[PATCH v4 06/13] dm snapshot: merge target should not allocate new exceptions

[Mike Snitzer <snitzer@redhat.com>]

From: Mikulas Patocka <mpatocka@redhat.com>

The snapshot-merge target should not allocate new exceptions because the
intent is to merge all of its exceptions as quickly and safely as
possible.

Introduce new method, snapshot_merge_map(), that won't allocate
exceptions.  Modify __origin_write() so that it doesn't allocate
exceptions in merging snapshots.

If a write request to a merging snapshot device is to be dispatched
directly to the origin (because the chunk is not remapped or was already
merged), snapshot_merge_map() must make exceptions in other snapshots.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 drivers/md/dm-snap.c |   49 ++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 48 insertions(+), 1 deletion(-)

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
@@ -127,6 +127,11 @@ static int bdev_equal(struct block_devic
 	return lhs == rhs;
 }
 
+static int snapshot_merge_map(struct dm_target *ti, struct bio *bio,
+			      union map_info *map_context);
+
+#define is_merge(ti) ((ti)->type->map == snapshot_merge_map)
+
 struct dm_snap_pending_exception {
 	struct dm_exception e;
 
@@ -1292,6 +1297,44 @@ static int snapshot_map(struct dm_target
 	return r;
 }
 
+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;
+
+	chunk = sector_to_chunk(s->store, bio->bi_sector);
+
+	down_read(&s->lock);
+
+	/* Full snapshots are not usable */
+	if (!s->valid) {
+		r = -EIO;
+		goto out_unlock;
+	}
+
+	/* If the block is already remapped - use that */
+	e = dm_lookup_exception(&s->complete, chunk);
+	if (e) {
+		remap_exception(s, e, bio, chunk);
+		goto out_unlock;
+	}
+
+	bio->bi_bdev = s->origin->bdev;
+
+	if (bio_rw(bio) == WRITE) {
+		up_read(&s->lock);
+		return do_origin(s->origin, bio);
+	}
+
+ out_unlock:
+	up_read(&s->lock);
+
+	return r;
+}
+
 static int snapshot_end_io(struct dm_target *ti, struct bio *bio,
 			   int error, union map_info *map_context)
 {
@@ -1442,6 +1485,10 @@ static int __origin_write(struct list_he
 	/* Do all the snapshots on this origin */
 	list_for_each_entry (snap, snapshots, list) {
 
+		/* Don't make new exceptions in a merging snapshot */
+		if (is_merge(snap->ti))
+			continue;
+
 		down_write(&snap->lock);
 
 		/* Only deal with valid and active snapshots */
@@ -1670,7 +1717,7 @@ static struct target_type merge_target =
 	.module  = THIS_MODULE,
 	.ctr     = snapshot_ctr,
 	.dtr     = snapshot_dtr,
-	.map     = snapshot_map,
+	.map     = snapshot_merge_map,
 	.end_io  = snapshot_end_io,
 	.postsuspend = snapshot_postsuspend,
 	.preresume  = snapshot_preresume,