[dm-devel] [PATCH] dm snapshot: ignore discards issued to the snapshot-origin target

[Mike Snitzer]

Discards pose a problem for the snapshot-origin target because they are
treated as writes.  Treating a discard as a write would trigger a
copyout to the snapshot.  Such copyout can prove too costly in the face
of otherwise benign scenarios (e.g. create a snapshot and then mkfs.ext4
the origin -- mkfs.ext4 discards the entire volume by default, which
would copyout the entire origin volume to the snapshot).

Signed-off-by: Mike Snitzer <snitzer at redhat.com>
---
 drivers/md/dm-snap.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index a2d3309..639af12 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -2076,6 +2076,7 @@ static int origin_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 
 	ti->private = dev;
 	ti->num_flush_requests = 1;
+	ti->num_discard_requests = 1;
 
 	return 0;
 }
@@ -2095,6 +2096,12 @@ static int origin_map(struct dm_target *ti, struct bio *bio,
 	if (bio->bi_rw & REQ_FLUSH)
 		return DM_MAPIO_REMAPPED;
 
+	if (bio->bi_rw & REQ_DISCARD) {
+		/* ignore discard requests */
+		bio_endio(bio, 0);
+		return DM_MAPIO_SUBMITTED;
+	}
+
 	/* Only tell snapshots if this is a write */
 	return (bio_rw(bio) == WRITE) ? do_origin(dev, bio) : DM_MAPIO_REMAPPED;
 }
@@ -2153,7 +2160,7 @@ static int origin_iterate_devices(struct dm_target *ti,
 
 static struct target_type origin_target = {
 	.name    = "snapshot-origin",
-	.version = {1, 7, 1},
+	.version = {1, 8, 0},
 	.module  = THIS_MODULE,
 	.ctr     = origin_ctr,
 	.dtr     = origin_dtr,