1 files changed, 1017 insertions, 262 deletions

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 3a3ba46e6d4b..ee8eb283650d 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -25,6 +25,11 @@
 
 #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 percentage increment we will wake up users at
  */
@@ -49,7 +54,7 @@
 #define DM_TRACKED_CHUNK_HASH(x)        ((unsigned long)(x) & \
                                          (DM_TRACKED_CHUNK_HASH_SIZE - 1))
 
-struct exception_table {
+struct dm_exception_table {
         uint32_t hash_mask;
         unsigned hash_shift;
         struct list_head *table;
@@ -59,22 +64,31 @@ 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) */
+        /*
+         * 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;
 
+        /* Whether or not owning mapped_device is suspended */
+        int suspended;
+
         mempool_t *pending_pool;
 
         atomic_t pending_exceptions_count;
 
-        struct exception_table pending;
-        struct exception_table complete;
+        struct dm_exception_table pending;
+        struct dm_exception_table complete;
 
         /*
          * pe_lock protects all pending_exception operations and access
@@ -95,8 +109,51 @@ struct dm_snapshot {
         mempool_t *tracked_chunk_pool;
         spinlock_t tracked_chunk_lock;
         struct hlist_head tracked_chunk_hash[DM_TRACKED_CHUNK_HASH_SIZE];
+
+        /*
+         * 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;
+
+        /* 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;
+
+        /*
+         * 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_cow(struct dm_snapshot *s)
+{
+        return s->cow;
+}
+EXPORT_SYMBOL(dm_snap_cow);
+
 static struct workqueue_struct *ksnapd;
 static void flush_queued_bios(struct work_struct *work);
 
@@ -116,7 +173,7 @@ static int bdev_equal(struct block_device *lhs, struct block_device *rhs)
 }
 
 struct dm_snap_pending_exception {
-        struct dm_snap_exception e;
+        struct dm_exception e;
 
         /*
          * Origin buffers waiting for this to complete are held
@@ -125,28 +182,6 @@ struct dm_snap_pending_exception {
         struct bio_list origin_bios;
         struct bio_list snapshot_bios;
 
-        /*
-         * Short-term queue of pending exceptions prior to submission.
-         */
-        struct list_head list;
-
-        /*
-         * The primary pending_exception is the one that holds
-         * the ref_count and the list of origin_bios for a
-         * group of pending_exceptions.  It is always last to get freed.
-         * These fields get set up when writing to the origin.
-         */
-        struct dm_snap_pending_exception *primary_pe;
-
-        /*
-         * Number of pending_exceptions processing this chunk.
-         * When this drops to zero we must complete the origin bios.
-         * If incrementing or decrementing this, hold pe->snap->lock for
-         * the sibling concerned and not pe->primary_pe->snap->lock unless
-         * they are the same.
-         */
-        atomic_t ref_count;
-
         /* Pointer back to snapshot context */
         struct dm_snapshot *snap;
 
@@ -222,6 +257,16 @@ static int __chunk_is_tracked(struct dm_snapshot *s, chunk_t chunk)
 }
 
 /*
+ * 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 {
@@ -243,6 +288,10 @@ struct origin {
 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;
@@ -291,22 +340,144 @@ static void __insert_origin(struct origin *o)
 }
 
 /*
+ * _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 dm_snapshot *l;
-        struct origin *o, *new_o;
+        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);
-        o = __lookup_origin(bdev);
 
+        r = __validate_exception_handover(snap);
+        if (r < 0) {
+                kfree(new_o);
+                goto out;
+        }
+
+        o = __lookup_origin(bdev);
         if (o)
                 kfree(new_o);
         else {
@@ -320,14 +491,27 @@ static int register_snapshot(struct dm_snapshot *snap)
                 __insert_origin(o);
         }
 
