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[dm-devel] [PATCH 28 of 29] dm-snap-snapshare-1.patch



Index: linux-2.6/drivers/md/dm-snap.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-snap.c
+++ linux-2.6/drivers/md/dm-snap.c
@@ -51,6 +51,17 @@
 #define DM_TRACKED_CHUNK_HASH(x)	((unsigned long)(x) & \
 					 (DM_TRACKED_CHUNK_HASH_SIZE - 1))
 
+/*
+ * Exception table hash sizes for pending exceptions
+ * The snapshot pending exception table holds pending exceptions
+ * that affect all snapshots in the share group (due to origin write).
+ * The snapshare pending exception table holds pending exceptions
+ * that affect just one snapshot in the share group (due to a
+ * write to one of the snapshots).
+ */
+#define DM_SNAPSHARE_HASH_SIZE 16
+#define DM_SNAPSHOT_HASH_SIZE 64
+
 struct dm_snapshot {
 	struct rw_semaphore lock;
 
@@ -67,19 +78,17 @@ struct dm_snapshot {
 
 	mempool_t *pending_pool;
 
-	atomic_t pending_exceptions_count;
-
 	struct dm_exception_table *pending;
 
+	uint64_t shared_uuid;
+	struct list_head shared_list;
+
 	/*
 	 * pe_lock protects all pending_exception operations and access
 	 * as well as the snapshot_bios list.
 	 */
 	spinlock_t pe_lock;
 
-	/* The on disk metadata handler */
-	struct dm_exception_store *store;
-
 	struct dm_kcopyd_client *kcopyd_client;
 
 	/* Queue of snapshot writes for ksnapd to flush */
@@ -98,6 +107,7 @@ struct dm_snapshare {
 	struct list_head shared_list;
 
 	atomic_t pending_exceptions_count;
+	struct dm_exception_table *pending;
 
 	/* The on disk metadata handler */
 	struct dm_exception_store *store;
@@ -106,6 +116,19 @@ struct dm_snapshare {
 static struct workqueue_struct *ksnapd;
 static void flush_queued_bios(struct work_struct *work);
 
+static struct dm_exception_store *get_first_store(struct dm_snapshot *s)
+{
+	struct dm_snapshare *ss;
+
+	list_for_each_entry(ss, &s->shared_list, shared_list)
+		return ss->store;
+
+	DMERR("No snapshares in snapshot");
+	BUG();
+
+	return NULL;
+}
+
 static sector_t chunk_to_sector(struct dm_exception_store *store,
 				chunk_t chunk)
 {
@@ -153,8 +176,12 @@ struct dm_snap_pending_exception {
 	 */
 	atomic_t ref_count;
 
-	/* Pointer back to snapshot context */
+	/*
+	 * Pointer back to snapshot or snapshare context
+	 * Only one of 'ss' or 'snap' may be populated.
+	 */
 	struct dm_snapshot *snap;
+	struct dm_snapshare *ss;
 
 	/*
 	 * 1 indicates the exception has already been sent to
@@ -296,13 +323,21 @@ static void __insert_origin(struct origi
 }
 
 /*
+ * register_snapshare
+ * @ss: snapshare - initialized and populated with 's'
+ *
  * Make a note of the snapshot and its origin so we can look it
  * up when the origin has a write on it.
+ *
+ * Returns: 0 on success, -Exxx on failure
  */
-static int register_snapshot(struct dm_snapshot *snap)
+static void dealloc_snapshot(struct dm_snapshot *s);
+static int register_snapshare(struct dm_snapshare *ss)
 {
+	int found = 0;
 	struct origin *o, *new_o;
-	struct block_device *bdev = snap->origin->bdev;
+	struct dm_snapshot *s;
+	struct block_device *bdev = ss->snap->origin->bdev;
 
 	new_o = kmalloc(sizeof(*new_o), GFP_KERNEL);
 	if (!new_o)
@@ -324,20 +359,61 @@ static int register_snapshot(struct dm_s
 		__insert_origin(o);
 	}
 
