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: NMikulas Patocka <mpatocka@redhat.com>
Reviewed-by: NMike Snitzer <snitzer@redhat.com>
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>

drivers/md/dm-snap.c

@@ -892,7 +892,7 @@ static void snapshot_merge_next_chunks(struct dm_snapshot *s)
 	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 @@ static int snapshot_merge_map(struct dm_target *ti, struct bio *bio,
 			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;
 	}