-        /* Sort the list according to chunk size, largest-first smallest-last */
-        list_for_each_entry(l, &o->snapshots, list)
-                if (l->store->chunk_size < snap->store->chunk_size)
-                        break;
-        list_add_tail(&snap->list, &l->list);
+        __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);
-        return 0;
 }
 
 static void unregister_snapshot(struct dm_snapshot *s)
@@ -338,7 +522,7 @@ static void unregister_snapshot(struct dm_snapshot *s)
         o = __lookup_origin(s->origin->bdev);
 
         list_del(&s->list);
-        if (list_empty(&o->snapshots)) {
+        if (o && list_empty(&o->snapshots)) {
                 list_del(&o->hash_list);
                 kfree(o);
         }
@@ -351,8 +535,8 @@ static void unregister_snapshot(struct dm_snapshot *s)
  * The lowest hash_shift bits of the chunk number are ignored, allowing
  * some consecutive chunks to be grouped together.
  */
-static int init_exception_table(struct exception_table *et, uint32_t size,
-                                unsigned hash_shift)
+static int dm_exception_table_init(struct dm_exception_table *et,
+                                   uint32_t size, unsigned hash_shift)
 {
         unsigned int i;
 
@@ -368,10 +552,11 @@ static int init_exception_table(struct exception_table *et, uint32_t size,
         return 0;
 }
 
-static void exit_exception_table(struct exception_table *et, struct kmem_cache *mem)
+static void dm_exception_table_exit(struct dm_exception_table *et,
+                                    struct kmem_cache *mem)
 {
         struct list_head *slot;
-        struct dm_snap_exception *ex, *next;
+        struct dm_exception *ex, *next;
         int i, size;
 
         size = et->hash_mask + 1;
@@ -385,19 +570,12 @@ static void exit_exception_table(struct exception_table *et, struct kmem_cache *
         vfree(et->table);
 }
 
-static uint32_t exception_hash(struct exception_table *et, chunk_t chunk)
+static uint32_t exception_hash(struct dm_exception_table *et, chunk_t chunk)
 {
         return (chunk >> et->hash_shift) & et->hash_mask;
 }
 
-static void insert_exception(struct exception_table *eh,
-                             struct dm_snap_exception *e)
-{
-        struct list_head *l = &eh->table[exception_hash(eh, e->old_chunk)];
-        list_add(&e->hash_list, l);
-}
-
-static void remove_exception(struct dm_snap_exception *e)
+static void dm_remove_exception(struct dm_exception *e)
 {
         list_del(&e->hash_list);
 }
@@ -406,11 +584,11 @@ static void remove_exception(struct dm_snap_exception *e)
  * Return the exception data for a sector, or NULL if not
  * remapped.
  */
-static struct dm_snap_exception *lookup_exception(struct exception_table *et,
-                                                  chunk_t chunk)
+static struct dm_exception *dm_lookup_exception(struct dm_exception_table *et,
+                                                chunk_t chunk)
 {
         struct list_head *slot;
-        struct dm_snap_exception *e;
+        struct dm_exception *e;
 
         slot = &et->table[exception_hash(et, chunk)];
         list_for_each_entry (e, slot, hash_list)
@@ -421,9 +599,9 @@ static struct dm_snap_exception *lookup_exception(struct exception_table *et,
         return NULL;
 }
 
-static struct dm_snap_exception *alloc_exception(void)
+static struct dm_exception *alloc_completed_exception(void)
 {
-        struct dm_snap_exception *e;
+        struct dm_exception *e;
 
         e = kmem_cache_alloc(exception_cache, GFP_NOIO);
         if (!e)
@@ -432,7 +610,7 @@ static struct dm_snap_exception *alloc_exception(void)
         return e;
 }
 
-static void free_exception(struct dm_snap_exception *e)
+static void free_completed_exception(struct dm_exception *e)
 {
         kmem_cache_free(exception_cache, e);
 }
@@ -457,12 +635,11 @@ static void free_pending_exception(struct dm_snap_pending_exception *pe)
         atomic_dec(&s->pending_exceptions_count);
 }
 
-static void insert_completed_exception(struct dm_snapshot *s,
-                                       struct dm_snap_exception *new_e)
+static void dm_insert_exception(struct dm_exception_table *eh,
+                                struct dm_exception *new_e)
 {
-        struct exception_table *eh = &s->complete;
         struct list_head *l;
-        struct dm_snap_exception *e = NULL;
+        struct dm_exception *e = NULL;
 
         l = &eh->table[exception_hash(eh, new_e->old_chunk)];
 
