dm snapshot: delay merging a chunk until writes to it complete

Track writes to chunks that are currently being merged and delay merging
a chunk until all writes to that chunk finish.

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

drivers/md/dm-snap.c

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 91a47c5..bc52776 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -892,7 +892,7 @@
 	s->num_merging_chunks = 1;
 	up_write(&s->lock);
 
-	/* !!! FIXME: wait until writes to this chunk drain */
+	__check_for_conflicting_io(s, old_chunk);
 
 	dm_kcopyd_copy(s->kcopyd_client, &src, 1, &dest, 0, merge_callback, s);
 	return;
@@ -1635,7 +1635,11 @@
 			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;
 	}