-	list_add_tail(&snap->list, &o->snapshots);
+	if (!ss->snap->shared_uuid)
+		goto new_snapshot;
+
+	list_for_each_entry(s, &o->snapshots, list) {
+		down_write(&s->lock);
+		if (s->shared_uuid == ss->snap->shared_uuid) {
+			DMERR("Putting origin because it is shared");
+			dm_put_device(ss->store->ti, ss->snap->origin);
+
+			DMERR("Adding share to existing snapshot");
+			list_add(&ss->shared_list, &s->shared_list);
+
+			DMERR("Deallocating duplicate snapshot");
+			dealloc_snapshot(ss->snap);
+
+			ss->snap = s;
+
+			up_write(&s->lock);
+			found = 1;
+			break;
+		}
+		up_write(&s->lock);
+	}
+
+new_snapshot:
+	if (!found)
+		list_add_tail(&ss->snap->list, &o->snapshots);
 
 	up_write(&_origins_lock);
 	return 0;
 }
 
-static void unregister_snapshot(struct dm_snapshot *s)
+static void unregister_snapshare(struct dm_snapshare *ss)
 {
 	struct origin *o;
 
+	/*
+	 * Always origin lock, then snapshot lock
+	 */
 	down_write(&_origins_lock);
-	o = __lookup_origin(s->origin->bdev);
+	o = __lookup_origin(ss->snap->origin->bdev);
+
+	down_write(&ss->snap->lock);
+
+	/*
+	 * Remove the snapshare, then if there are no
+	 * more snapshares left, remove the snapshot
+	 * from the origin's list
+	 */
+	list_del(&ss->shared_list);
+
+	if (list_empty(&ss->snap->shared_list))
+		list_del(&ss->snap->list);
+	up_write(&ss->snap->lock);
 
-	list_del(&s->list);
 	if (list_empty(&o->snapshots)) {
 		list_del(&o->hash_list);
 		kfree(o);
@@ -349,11 +425,14 @@ static void unregister_snapshot(struct d
 static struct dm_exception *alloc_pending_exception(void *context)
 {
 	struct dm_snapshot *s = context;
+	struct dm_snapshare *ss;
 	struct dm_snap_pending_exception *pe = mempool_alloc(s->pending_pool,
 							     GFP_NOIO);
 
-	atomic_inc(&s->pending_exceptions_count);
+	list_for_each_entry(ss, &s->shared_list, shared_list)
+		atomic_inc(&ss->pending_exceptions_count);
 	pe->snap = s;
+	pe->ss = NULL;
 
 	return &pe->e;
 }
@@ -362,25 +441,43 @@ static void free_pending_exception(struc
 {
 	struct dm_snap_pending_exception *pe;
 	struct dm_snapshot *s;
+	struct dm_snapshare *ss;
 
 	pe = container_of(e, struct dm_snap_pending_exception, e);
 	s = pe->snap;
 
 	mempool_free(pe, s->pending_pool);
 	smp_mb__before_atomic_dec();
-	atomic_dec(&s->pending_exceptions_count);
+
+	list_for_each_entry(ss, &s->shared_list, shared_list)
+		atomic_dec(&ss->pending_exceptions_count);
 }
 
-/*
- * Hard coded magic.
- */
-static int calc_max_buckets(void)
+static struct dm_exception *alloc_snapshare_pending_exception(void *context)
 {
-	/* use a fixed size of 2MB */
-	unsigned long mem = 2 * 1024 * 1024;
-	mem /= sizeof(struct list_head);
+	struct dm_snapshare *ss = context;
+	struct dm_snap_pending_exception *pe;
+
+	pe = mempool_alloc(ss->snap->pending_pool, GFP_NOIO);
+
+	atomic_inc(&ss->pending_exceptions_count);
+	pe->ss = ss;
+	pe->snap = NULL;
+
+	return &pe->e;
+}
+
+static void free_snapshare_pending_exception(struct dm_exception *e, void *unused)
+{
+	struct dm_snap_pending_exception *pe;
+	struct dm_snapshare *ss;
+
+	pe = container_of(e, struct dm_snap_pending_exception, e);
+	ss = pe->ss;
 
-	return mem;
+	mempool_free(pe, ss->snap->pending_pool);
+	smp_mb__before_atomic_dec();
+	atomic_dec(&ss->pending_exceptions_count);
 }
 