@@ -478,7 +655,7 @@ static void insert_completed_exception(struct dm_snapshot *s,
                     new_e->new_chunk == (dm_chunk_number(e->new_chunk) +
                                          dm_consecutive_chunk_count(e) + 1)) {
                         dm_consecutive_chunk_count_inc(e);
-                        free_exception(new_e);
+                        free_completed_exception(new_e);
                         return;
                 }
 
@@ -488,7 +665,7 @@ static void insert_completed_exception(struct dm_snapshot *s,
                         dm_consecutive_chunk_count_inc(e);
                         e->old_chunk--;
                         e->new_chunk--;
-                        free_exception(new_e);
+                        free_completed_exception(new_e);
                         return;
                 }
 
@@ -507,9 +684,9 @@ out:
 static int dm_add_exception(void *context, chunk_t old, chunk_t new)
 {
         struct dm_snapshot *s = context;
-        struct dm_snap_exception *e;
+        struct dm_exception *e;
 
-        e = alloc_exception();
+        e = alloc_completed_exception();
         if (!e)
                 return -ENOMEM;
 
@@ -518,11 +695,30 @@ static int dm_add_exception(void *context, chunk_t old, chunk_t new)
         /* Consecutive_count is implicitly initialised to zero */
         e->new_chunk = new;
 
-        insert_completed_exception(s, e);
+        dm_insert_exception(&s->complete, e);
 
         return 0;
 }
 
+#define min_not_zero(l, r) (((l) == 0) ? (r) : (((r) == 0) ? (l) : min(l, r)))
+
+/*
+ * 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.
  */
@@ -546,16 +742,18 @@ static int init_hash_tables(struct dm_snapshot *s)
          * Calculate based on the size of the original volume or
          * the COW volume...
          */
-        cow_dev_size = get_dev_size(s->store->cow->bdev);
+        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 (init_exception_table(&s->complete, hash_size,
-                                 DM_CHUNK_CONSECUTIVE_BITS))
+        if (dm_exception_table_init(&s->complete, hash_size,
+                                    DM_CHUNK_CONSECUTIVE_BITS))
                 return -ENOMEM;
 
         /*
@@ -566,14 +764,284 @@ static int init_hash_tables(struct dm_snapshot *s)
         if (hash_size < 64)
                 hash_size = 64;
 
-        if (init_exception_table(&s->pending, hash_size, 0)) {
-                exit_exception_table(&s->complete, exception_cache);
+        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>
  */
@@ -582,50 +1050,73 @@ 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;
-        struct dm_exception_store *store;
-        unsigned args_used;
+        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_args;
+                goto bad;
+        }
+
+        if (dm_target_is_snapshot_merge(ti)) {
+                num_flush_requests = 2;
+                origin_mode = FMODE_WRITE;
         }
 
         origin_path = argv[0];
         argv++;
         argc--;
 
-        r = dm_exception_store_create(ti, argc, argv, &args_used, &store);
+        s = kmalloc(sizeof(*s), GFP_KERNEL);
+        if (!s) {
+                ti->error = "Cannot allocate snapshot context private "
+                    "structure";
+                r = -ENOMEM;
+                goto bad;
+        }
+
+        cow_path = argv[0];
+        argv++;
+        argc--;
+
+        r = dm_get_device(ti, cow_path, 0, 0,
+                          FMODE_READ | FMODE_WRITE, &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_args;
+                goto bad_store;
         }
 
         argv += args_used;
         argc -= args_used;
 
-        s = kmalloc(sizeof(*s), GFP_KERNEL);
-        if (!s) {
-                ti->error = "Cannot allocate snapshot context private "
-                    "structure";
-                r = -ENOMEM;
-                goto bad_snap;
-        }
-
-        r = dm_get_device(ti, origin_path, 0, ti->len, FMODE_READ, &s->origin);
+        r = dm_get_device(ti, origin_path, 0, ti->len, origin_mode, &s->origin);
         if (r) {
                 ti->error = "Cannot get origin device";
                 goto bad_origin;
         }
 
