapshot *snap;
    struct dm_exception *e;
    struct dm_snap_pending_exception *pe;
    struct dm_snap_pending_exception *pe_to_start_now = NULL;
    struct dm_snap_pending_exception *pe_to_start_last = NULL;
    chunk_t chunk;

    /* Do all the snapshots on this origin */
    list_for_each_entry (snap, snapshots, list) {
        /*
         * Don't make new exceptions in a merging snapshot
         * because it has effectively been deleted
         */
        if (dm_target_is_snapshot_merge(snap->ti))
            continue;

        down_write(&snap->lock);

        /* Only deal with valid and active snapshots */
        if (!snap->valid || !snap->active)
            goto next_snapshot;

        /* Nothing to do if writing beyond end of snapshot */
        if (sector >= dm_table_get_size(snap->ti->table))
            goto next_snapshot;

        /*
         * Remember, different snapshots can have
         * different chunk sizes.
         */
        chunk = sector_to_chunk(snap->store, sector);

        /*
         * Check exception table to see if block
         * is already remapped in this snapshot
         * and trigger an exception if not.
         */
        e = dm_lookup_exception(&snap->complete, chunk);
        if (e)
            goto next_snapshot;

        pe = __lookup_pending_exception(snap, chunk);
        if (!pe) {
            up_write(&snap->lock);
            pe = alloc_pending_exception(snap);
            down_write(&snap->lock);

            if (!snap->valid) {
                free_pending_exception(pe);
                goto next_snapshot;
            }

            e = dm_lookup_exception(&snap->complete, chunk);
            if (e) {
                free_pending_exception(pe);
                goto next_snapshot;
            }

            pe = __find_pending_exception(snap, pe, chunk);
            if (!pe) {
                __invalidate_snapshot(snap, -ENOMEM);
                goto next_snapshot;
            }
        }

        r = DM_MAPIO_SUBMITTED;

        /*
         * If an origin bio was supplied, queue it to wait for the
         * completion of this exception, and start this one last,
         * at the end of the function.
         */
        if (bio) {
            bio_list_add(&pe->origin_bios, bio);
            bio = NULL;

            if (!pe->started) {
                pe->started = 1;
                pe_to_start_last = pe;
            }
        }

        if (!pe->started) {
            pe->started = 1;
            pe_to_start_now = pe;
        }

next_snapshot:
        up_write(&snap->lock);

        if (pe_to_start_now) {
            start_copy(pe_to_start_now);
            pe_to_start_now = NULL;
        }
    }

    /*
     * Submit the exception against which the bio is queued last,
     * to give the other exceptions a head start.
     */
    if (pe_to_start_last)
        start_copy(pe_to_start_last);

    return r;
}

/*
 * Called on a write from the origin driver.
 */
static int do_origin(struct dm_dev *origin, struct bio *bio)
{
    struct origin *o;
    int r = DM_MAPIO_REMAPPED;

    down_read(&_origins_lock);
    o = __lookup_origin(origin->bdev);
    if (o)
        r = __origin_write(&o->snapshots, bio->bi_sector, bio);
    up_read(&_origins_lock);

    return r;
}

/*
 * Trigger exceptions in all non-merging snapshots.
 *
 * The chunk size of the merging snapshot may be larger than the chunk
 * size of some other snapshot so we may need to reallocate multiple
 * chunks in other snapshots.
 *
 * We scan all the overlapping exceptions in the other snapshots.
 * Returns 1 if anything was reallocated and must be waited for,
 * otherwise returns 0.
 *
 * size must be a multiple of merging_snap's chunk_size.
 */
static int origin_write_extent(struct dm_snapshot *merging_snap,
                   sector_t sector, unsigned size)
{
    int must_wait = 0;
    sector_t n;
    struct origin *o;

    /*
     * The origin's __minimum_chunk_size() got stored in max_io_len
     * by snapshot_merge_resume().
     */
    down_read(&_origins_lock);
    o = __lookup_origin(merging_snap->origin->bdev);
    for (n = 0; n < size; n += merging_snap->ti->max_io_len)
        if (__origin_write(&o->snapshots, sector + n, NULL) ==
            DM_MAPIO_SUBMITTED)
            must_wait = 1;
    up_read(&_origins_lock);

    return must_wait;
}

/*
 * Origin: maps a linear range of a device, with hooks for snapshotting.
 */

/*
 * Construct an origin mapping: <dev_path>
 * The context for an origin is merely a 'struct dm_dev *'
 * pointing to the real device.
 */
static int origin_ctr(struct dm_target *ti, unsigned int argc, char **argv)
{
    int r;
    struct dm_dev *dev;

    if (argc != 1) {
        ti->error = "origin: incorrect number of arguments";
        return -EINVAL;
    }

    r = dm_get_device(ti, argv[0], dm_table_get_mode(ti->table), &dev);
    if (r) {
        ti->error = "Cannot get target device";
        return r;
    }

    ti->private = dev;
    ti->num_flush_requests = 1;

    return 0;
}

static void origin_dtr(struct dm_target *ti)
{
    struct dm_dev *dev = ti->private;
    dm_put_device(ti, dev);
}

static int origin_map(struct dm_target *ti, struct bio *bio,
              union map_info *map_context)
{
    struct dm_dev *dev = ti->private;
    bio->bi_bdev = dev->bdev;

    if (bio->bi_rw & REQ_FLUSH)
        return DM_MAPIO_REMAPPED;

    /* Only tell snapshots if this is a write */
    return (bio_rw(bio) == WRITE) ? do_origin(dev, bio) : DM_MAPIO_REMAPPED;
}