 /*
@@ -442,7 +539,7 @@ static int create_exception_store(struct
 					 argv + 2, store);
 }
 
-static struct dm_snapshot *alloc_snapshot(sector_t hash_size)
+static struct dm_snapshot *alloc_snapshot(void)
 {
 	int r, i;
 	struct dm_snapshot *s;
@@ -453,14 +550,14 @@ static struct dm_snapshot *alloc_snapsho
 		return NULL;
 	}
 
+	INIT_LIST_HEAD(&s->shared_list);
 	s->valid = 1;
 	s->active = 0;
-	atomic_set(&s->pending_exceptions_count, 0);
 	init_rwsem(&s->lock);
 	spin_lock_init(&s->pe_lock);
 
 	/* Allocate hash table for pending COW data */
-	s->pending = dm_exception_table_create(hash_size, 0,
+	s->pending = dm_exception_table_create(DM_SNAPSHOT_HASH_SIZE, 0,
 					       alloc_pending_exception, s,
 					       free_pending_exception, NULL);
 	if (!s->pending) {
@@ -539,11 +636,9 @@ static void dealloc_snapshot(struct dm_s
  */
 static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 {
-	sector_t hash_size, cow_dev_size, origin_dev_size, max_buckets;
 	struct dm_dev *origin;
 	struct dm_snapshare *ss;
 	struct dm_snapshot *s;
-	int i;
 	int r = -EINVAL;
 	char *origin_path;
 	struct dm_exception_store *store;
@@ -566,6 +661,14 @@ static int snapshot_ctr(struct dm_target
 	INIT_LIST_HEAD(&ss->shared_list);
 	atomic_set(&ss->pending_exceptions_count, 0);
 
+	ss->pending = dm_exception_table_create(DM_SNAPSHARE_HASH_SIZE, 0,
+						alloc_snapshare_pending_exception, ss,
+						free_snapshare_pending_exception, NULL);
+	if (!ss->pending) {
+		ti->error = "Failed to allocate exception hash table";
+		goto bad_hash_table;
+	}
+
 	r = create_exception_store(ti, argc, argv, &args_used, &store);
 	if (r) {
 		ti->error = "Failed to create snapshot exception store";
@@ -583,25 +686,9 @@ static int snapshot_ctr(struct dm_target
 	}
 
 	/*
-	 * Calculate based on the size of the original volume or
-	 * the COW volume...
-	 */
-	cow_dev_size = get_dev_size(store->cow->bdev);
-	origin_dev_size = get_dev_size(origin->bdev);
-	max_buckets = calc_max_buckets();
-
-	hash_size = min(origin_dev_size, cow_dev_size) >> store->chunk_shift;
-	hash_size = min(hash_size, max_buckets);
-
-	hash_size = rounddown_pow_of_two(hash_size);
-	hash_size >>= 3;
-	if (hash_size < 64)
-		hash_size = 64;
-
-	/*
 	 * Allocate the snapshot
 	 */
-	s = alloc_snapshot(hash_size);
+	s = alloc_snapshot();
 	if (!s) {
 		r = -ENOMEM;
 		ti->error = "Failed to create snapshot structure";
@@ -609,11 +696,12 @@ static int snapshot_ctr(struct dm_target
 	}
 	ss->snap = s;
 	s->origin = origin;
-	s->store = ss->store;
+	s->shared_uuid = store->shared_uuid;
+	list_add(&ss->shared_list, &s->shared_list);
 
 	/* Add snapshot to the list of snapshots for this origin */
 	/* Exceptions aren't triggered till snapshot_resume() is called */
-	if (register_snapshot(s)) {
+	if (register_snapshare(ss)) {
 		r = -EINVAL;
 		ti->error = "Cannot register snapshot with origin";
 		goto bad_load_and_register;
@@ -634,6 +722,9 @@ bad_origin:
 	dm_exception_store_destroy(store);
 
 bad_exception_store:
+	dm_exception_table_destroy(ss->pending);
+
+bad_hash_table:
 	kfree(ss);
 
 	return r;
@@ -651,9 +742,9 @@ static void snapshot_dtr(struct dm_targe
 
 	/* Prevent further origin writes from using this snapshot. */
 	/* After this returns there can be no new kcopyd jobs. */
-	unregister_snapshot(s);
+	unregister_snapshare(ss);
 