-        s->store = store;
+        s->ti = ti;
         s->valid = 1;
         s->active = 0;
+        s->suspended = 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)) {
@@ -659,39 +1150,55 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 
         spin_lock_init(&s->tracked_chunk_lock);
 
-        /* Metadata must only be loaded into one table at once */
+        bio_list_init(&s->queued_bios);
+        INIT_WORK(&s->queued_bios_work, flush_queued_bios);
+
+        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_load_and_register;
+                goto bad_read_metadata;
         } else if (r > 0) {
                 s->valid = 0;
                 DMWARN("Snapshot is marked invalid.");
         }
 
-        bio_list_init(&s->queued_bios);
-        INIT_WORK(&s->queued_bios_work, flush_queued_bios);
-
         if (!s->store->chunk_size) {
                 ti->error = "Chunk size not set";
-                goto bad_load_and_register;
-        }
-
-        /* Add snapshot to the list of snapshots for this origin */
-        /* Exceptions aren't triggered till snapshot_resume() is called */
-        if (register_snapshot(s)) {
-                r = -EINVAL;
-                ti->error = "Cannot register snapshot origin";
-                goto bad_load_and_register;
+                goto bad_read_metadata;
         }
-
-        ti->private = s;
         ti->split_io = s->store->chunk_size;
-        ti->num_flush_requests = 1;
 
         return 0;
 
+bad_read_metadata:
+        unregister_snapshot(s);
+
 bad_load_and_register:
         mempool_destroy(s->tracked_chunk_pool);
 
@@ -702,19 +1209,22 @@ bad_pending_pool:
         dm_kcopyd_client_destroy(s->kcopyd_client);
 
 bad_kcopyd:
-        exit_exception_table(&s->pending, pending_cache);
-        exit_exception_table(&s->complete, exception_cache);
+        dm_exception_table_exit(&s->pending, pending_cache);
+        dm_exception_table_exit(&s->complete, exception_cache);
 
 bad_hash_tables:
         dm_put_device(ti, s->origin);
 
 bad_origin:
-        kfree(s);
+        dm_exception_store_destroy(s->store);
 
-bad_snap:
-        dm_exception_store_destroy(store);
+bad_store:
+        dm_put_device(ti, s->cow);
+
+bad_cow:
+        kfree(s);
 
-bad_args:
+bad:
         return r;
 }
 
@@ -723,8 +1233,39 @@ static void __free_exceptions(struct dm_snapshot *s)
         dm_kcopyd_client_destroy(s->kcopyd_client);
         s->kcopyd_client = NULL;
 
-        exit_exception_table(&s->pending, pending_cache);
-        exit_exception_table(&s->complete, exception_cache);
+        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)
@@ -733,9 +1274,24 @@ static void snapshot_dtr(struct dm_target *ti)
         int i;
 #endif
         struct dm_snapshot *s = ti->private;
+        struct dm_snapshot *snap_src = NULL, *snap_dest = NULL;
 
         flush_workqueue(ksnapd);
 
+        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);
@@ -763,6 +1319,8 @@ static void snapshot_dtr(struct dm_target *ti)
 
         dm_exception_store_destroy(s->store);
 
+        dm_put_device(ti, s->cow);
+
         kfree(s);
 }
 
@@ -795,6 +1353,26 @@ static void flush_queued_bios(struct work_struct *work)
         flush_bios(queued_bios);
 }
 
+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.
  */
@@ -825,45 +1403,12 @@ static void __invalidate_snapshot(struct dm_snapshot *s, int err)
 
         s->valid = 0;
 
