rbd: store snapshot id instead of index

When a device was open at a snapshot, and snapshots were deleted or
added, data from the wrong snapshot could be read. Instead of
assuming the snap context is constant, store the actual snap id when
the device is initialized, and rely on the OSDs to signal an error
if we try reading from a snapshot that was deleted.

Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>
Reviewed-by: Alex Elder <elder@dreamhost.com>
Reviewed-by: Yehuda Sadeh <yehuda@hq.newdream.net>

drivers/block/rbd.c

@@ -175,8 +175,7 @@ struct rbd_device {
 	/* protects updating the header */
 	struct rw_semaphore     header_rwsem;
 	char                    snap_name[RBD_MAX_SNAP_NAME_LEN];
-	u32 cur_snap;	/* index+1 of current snapshot within snap context
-			   0 - for the head */
+	u64                     snap_id;	/* current snapshot id */
 	int read_only;
 
 	struct list_head	node;
@@ -554,21 +553,6 @@ static int rbd_header_from_disk(struct rbd_image_header *header,
 	return -ENOMEM;
 }
 
-static int snap_index(struct rbd_image_header *header, int snap_num)
-{
-	return header->total_snaps - snap_num;
-}
-
-static u64 cur_snap_id(struct rbd_device *rbd_dev)
-{
-	struct rbd_image_header *header = &rbd_dev->header;
-
-	if (!rbd_dev->cur_snap)
-		return 0;
-
-	return header->snapc->snaps[snap_index(header, rbd_dev->cur_snap)];
-}
-
 static int snap_by_name(struct rbd_image_header *header, const char *snap_name,
 			u64 *seq, u64 *size)
 {
@@ -607,16 +591,15 @@ static int rbd_header_set_snap(struct rbd_device *dev, u64 *size)
 			snapc->seq = header->snap_seq;
 		else
 			snapc->seq = 0;
-		dev->cur_snap = 0;
+		dev->snap_id = CEPH_NOSNAP;
 		dev->read_only = 0;
 		if (size)
 			*size = header->image_size;
 	} else {
 		ret = snap_by_name(header, dev->snap_name, &snapc->seq, size);
 		if (ret < 0)
 			goto done;
-
-		dev->cur_snap = header->total_snaps - ret;
+		dev->snap_id = snapc->seq;
 		dev->read_only = 1;
 	}
 
@@ -1522,7 +1505,7 @@ static void rbd_rq_fn(struct request_queue *q)
 					      coll, cur_seg);
 			else
 				rbd_req_read(rq, rbd_dev,
-					     cur_snap_id(rbd_dev),
+					     rbd_dev->snap_id,
 					     ofs,
 					     op_size, bio,
 					     coll, cur_seg);
@@ -1657,7 +1640,7 @@ static int rbd_header_add_snap(struct rbd_device *dev,
 	struct ceph_mon_client *monc;
 
 	/* we should create a snapshot only if we're pointing at the head */
-	if (dev->cur_snap)
+	if (dev->snap_id != CEPH_NOSNAP)
 		return -EINVAL;
 
 	monc = &dev->rbd_client->client->monc;