-	while (atomic_read(&s->pending_exceptions_count))
+	while (atomic_read(&ss->pending_exceptions_count))
 		msleep(1);
 	/*
 	 * Ensure instructions in mempool_destroy aren't reordered
@@ -672,6 +763,8 @@ static void snapshot_dtr(struct dm_targe
 
 	dm_exception_store_destroy(ss->store);
 
+	dm_exception_table_destroy(ss->pending);
+
 	kfree(ss);
 }
 
@@ -721,6 +814,7 @@ static void error_bios(struct bio *bio)
 
 static void __invalidate_snapshot(struct dm_snapshot *s, int err)
 {
+	struct dm_snapshare *ss;
 	char *tmp_str = "ES_INVALIDATE";
 
 	if (!s->valid)
@@ -731,12 +825,15 @@ static void __invalidate_snapshot(struct
 	else if (err == -ENOMEM)
 		DMERR("Invalidating snapshot: Unable to allocate exception.");
 
-	if (s->store->type->message)
-		s->store->type->message(s->store, 1, &tmp_str);
+
+	/* Invalidating the snapshot will invalidate all snapshares. */
+	list_for_each_entry(ss, &s->shared_list, shared_list)
+		if (ss->store->type->message)
+			ss->store->type->message(ss->store, 1, &tmp_str);
 
 	s->valid = 0;
 
-	dm_table_event(s->store->ti->table);
+	dm_table_event(ss->store->ti->table);
 }
 
 static void get_pending_exception(struct dm_snap_pending_exception *pe)
@@ -747,7 +844,6 @@ static void get_pending_exception(struct
 static struct bio *put_pending_exception(struct dm_snap_pending_exception *pe)
 {
 	struct dm_snap_pending_exception *primary_pe;
-	struct dm_snapshot *s = pe->snap;
 	struct bio *origin_bios = NULL;
 
 	primary_pe = pe->primary_pe;
@@ -760,7 +856,9 @@ static struct bio *put_pending_exception
 	if (primary_pe &&
 	    atomic_dec_and_test(&primary_pe->ref_count)) {
 		origin_bios = bio_list_get(&primary_pe->origin_bios);
-		dm_free_exception(s->pending, &primary_pe->e);
+		dm_free_exception(primary_pe->ss ? primary_pe->ss->pending :
+				  primary_pe->snap->pending,
+				  &primary_pe->e);
 	}
 
 	/*
@@ -768,14 +866,15 @@ static struct bio *put_pending_exception
 	 * it's not itself a primary pe.
 	 */
 	if (!primary_pe || primary_pe != pe)
-		dm_free_exception(s->pending, &pe->e);
+		dm_free_exception(pe->ss ? pe->ss->pending : pe->snap->pending,
+				  &pe->e);
 
 	return origin_bios;
 }
 
 static void pending_complete(struct dm_snap_pending_exception *pe, int success)
 {
-	struct dm_snapshot *s = pe->snap;
+	struct dm_snapshot *s = pe->snap ? pe->snap : pe->ss->snap;
 	struct bio *origin_bios = NULL;
 	struct bio *snapshot_bios = NULL;
 	int error = 0;
@@ -825,15 +924,17 @@ static void commit_callback(void *contex
 static void copy_callback(int read_err, unsigned long write_err, void *context)
 {
 	struct dm_snap_pending_exception *pe = context;
-	struct dm_snapshot *s = pe->snap;
+	struct dm_exception_store *store;
+
+	store = pe->ss ? pe->ss->store : get_first_store(pe->snap);
 
 	if (read_err || write_err)
 		pending_complete(pe, 0);
 
 	else
 		/* Update the metadata if we are persistent */
-		s->store->type->commit_exception(s->store, &pe->e,
-						 commit_callback, pe);
+		store->type->commit_exception(store, &pe->e,
+					      commit_callback, pe);
 }
 