-        dm_table_event(s->store->ti->table);
-}
-
-static void get_pending_exception(struct dm_snap_pending_exception *pe)
-{
-        atomic_inc(&pe->ref_count);
-}
-
-static struct bio *put_pending_exception(struct dm_snap_pending_exception *pe)
-{
-        struct dm_snap_pending_exception *primary_pe;
-        struct bio *origin_bios = NULL;
-
-        primary_pe = pe->primary_pe;
-
-        /*
-         * If this pe is involved in a write to the origin and
-         * it is the last sibling to complete then release
-         * the bios for the original write to the origin.
-         */
-        if (primary_pe &&
-            atomic_dec_and_test(&primary_pe->ref_count)) {
-                origin_bios = bio_list_get(&primary_pe->origin_bios);
-                free_pending_exception(primary_pe);
-        }
-
-        /*
-         * Free the pe if it's not linked to an origin write or if
-         * it's not itself a primary pe.
-         */
-        if (!primary_pe || primary_pe != pe)
-                free_pending_exception(pe);
-
-        return origin_bios;
+        dm_table_event(s->ti->table);
 }
 
 static void pending_complete(struct dm_snap_pending_exception *pe, int success)
 {
-        struct dm_snap_exception *e;
+        struct dm_exception *e;
         struct dm_snapshot *s = pe->snap;
         struct bio *origin_bios = NULL;
         struct bio *snapshot_bios = NULL;
@@ -877,7 +1422,7 @@ static void pending_complete(struct dm_snap_pending_exception *pe, int success)
                 goto out;
         }
 
-        e = alloc_exception();
+        e = alloc_completed_exception();
         if (!e) {
                 down_write(&s->lock);
                 __invalidate_snapshot(s, -ENOMEM);
@@ -888,28 +1433,27 @@ static void pending_complete(struct dm_snap_pending_exception *pe, int success)
 
         down_write(&s->lock);
         if (!s->valid) {
-                free_exception(e);
+                free_completed_exception(e);
                 error = 1;
                 goto out;
         }
 
-        /*
-         * Check for conflicting reads. This is extremely improbable,
-         * so msleep(1) is sufficient and there is no need for a wait queue.
-         */
-        while (__chunk_is_tracked(s, pe->e.old_chunk))
-                msleep(1);
+        /* 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.
          */
-        insert_completed_exception(s, e);
+        dm_insert_exception(&s->complete, e);
 
  out:
-        remove_exception(&pe->e);
+        dm_remove_exception(&pe->e);
         snapshot_bios = bio_list_get(&pe->snapshot_bios);
-        origin_bios = put_pending_exception(pe);
+        origin_bios = bio_list_get(&pe->origin_bios);
+        free_pending_exception(pe);
+
+        increment_pending_exceptions_done_count();
 
         up_write(&s->lock);
 
@@ -919,7 +1463,7 @@ static void pending_complete(struct dm_snap_pending_exception *pe, int success)
         else
                 flush_bios(snapshot_bios);
 
-        flush_bios(origin_bios);
+        retry_origin_bios(s, origin_bios);
 }
 
 static void commit_callback(void *context, int success)
@@ -963,7 +1507,7 @@ static void start_copy(struct dm_snap_pending_exception *pe)
         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->store->cow->bdev;
+        dest.bdev = s->cow->bdev;
         dest.sector = chunk_to_sector(s->store, pe->e.new_chunk);
         dest.count = src.count;
 
@@ -975,7 +1519,7 @@ static void start_copy(struct dm_snap_pending_exception *pe)
 static struct dm_snap_pending_exception *
 __lookup_pending_exception(struct dm_snapshot *s, chunk_t chunk)
 {
-        struct dm_snap_exception *e = lookup_exception(&s->pending, chunk);
+        struct dm_exception *e = dm_lookup_exception(&s->pending, chunk);
 
         if (!e)
                 return NULL;
@@ -1006,8 +1550,6 @@ __find_pending_exception(struct dm_snapshot *s,
         pe->e.old_chunk = chunk;
         bio_list_init(&pe->origin_bios);
         bio_list_init(&pe->snapshot_bios);
-        pe->primary_pe = NULL;
-        atomic_set(&pe->ref_count, 0);
         pe->started = 0;
 
         if (s->store->type->prepare_exception(s->store, &pe->e)) {
@@ -1015,16 +1557,15 @@ __find_pending_exception(struct dm_snapshot *s,
                 return NULL;
         }
 
