dm snapshot: support barriers in snapshot merge target

Sets num_flush_requests=2 to support flushing both the origin and cow
devices used by the snapshot-merge target.

Also, snapshot_ctr() now gets the origin device using FMODE_WRITE if the
target is snapshot-merge (which writes to the origin device).

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 18 insertions, 3 deletions

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 59d9ef6f5051..23e3396e43b9 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -709,7 +709,8 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
         int i;
         int r = -EINVAL;
         char *origin_path, *cow_path;
-        unsigned args_used;
+        unsigned args_used, num_flush_requests = 1;
+        fmode_t origin_mode = FMODE_READ;
 
         if (argc != 4) {
                 ti->error = "requires exactly 4 arguments";
@@ -717,6 +718,11 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
                 goto bad;
         }
 
+        if (dm_target_is_snapshot_merge(ti)) {
+                num_flush_requests = 2;
+                origin_mode = FMODE_WRITE;
+        }
+
         origin_path = argv[0];
         argv++;
         argc--;
@@ -750,7 +756,7 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
         argv += args_used;
         argc -= args_used;
 
-        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;
@@ -801,7 +807,7 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
         INIT_WORK(&s->queued_bios_work, flush_queued_bios);
 
         ti->private = s;
-        ti->num_flush_requests = 1;
+        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 */
@@ -1326,6 +1332,15 @@ static int snapshot_merge_map(struct dm_target *ti, struct bio *bio,
         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_read(&s->lock);