dm-snapshot: fix performance degradation due to small hash size

[pandora-kernel.git] / drivers / md / dm-snap.c

/*
 * 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 sector_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 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, max_buckets;

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

        hash_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";
                r = -ENOMEM;
                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;
        }
        ti->split_io = s->store->chunk_size;

        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->split_io = 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 sector_t get_origin_minimum_chunksize(struct block_device *bdev)
{
        sector_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->split_io
         */
        ti->split_io = get_origin_minimum_chunksize(s->origin->bdev);

        start_merge(s);
}

static void snapshot_status(struct dm_target *ti, status_type_t type,
                            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;
        }
}

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 split_io
         * by snapshot_merge_resume().
         */
        down_read(&_origins_lock);
        o = __lookup_origin(merging_snap->origin->bdev);
        for (n = 0; n < size; n += merging_snap->ti->split_io)
                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 "split_io" 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->split_io = get_origin_minimum_chunksize(dev->bdev);
}

static void origin_status(struct dm_target *ti, status_type_t type,
                          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;
        }
}

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;
        bvm->bi_sector = bvm->bi_sector;

        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, 2},
        .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, 1},
        .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");
MODULE_ALIAS("dm-snapshot-origin");
MODULE_ALIAS("dm-snapshot-merge");