-        get_pending_exception(pe);
-        insert_exception(&s->pending, &pe->e);
+        dm_insert_exception(&s->pending, &pe->e);
 
         return pe;
 }
 
-static void remap_exception(struct dm_snapshot *s, struct dm_snap_exception *e,
+static void remap_exception(struct dm_snapshot *s, struct dm_exception *e,
                             struct bio *bio, chunk_t chunk)
 {
-        bio->bi_bdev = s->store->cow->bdev;
+        bio->bi_bdev = s->cow->bdev;
         bio->bi_sector = chunk_to_sector(s->store,
                                          dm_chunk_number(e->new_chunk) +
                                          (chunk - e->old_chunk)) +
@@ -1035,14 +1576,14 @@ static void remap_exception(struct dm_snapshot *s, struct dm_snap_exception *e,
 static int snapshot_map(struct dm_target *ti, struct bio *bio,
                         union map_info *map_context)
 {
-        struct dm_snap_exception *e;
+        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 (unlikely(bio_empty_barrier(bio))) {
-                bio->bi_bdev = s->store->cow->bdev;
+                bio->bi_bdev = s->cow->bdev;
                 return DM_MAPIO_REMAPPED;
         }
 
@@ -1063,7 +1604,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio,
         }
 
         /* If the block is already remapped - use that, else remap it */
-        e = lookup_exception(&s->complete, chunk);
+        e = dm_lookup_exception(&s->complete, chunk);
         if (e) {
                 remap_exception(s, e, bio, chunk);
                 goto out_unlock;
@@ -1087,7 +1628,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio,
                                 goto out_unlock;
                         }
 
-                        e = lookup_exception(&s->complete, chunk);
+                        e = dm_lookup_exception(&s->complete, chunk);
                         if (e) {
                                 free_pending_exception(pe);
                                 remap_exception(s, e, bio, chunk);
@@ -1125,6 +1666,78 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio,
         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 (unlikely(bio_empty_barrier(bio))) {
+                if (!map_context->flush_request)
+                        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)
 {
@@ -1137,40 +1750,135 @@ static int snapshot_end_io(struct dm_target *ti, struct bio *bio,
         return 0;
 }
 
+static void snapshot_merge_presuspend(struct dm_target *ti)
+{
+        struct dm_snapshot *s = ti->private;
+
+        stop_merge(s);
+}
+
+static void snapshot_postsuspend(struct dm_target *ti)
+{
+        struct dm_snapshot *s = ti->private;
+
+        down_write(&s->lock);
+        s->suspended = 1;
+        up_write(&s->lock);
+}
+
+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 (!snap_src->suspended) {
+                        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;
+        s->suspended = 0;
         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 int snapshot_status(struct dm_target *ti, status_type_t type,
                            char *result, unsigned int maxlen)
 {
         unsigned sz = 0;
         struct dm_snapshot *snap = ti->private;
 
-        down_write(&snap->lock);
-
         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->fraction_full) {
-                                sector_t numerator, denominator;
-                                snap->store->type->fraction_full(snap->store,
-                                                                 &numerator,
-                                                                 &denominator);
-                                DMEMIT("%llu/%llu",
-                                       (unsigned long long)numerator,
-                                       (unsigned long long)denominator);
+                        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:
@@ -1179,14 +1887,12 @@ static int snapshot_status(struct dm_target *ti, status_type_t type,
                  * to make private copies if the output is to
                  * make sense.
                  */
-                DMEMIT("%s", snap->origin->name);
+                DMEMIT("%s %s", snap->origin->name, snap->cow->name);
                 snap->store->type->status(snap->store, type, result + sz,
                                           maxlen - sz);
                 break;
         }
 
-        up_write(&snap->lock);
-
         return 0;
 }
 
@@ -1202,17 +1908,36 @@ static int snapshot_iterate_devices(struct dm_target *ti,
 /*-----------------------------------------------------------------
  * Origin methods
  *---------------------------------------------------------------*/
-static int __origin_write(struct list_head *snapshots, struct bio *bio)
+
+/*
+ * 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, first = 0;
+        int r = DM_MAPIO_REMAPPED;
         struct dm_snapshot *snap;
-        struct dm_snap_exception *e;
-        struct dm_snap_pending_exception *pe, *next_pe, *primary_pe = NULL;
+        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;
-        LIST_HEAD(pe_queue);
 