/*
 * Set the target "max_io_len" field to the minimum of all the snapshots'
 * chunk sizes.
 */
static void origin_resume(struct dm_target *ti)
{
    struct dm_dev *dev = ti->private;

    ti->max_io_len = get_origin_minimum_chunksize(dev->bdev);
}

static int origin_status(struct dm_target *ti, status_type_t type,
             unsigned status_flags, char *result, unsigned maxlen)
{
    struct dm_dev *dev = ti->private;

    switch (type) {
    case STATUSTYPE_INFO:
        result[0] = '\0';
        break;

    case STATUSTYPE_TABLE:
        snprintf(result, maxlen, "%s", dev->name);
        break;
    }

    return 0;
}

static int origin_merge(struct dm_target *ti, struct bvec_merge_data *bvm,
            struct bio_vec *biovec, int max_size)
{
    struct dm_dev *dev = ti->private;
    struct request_queue *q = bdev_get_queue(dev->bdev);

    if (!q->merge_bvec_fn)
        return max_size;

    bvm->bi_bdev = dev->bdev;

    return min(max_size, q->merge_bvec_fn(q, bvm, biovec));
}

static int origin_iterate_devices(struct dm_target *ti,
                  iterate_devices_callout_fn fn, void *data)
{
    struct dm_dev *dev = ti->private;

    return fn(ti, dev, 0, ti->len, data);
}

static struct target_type origin_target = {
    .name    = "snapshot-origin",
    .version = {1, 7, 1},
    .module  = THIS_MODULE,
    .ctr     = origin_ctr,
    .dtr     = origin_dtr,
    .map     = origin_map,
    .resume  = origin_resume,
    .status  = origin_status,
    .merge   = origin_merge,
    .iterate_devices = origin_iterate_devices,
};

static struct target_type snapshot_target = {
    .name    = "snapshot",
    .version = {1, 10, 0},
    .module  = THIS_MODULE,
    .ctr     = snapshot_ctr,
    .dtr     = snapshot_dtr,
    .map     = snapshot_map,
    .end_io  = snapshot_end_io,
    .preresume  = snapshot_preresume,
    .resume  = snapshot_resume,
    .status  = snapshot_status,
    .iterate_devices = snapshot_iterate_devices,
};

static struct target_type merge_target = {
    .name    = dm_snapshot_merge_target_name,
    .version = {1, 1, 0},
    .module  = THIS_MODULE,
    .ctr     = snapshot_ctr,
    .dtr     = snapshot_dtr,
    .map     = snapshot_merge_map,
    .end_io  = snapshot_end_io,
    .presuspend = snapshot_merge_presuspend,
    .preresume  = snapshot_preresume,
    .resume  = snapshot_merge_resume,
    .status  = snapshot_status,
    .iterate_devices = snapshot_iterate_devices,
};

static int __init dm_snapshot_init(void)
{
    int r;

    r = dm_exception_store_init();
    if (r) {
        DMERR("Failed to initialize exception stores");
        return r;
    }

    r = dm_register_target(&snapshot_target);
    if (r < 0) {
        DMERR("snapshot target register failed %d", r);
        goto bad_register_snapshot_target;
    }

    r = dm_register_target(&origin_target);
    if (r < 0) {
        DMERR("Origin target register failed %d", r);
        goto bad_register_origin_target;
    }

    r = dm_register_target(&merge_target);
    if (r < 0) {
        DMERR("Merge target register failed %d", r);
        goto bad_register_merge_target;
    }

    r = init_origin_hash();
    if (r) {
        DMERR("init_origin_hash failed.");
        goto bad_origin_hash;
    }

    exception_cache = KMEM_CACHE(dm_exception, 0);
    if (!exception_cache) {
        DMERR("Couldn't create exception cache.");
        r = -ENOMEM;
        goto bad_exception_cache;
    }

    pending_cache = KMEM_CACHE(dm_snap_pending_exception, 0);
    if (!pending_cache) {
        DMERR("Couldn't create pending cache.");
        r = -ENOMEM;
        goto bad_pending_cache;
    }

    tracked_chunk_cache = KMEM_CACHE(dm_snap_tracked_chunk, 0);
    if (!tracked_chunk_cache) {
        DMERR("Couldn't create cache to track chunks in use.");
        r = -ENOMEM;
        goto bad_tracked_chunk_cache;
    }

    return 0;

bad_tracked_chunk_cache:
    kmem_cache_destroy(pending_cache);
bad_pending_cache:
    kmem_cache_destroy(exception_cache);
bad_exception_cache:
    exit_origin_hash();
bad_origin_hash:
    dm_unregister_target(&merge_target);
bad_register_merge_target:
    dm_unregister_target(&origin_target);
bad_register_origin_target:
    dm_unregister_target(&snapshot_target);
bad_register_snapshot_target:
    dm_exception_store_exit();

    return r;
}

static void __exit dm_snapshot_exit(void)
{
    dm_unregister_target(&snapshot_target);
    dm_unregister_target(&origin_target);
    dm_unregister_target(&merge_target);

    exit_origin_hash();
    kmem_cache_destroy(pending_cache);
    kmem_cache_destroy(exception_cache);
    kmem_cache_destroy(tracked_chunk_cache);

    dm_exception_store_exit();
}

/* Module hooks */
module_init(dm_snapshot_init);
module_exit(dm_snapshot_exit);

MODULE_DESCRIPTION(DM_NAME " snapshot target");
MODULE_AUTHOR("Joe Thornber");
MODULE_LICENSE("GPL");