rbd: remove snapshot existence validation code

RBD_DEV_FLAG_EXISTS check in rbd_queue_workfn() is racy and leads to
inconsistent behaviour.  If the object (or its snapshot) isn't there,
the OSD returns ENOENT.  A read submitted before the snapshot removal
notification is processed would be zero-filled and ended with status
OK, while future reads would be failed with IOERR.  It also doesn't
handle a case when an image that is mapped read-only is removed.

On top of this, because watch is no longer established for read-only
mappings, we no longer get notifications, so rbd_exists_validate() is
effectively dead code.  While failing requests rather than returning
zeros is a good thing, RBD_DEV_FLAG_EXISTS is not it.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>

drivers/block/rbd.c

@@ -462,7 +462,7 @@ struct rbd_device {
  *   by rbd_dev->lock
  */
 enum rbd_dev_flags {
-	RBD_DEV_FLAG_EXISTS,	/* mapped snapshot has not been deleted */
+	RBD_DEV_FLAG_EXISTS,	/* rbd_dev_device_setup() ran */
 	RBD_DEV_FLAG_REMOVING,	/* this mapping is being removed */
 	RBD_DEV_FLAG_READONLY,  /* -o ro or snapshot */
 };
@@ -4865,19 +4865,6 @@ static void rbd_queue_workfn(struct work_struct *work)
 		rbd_assert(!rbd_is_snap(rbd_dev));
 	}
 
-	/*
-	 * Quit early if the mapped snapshot no longer exists.  It's
-	 * still possible the snapshot will have disappeared by the
-	 * time our request arrives at the osd, but there's no sense in
-	 * sending it if we already know.
-	 */
-	if (!test_bit(RBD_DEV_FLAG_EXISTS, &rbd_dev->flags)) {
-		dout("request for non-existent snapshot");
-		rbd_assert(rbd_is_snap(rbd_dev));
-		result = -ENXIO;
-		goto err_rq;
-	}
-
 	if (offset && length > U64_MAX - offset + 1) {
 		rbd_warn(rbd_dev, "bad request range (%llu~%llu)", offset,
 			 length);
@@ -5057,25 +5044,6 @@ static int rbd_dev_v1_header_info(struct rbd_device *rbd_dev)
 	return ret;
 }
 
-/*
- * Clear the rbd device's EXISTS flag if the snapshot it's mapped to
- * has disappeared from the (just updated) snapshot context.
- */
-static void rbd_exists_validate(struct rbd_device *rbd_dev)
-{
-	u64 snap_id;
-
-	if (!test_bit(RBD_DEV_FLAG_EXISTS, &rbd_dev->flags))
-		return;
-
-	snap_id = rbd_dev->spec->snap_id;
-	if (snap_id == CEPH_NOSNAP)
-		return;
-
-	if (rbd_dev_snap_index(rbd_dev, snap_id) == BAD_SNAP_INDEX)
-		clear_bit(RBD_DEV_FLAG_EXISTS, &rbd_dev->flags);
-}
-
 static void rbd_dev_update_size(struct rbd_device *rbd_dev)
 {
 	sector_t size;
@@ -5116,12 +5084,8 @@ static int rbd_dev_refresh(struct rbd_device *rbd_dev)
 			goto out;
 	}
 
-	if (!rbd_is_snap(rbd_dev)) {
-		rbd_dev->mapping.size = rbd_dev->header.image_size;
-	} else {
-		/* validate mapped snapshot's EXISTS flag */
-		rbd_exists_validate(rbd_dev);
-	}
+	rbd_assert(!rbd_is_snap(rbd_dev));
+	rbd_dev->mapping.size = rbd_dev->header.image_size;
 
 out:
 	up_write(&rbd_dev->header_rwsem);