         /* 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);
 
@@ -1221,24 +1946,21 @@ static int __origin_write(struct list_head *snapshots, struct bio *bio)
                         goto next_snapshot;
 
                 /* Nothing to do if writing beyond end of snapshot */
-                if (bio->bi_sector >= dm_table_get_size(snap->store->ti->table))
+                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, bio->bi_sector);
+                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.
-                 *
-                 * ref_count is initialised to 1 so pending_complete()
-                 * won't destroy the primary_pe while we're inside this loop.
                  */
-                e = lookup_exception(&snap->complete, chunk);
+                e = dm_lookup_exception(&snap->complete, chunk);
                 if (e)
                         goto next_snapshot;
 
@@ -1253,7 +1975,7 @@ static int __origin_write(struct list_head *snapshots, struct bio *bio)
                                 goto next_snapshot;
                         }
 
-                        e = lookup_exception(&snap->complete, chunk);
+                        e = dm_lookup_exception(&snap->complete, chunk);
                         if (e) {
                                 free_pending_exception(pe);
                                 goto next_snapshot;
@@ -1266,59 +1988,43 @@ static int __origin_write(struct list_head *snapshots, struct bio *bio)
                         }
                 }
 
-                if (!primary_pe) {
-                        /*
-                         * Either every pe here has same
-                         * primary_pe or none has one yet.
-                         */
-                        if (pe->primary_pe)
-                                primary_pe = pe->primary_pe;
-                        else {
-                                primary_pe = pe;
-                                first = 1;
-                        }
-
-                        bio_list_add(&primary_pe->origin_bios, bio);
+                r = DM_MAPIO_SUBMITTED;
 
-                        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->primary_pe) {
-                        pe->primary_pe = primary_pe;
-                        get_pending_exception(primary_pe);
+                        if (!pe->started) {
+                                pe->started = 1;
+                                pe_to_start_last = pe;
+                        }
                 }
 
                 if (!pe->started) {
                         pe->started = 1;
-                        list_add_tail(&pe->list, &pe_queue);
+                        pe_to_start_now = pe;
                 }
 
  next_snapshot:
                 up_write(&snap->lock);
-        }
 
-        if (!primary_pe)
-                return r;
-
-        /*
-         * If this is the first time we're processing this chunk and
-         * ref_count is now 1 it means all the pending exceptions
-         * got completed while we were in the loop above, so it falls to
-         * us here to remove the primary_pe and submit any origin_bios.
-         */
-
-        if (first && atomic_dec_and_test(&primary_pe->ref_count)) {
-                flush_bios(bio_list_get(&primary_pe->origin_bios));
-                free_pending_exception(primary_pe);
-                /* If we got here, pe_queue is necessarily empty. */
-                return r;
+                if (pe_to_start_now) {
+                        start_copy(pe_to_start_now);
+                        pe_to_start_now = NULL;
+                }
         }
 
         /*
-         * Now that we have a complete pe list we can start the copying.
+         * Submit the exception against which the bio is queued last,
+         * to give the other exceptions a head start.
          */
-        list_for_each_entry_safe(pe, next_pe, &pe_queue, list)
-                start_copy(pe);
+        if (pe_to_start_last)
+                start_copy(pe_to_start_last);
 
         return r;
 }
@@ -1334,13 +2040,48 @@ static int do_origin(struct dm_dev *origin, struct bio *bio)
         down_read(&_origins_lock);
         o = __lookup_origin(origin->bdev);
         if (o)
-                r = __origin_write(&o->snapshots, bio);
+                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.
  */
 
@@ -1391,8 +2132,6 @@ static int origin_map(struct dm_target *ti, struct bio *bio,
         return (bio_rw(bio) == WRITE) ? do_origin(dev, bio) : DM_MAPIO_REMAPPED;
 }
 