 /*
@@ -841,19 +942,24 @@ static void copy_callback(int read_err, 
  */
 static void start_copy(struct dm_snap_pending_exception *pe)
 {
-	struct dm_snapshot *s = pe->snap;
+	struct dm_exception_store *store;
+	struct dm_snapshot *s;
 	struct dm_io_region src, dest;
-	struct block_device *bdev = s->origin->bdev;
+	struct block_device *bdev;
 	sector_t dev_size;
 
+	store = (pe->ss) ? pe->ss->store : get_first_store(pe->snap);
+	s = pe->snap ? pe->snap : pe->ss->snap;
+	bdev = s->origin->bdev;
+
 	dev_size = get_dev_size(bdev);
 
 	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.sector = chunk_to_sector(store, pe->e.old_chunk);
+	src.count = min(store->chunk_size, dev_size - src.sector);
 
-	dest.bdev = s->store->cow->bdev;
-	dest.sector = chunk_to_sector(s->store, pe->e.new_chunk);
+	dest.bdev = store->cow->bdev;
+	dest.sector = chunk_to_sector(store, pe->e.new_chunk);
 	dest.count = src.count;
 
 	/* Hand over to kcopyd */
@@ -873,14 +979,17 @@ static struct dm_snap_pending_exception 
 __find_pending_exception(struct dm_snapshot *s, struct bio *bio,
 			 struct dm_snapshare *ss)
 {
+	int r;
 	struct dm_exception *e, *tmp_e;
 	struct dm_snap_pending_exception *pe;
-	chunk_t chunk = sector_to_chunk(s->store, bio->bi_sector);
+	struct dm_exception_store *store = ss ? ss->store : get_first_store(s);
+	struct dm_exception_table *table = ss ? ss->pending : s->pending;
+	chunk_t chunk = sector_to_chunk(store, bio->bi_sector);
 
 	/*
 	 * Is there a pending exception for this already ?
 	 */
-	e = dm_lookup_exception(s->pending, chunk);
+	e = dm_lookup_exception(table, chunk);
 	if (e) {
 		/* cast the exception to a pending exception */
 		pe = container_of(e, struct dm_snap_pending_exception, e);
@@ -892,18 +1001,18 @@ __find_pending_exception(struct dm_snaps
 	 * to hold the lock while we do this.
 	 */
 	up_write(&s->lock);
-	tmp_e = dm_alloc_exception(s->pending);
+	tmp_e = dm_alloc_exception(table);
 	pe = container_of(tmp_e, struct dm_snap_pending_exception, e);
 	down_write(&s->lock);
 
 	if (!s->valid) {
-		dm_free_exception(s->pending, &pe->e);
+		dm_free_exception(table, &pe->e);
 		return NULL;
 	}
 
-	e = dm_lookup_exception(s->pending, chunk);
+	e = dm_lookup_exception(table, chunk);
 	if (e) {
-		dm_free_exception(s->pending, &pe->e);
+		dm_free_exception(table, &pe->e);
 		pe = container_of(e, struct dm_snap_pending_exception, e);
 		goto out;
 	}
@@ -915,23 +1024,24 @@ __find_pending_exception(struct dm_snaps
 	atomic_set(&pe->ref_count, 0);
 	pe->started = 0;
 
-	if (s->store->type->prepare_exception(s->store, &pe->e, ss ? 0 : 1)) {
-		dm_free_exception(s->pending, &pe->e);
+	r = store->type->prepare_exception(store, &pe->e, ss ? 0 : 1);
+	if (r) {
+		dm_free_exception(table, &pe->e);
 		return NULL;
 	}
 
 	get_pending_exception(pe);
-	dm_insert_exception(s->pending, &pe->e);
+	dm_insert_exception(table, &pe->e);
 
 out:
 	return pe;
 }
 
-static void remap_exception(struct dm_snapshot *s, struct bio *bio, chunk_t chunk)
+static void remap_exception(struct dm_snapshare *ss, struct bio *bio, chunk_t chunk)
 {
-	bio->bi_bdev = s->store->cow->bdev;
-	bio->bi_sector = chunk_to_sector(s->store, dm_chunk_number(chunk)) +
-		(bio->bi_sector & s->store->chunk_mask);
+	bio->bi_bdev = ss->store->cow->bdev;
+	bio->bi_sector = chunk_to_sector(ss->store, dm_chunk_number(chunk)) +
+		(bio->bi_sector & ss->store->chunk_mask);
 }
 
