X-Git-Url: http://ftp.safe.ca/?a=blobdiff_plain;f=drivers%2Fmd%2Fdm-snap.c;h=54853773510c41216c263c5096164559df7782b8;hb=c2f0e8c803ceba530060ec9bb9c74a06c2c3d833;hp=53f4063f7ea4dfe0490d7b2d54be922136023220;hpb=034a186d29dbcef099e57ab23ec39440596be911;p=safe%2Fjmp%2Flinux-2.6

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 53f4063..5485377 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;
 
-	mempool_t *pending_pool;
+	/* Whether or not owning mapped_device is suspended */
+	int suspended;
 
 	atomic_t pending_exceptions_count;
 
-	struct exception_table pending;
-	struct exception_table complete;
+	mempool_t *pending_pool;
+
+	struct dm_exception_table pending;
+	struct dm_exception_table complete;
 
 	/*
 	 * pe_lock protects all pending_exception operations and access
@@ -82,6 +96,11 @@ struct dm_snapshot {
 	 */
 	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;
 
@@ -91,12 +110,50 @@ struct dm_snapshot {
 	struct bio_list queued_bios;
 	struct work_struct queued_bios_work;
 
-	/* Chunks with outstanding reads */
-	mempool_t *tracked_chunk_pool;
-	spinlock_t tracked_chunk_lock;
-	struct hlist_head tracked_chunk_hash[DM_TRACKED_CHUNK_HASH_SIZE];
+	/* 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_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 +1050,72 @@ 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;
+	}
+
+	if (dm_target_is_snapshot_merge(ti)) {
+		num_flush_requests = 2;
+		origin_mode = FMODE_WRITE;
+	}
+
+	origin_path = argv[0];
+	argv++;
+	argc--;
 
-	if (argc != 4) {
-		ti->error = "requires exactly 4 arguments";
-		r = -EINVAL;
-		goto bad_args;
+	s = kmalloc(sizeof(*s), GFP_KERNEL);
+	if (!s) {
+		ti->error = "Cannot allocate snapshot context private "
+		    "structure";
+		r = -ENOMEM;
+		goto bad;
 	}
 
-	origin_path = argv[0];
+	cow_path = argv[0];
 	argv++;
 	argc--;
 
-	r = dm_exception_store_create(ti, argc, argv, &args_used, &store);
+	r = dm_get_device(ti, cow_path, 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, 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)) {
@@ -659,34 +1149,55 @@ 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);
-
-	/* 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;
+	if (!s->store->chunk_size) {
+		ti->error = "Chunk size not set";
+		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);
 
@@ -697,19 +1208,22 @@ 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_store:
+	dm_put_device(ti, s->cow);
 
-bad_snap:
-	dm_exception_store_destroy(store);
+bad_cow:
+	kfree(s);
 
-bad_args:
+bad:
 	return r;
 }
 
@@ -718,8 +1232,39 @@ 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)
@@ -728,9 +1273,24 @@ 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);
@@ -758,6 +1318,8 @@ static void snapshot_dtr(struct dm_target *ti)
 
 	dm_exception_store_destroy(s->store);
 
+	dm_put_device(ti, s->cow);
+
 	kfree(s);
 }
 
@@ -790,6 +1352,26 @@ 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.
  */
@@ -820,45 +1402,12 @@ 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;
@@ -872,7 +1421,7 @@ 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);
@@ -883,28 +1432,27 @@ 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);
 
@@ -914,7 +1462,7 @@ 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)
@@ -956,9 +1504,9 @@ static void start_copy(struct dm_snap_pending_exception *pe)
 
 	src.bdev = bdev;
 	src.sector = chunk_to_sector(s->store, pe->e.old_chunk);
-	src.count = min(s->store->chunk_size, dev_size - src.sector);
+	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;
 
@@ -970,7 +1518,7 @@ 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;
@@ -1001,8 +1549,6 @@ __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)) {
@@ -1010,16 +1556,15 @@ __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)) +
@@ -1030,14 +1575,14 @@ 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;
 	}
 
@@ -1058,7 +1603,7 @@ 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;
@@ -1082,7 +1627,7 @@ 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);
@@ -1120,6 +1665,78 @@ 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)
 {
@@ -1132,15 +1749,101 @@ 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)
 {
@@ -1149,21 +1852,32 @@ static int snapshot_status(struct dm_target *ti, status_type_t type,
 
 	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:
@@ -1172,7 +1886,7 @@ 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;
@@ -1193,17 +1907,36 @@ 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);
 
@@ -1212,24 +1945,21 @@ 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;
 
@@ -1244,7 +1974,7 @@ 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;
@@ -1257,59 +1987,43 @@ 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;
 }
@@ -1325,13 +2039,48 @@ 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.
  */
 
@@ -1350,8 +2099,7 @@ static int origin_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 		return -EINVAL;
 	}
 
-	r = dm_get_device(ti, argv[0], 0, ti->len,
-			  dm_table_get_mode(ti->table), &dev);
+	r = dm_get_device(ti, argv[0], dm_table_get_mode(ti->table), &dev);
 	if (r) {
 		ti->error = "Cannot get target device";
 		return r;
@@ -1382,8 +2130,6 @@ 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.
@@ -1391,19 +2137,8 @@ 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;
-	chunk_t 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,
@@ -1446,17 +2181,35 @@ 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;
@@ -1468,7 +2221,7 @@ 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;
 	}
@@ -1476,34 +2229,40 @@ 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");
@@ -1517,19 +2276,21 @@ 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;
 }
 
@@ -1539,6 +2300,7 @@ 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);