rbd: return errors for mapped but deleted snapshot

When a snapshot is deleted, the OSD will return ENOENT when reading
from it. This is normally interpreted as a hole by rbd, which will
return zeroes. To minimize the time in which this can happen, stop
requests early when we are notified that our snapshot no longer
exists.

[elder@inktank.com: updated __rbd_init_snaps_header() logic]
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>

drivers/block/rbd.c

@@ -172,8 +172,12 @@ struct rbd_device {
 
 	/* protects updating the header */
 	struct rw_semaphore     header_rwsem;
+	/* name of the snapshot this device reads from */
 	char                    *snap_name;
+	/* id of the snapshot this device reads from */
 	u64                     snap_id;	/* current snapshot id */
+	/* whether the snap_id this device reads from still exists */
+	bool                    snap_exists;
 	int                     read_only;
 
 	struct list_head	node;
@@ -597,6 +601,7 @@ static int rbd_header_set_snap(struct rbd_device *rbd_dev, u64 *size)
 		else
 			snapc->seq = 0;
 		rbd_dev->snap_id = CEPH_NOSNAP;
+		rbd_dev->snap_exists = false;
 		rbd_dev->read_only = 0;
 		if (size)
 			*size = header->image_size;
@@ -606,6 +611,7 @@ static int rbd_header_set_snap(struct rbd_device *rbd_dev, u64 *size)
 		if (ret < 0)
 			goto done;
 		rbd_dev->snap_id = snapc->seq;
+		rbd_dev->snap_exists = true;
 		rbd_dev->read_only = 1;
 	}
 
@@ -1468,6 +1474,21 @@ static void rbd_rq_fn(struct request_queue *q)
 
 		spin_unlock_irq(q->queue_lock);
 
+		if (rbd_dev->snap_id != CEPH_NOSNAP) {
+			bool snap_exists;
+
+			down_read(&rbd_dev->header_rwsem);
+			snap_exists = rbd_dev->snap_exists;
+			up_read(&rbd_dev->header_rwsem);
+
+			if (!snap_exists) {
+				dout("request for non-existent snapshot");
+				spin_lock_irq(q->queue_lock);
+				__blk_end_request_all(rq, -ENXIO);
+				continue;
+			}
+		}
+
 		dout("%s 0x%x bytes at 0x%llx\n",
 		     do_write ? "write" : "read",
 		     size, blk_rq_pos(rq) * SECTOR_SIZE);
@@ -2088,7 +2109,14 @@ static int __rbd_init_snaps_header(struct rbd_device *rbd_dev)
 			cur_id = rbd_dev->header.snapc->snaps[i - 1];
 
 		if (!i || old_snap->id < cur_id) {
-			/* old_snap->id was skipped, thus was removed */
+			/*
+			 * old_snap->id was skipped, thus was
+			 * removed.  If this rbd_dev is mapped to
+			 * the removed snapshot, record that it no
+			 * longer exists, to prevent further I/O.
+			 */
+			if (rbd_dev->snap_id == old_snap->id)
+				rbd_dev->snap_exists = false;
 			__rbd_remove_snap_dev(rbd_dev, old_snap);
 			continue;
 		}