 static int snapshot_map(struct dm_target *ti, struct bio *bio,
@@ -962,7 +1072,7 @@ static int snapshot_map(struct dm_target
 	/* If the block is already remapped - use that, else remap it */
 	rtn = ss->store->type->lookup_exception(ss->store, chunk, &new_chunk, 0);
 	if (!rtn) {
-		remap_exception(s, bio, new_chunk);
+		remap_exception(ss, bio, new_chunk);
 		goto out_unlock;
 	}
 
@@ -986,7 +1096,7 @@ static int snapshot_map(struct dm_target
 			goto out_unlock;
 		}
 
-		remap_exception(s, bio, pe->e.new_chunk);
+		remap_exception(ss, bio, pe->e.new_chunk);
 		bio_list_add(&pe->snapshot_bios, bio);
 
 		r = DM_MAPIO_SUBMITTED;
@@ -1112,13 +1222,38 @@ static int snapshot_message(struct dm_ta
 	return r;
 }
 
+static int is_completely_remapped(struct dm_snapshot *s, chunk_t chunk)
+{
+	int r;
+	struct dm_snapshare *ss;
+
+	list_for_each_entry(ss, &s->shared_list, shared_list) {
+		r = ss->store->type->lookup_exception(ss->store, chunk,
+						      NULL, 0);
+		switch (r) {
+		case 0:
+			continue;
+		case -ENOENT:
+			return 0;
+		case -EWOULDBLOCK:
+			DMERR("Unable to handle blocking exception stores");
+			BUG();
+		default:
+			DMERR("Invalid return from exception store lookup");
+			BUG();
+		}
+	}
+	return 1;
+}
+
 /*-----------------------------------------------------------------
  * Origin methods
  *---------------------------------------------------------------*/
 static int __origin_write(struct list_head *snapshots, struct bio *bio)
 {
-	int rtn, r = DM_MAPIO_REMAPPED, first = 0;
+	int r = DM_MAPIO_REMAPPED, first = 0;
 	struct dm_snapshot *snap;
+	struct dm_exception_store *store;
 	struct dm_snap_pending_exception *pe, *next_pe, *primary_pe = NULL;
 	chunk_t chunk;
 	LIST_HEAD(pe_queue);
@@ -1132,36 +1267,28 @@ static int __origin_write(struct list_he
 		if (!snap->valid || !snap->active)
 			goto next_snapshot;
 
+		store = get_first_store(snap);
+
 		/* Nothing to do if writing beyond end of snapshot */
-		if (bio->bi_sector >= dm_table_get_size(snap->store->ti->table))
+		if (bio->bi_sector >= dm_table_get_size(store->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(store, bio->bi_sector);
 
 		/*
-		 * Check exception table to see if block
-		 * is already remapped in this snapshot
-		 * and trigger an exception if not.
+		 * 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.
 		 */
-		rtn = snap->store->type->lookup_exception(snap->store, chunk,
-							  NULL, 0);
-		if (!rtn)
+		if (is_completely_remapped(snap, chunk))
 			goto next_snapshot;
 
-		/*
-		 * Could be -EWOULDBLOCK, but we don't handle that yet
-		 * and there are currently no exception store
-		 * implementations that would require us to.
-		 */
-		BUG_ON(rtn != -ENOENT);
-
 		pe = __find_pending_exception(snap, bio, NULL);
 		if (!pe) {
 			__invalidate_snapshot(snap, -ENOMEM);
@@ -1299,15 +1426,18 @@ static void origin_resume(struct dm_targ
 {
 	struct dm_dev *dev = ti->private;
 	struct dm_snapshot *snap;
+	struct dm_exception_store *store;
 	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)
+		list_for_each_entry (snap, &o->snapshots, list) {
+			store = get_first_store(snap);
 			chunk_size = min_not_zero(chunk_size,
-						  snap->store->chunk_size);
+						  store->chunk_size);
+		}
 	up_read(&_origins_lock);
 
 	ti->split_io = chunk_size;


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