-#define min_not_zero(l, r) (l == 0) ? r : ((r == 0) ? l : min(l, r))
-
 /*
  * Set the target "split_io" field to the minimum of all the snapshots'
  * chunk sizes.
@@ -1400,19 +2139,8 @@ static int origin_map(struct dm_target *ti, struct bio *bio,
 static void origin_resume(struct dm_target *ti)
 {
         struct dm_dev *dev = ti->private;
-        struct dm_snapshot *snap;
-        struct origin *o;
-        unsigned chunk_size = 0;
-
-        down_read(&_origins_lock);
-        o = __lookup_origin(dev->bdev);
-        if (o)
-                list_for_each_entry (snap, &o->snapshots, list)
-                        chunk_size = min_not_zero(chunk_size,
-                                                  snap->store->chunk_size);
-        up_read(&_origins_lock);
 
-        ti->split_io = chunk_size;
+        ti->split_io = get_origin_minimum_chunksize(dev->bdev);
 }
 
 static int origin_status(struct dm_target *ti, status_type_t type, char *result,
@@ -1455,17 +2183,35 @@ static struct target_type origin_target = {
 
 static struct target_type snapshot_target = {
         .name    = "snapshot",
-        .version = {1, 7, 0},
+        .version = {1, 9, 0},
         .module  = THIS_MODULE,
         .ctr     = snapshot_ctr,
         .dtr     = snapshot_dtr,
         .map     = snapshot_map,
         .end_io  = snapshot_end_io,
+        .postsuspend = snapshot_postsuspend,
+        .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, 0, 0},
+        .module  = THIS_MODULE,
+        .ctr     = snapshot_ctr,
+        .dtr     = snapshot_dtr,
+        .map     = snapshot_merge_map,
+        .end_io  = snapshot_end_io,
+        .presuspend = snapshot_merge_presuspend,
+        .postsuspend = snapshot_postsuspend,
+        .preresume  = snapshot_preresume,
+        .resume  = snapshot_merge_resume,
+        .status  = snapshot_status,
+        .iterate_devices = snapshot_iterate_devices,
+};
+
 static int __init dm_snapshot_init(void)
 {
         int r;
@@ -1477,7 +2223,7 @@ static int __init dm_snapshot_init(void)
         }
 
         r = dm_register_target(&snapshot_target);
-        if (r) {
+        if (r < 0) {
                 DMERR("snapshot target register failed %d", r);
                 goto bad_register_snapshot_target;
         }
@@ -1485,34 +2231,40 @@ static int __init dm_snapshot_init(void)
         r = dm_register_target(&origin_target);
         if (r < 0) {
                 DMERR("Origin target register failed %d", r);
-                goto bad1;
+                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 bad2;
+                goto bad_origin_hash;
         }
 
-        exception_cache = KMEM_CACHE(dm_snap_exception, 0);
+        exception_cache = KMEM_CACHE(dm_exception, 0);
         if (!exception_cache) {
                 DMERR("Couldn't create exception cache.");
                 r = -ENOMEM;
-                goto bad3;
+                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 bad4;
+                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 bad5;
+                goto bad_tracked_chunk_cache;
         }
 
         ksnapd = create_singlethread_workqueue("ksnapd");
@@ -1526,19 +2278,21 @@ static int __init dm_snapshot_init(void)
 
 bad_pending_pool:
         kmem_cache_destroy(tracked_chunk_cache);
-bad5:
+bad_tracked_chunk_cache:
         kmem_cache_destroy(pending_cache);
-bad4:
+bad_pending_cache:
         kmem_cache_destroy(exception_cache);
-bad3:
+bad_exception_cache:
         exit_origin_hash();
-bad2:
+bad_origin_hash:
+        dm_unregister_target(&merge_target);
+bad_register_merge_target:
         dm_unregister_target(&origin_target);
-bad1:
+bad_register_origin_target:
         dm_unregister_target(&snapshot_target);
-
 bad_register_snapshot_target:
         dm_exception_store_exit();
+
         return r;
 }
 
@@ -1548,6 +2302,7 @@ 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);