rbd: get parent info on refresh

Get parent info for format 2 images on every refresh (rather than
just during the initial probe).  This will be needed to detect the
disappearance of the parent image in the event a mapped image
becomes unlayered (i.e., flattened).  Avoid leaking the previous
parent spec on the second and subsequent times this information is
requested by dropping the previous one (if any) before updating it.
(Also, extract the pool id into a local variable before assigning
it into the parent spec.)

Switch to using a non-zero parent overlap value rather than the
existence of a parent (a non-null parent_spec pointer) to determine
whether to mark a request layered.  It will soon be possible for
a layered image to become unlayered while a request is in flight.

This means that the layered flag for an image request indicates that
there was a non-zero parent overlap at the time the image request
was created.  The parent overlap can change thereafter, which may
lead to special handling at request submission or completion time.

This and the next several patches are related to:
    http://tracker.ceph.com/issues/3763

NOTE:
If an error occurs while refreshing the parent info (i.e.,
requesting it after initial probe), the old parent info will
persist.  This is not really correct, and is a scenario that needs
to be addressed.  For now we'll assert that the failure mode is
unlikely, but the issue has been documented in tracker issue 5040.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -1873,7 +1873,7 @@ static struct rbd_img_request *rbd_img_request_create(
 	}
 	if (child_request)
 		img_request_child_set(img_request);
-	if (rbd_dev->parent_spec)
+	if (rbd_dev->parent_overlap)
 		img_request_layered_set(img_request);
 	spin_lock_init(&img_request->completion_lock);
 	img_request->next_completion = 0;
@@ -3613,6 +3613,7 @@ static int rbd_dev_v2_parent_info(struct rbd_device *rbd_dev)
 	__le64 snapid;
 	void *p;
 	void *end;
+	u64 pool_id;
 	char *image_id;
 	u64 overlap;
 	int ret;
@@ -3643,18 +3644,19 @@ static int rbd_dev_v2_parent_info(struct rbd_device *rbd_dev)
 	p = reply_buf;
 	end = reply_buf + ret;
 	ret = -ERANGE;
-	ceph_decode_64_safe(&p, end, parent_spec->pool_id, out_err);
-	if (parent_spec->pool_id == CEPH_NOPOOL)
+	ceph_decode_64_safe(&p, end, pool_id, out_err);
+	if (pool_id == CEPH_NOPOOL)
 		goto out;	/* No parent?  No problem. */
 
 	/* The ceph file layout needs to fit pool id in 32 bits */
 
 	ret = -EIO;
-	if (parent_spec->pool_id > (u64)U32_MAX) {
+	if (pool_id > (u64)U32_MAX) {
 		rbd_warn(NULL, "parent pool id too large (%llu > %u)\n",
-			(unsigned long long)parent_spec->pool_id, U32_MAX);
+			(unsigned long long)pool_id, U32_MAX);
 		goto out_err;
 	}
+	parent_spec->pool_id = pool_id;
 
 	image_id = ceph_extract_encoded_string(&p, end, NULL, GFP_KERNEL);
 	if (IS_ERR(image_id)) {
@@ -3666,6 +3668,7 @@ static int rbd_dev_v2_parent_info(struct rbd_device *rbd_dev)
 	ceph_decode_64_safe(&p, end, overlap, out_err);
 
 	if (overlap) {
+		rbd_spec_put(rbd_dev->parent_spec);
 		rbd_dev->parent_spec = parent_spec;
 		parent_spec = NULL;	/* rbd_dev now owns this */
 		rbd_dev->parent_overlap = overlap;
@@ -4034,17 +4037,43 @@ static int rbd_dev_v2_header_info(struct rbd_device *rbd_dev)
 			goto out;
 	}
 
+	/*
+	 * If the image supports layering, get the parent info.  We
+	 * need to probe the first time regardless.  Thereafter we
+	 * only need to if there's a parent, to see if it has
+	 * disappeared due to the mapped image getting flattened.
+	 */
+	if (rbd_dev->header.features & RBD_FEATURE_LAYERING &&
+			(first_time || rbd_dev->parent_spec)) {
+		bool warn;
+
+		ret = rbd_dev_v2_parent_info(rbd_dev);
+		if (ret)
+			goto out;
+
+		/*
+		 * Print a warning if this is the initial probe and
+		 * the image has a parent.  Don't print it if the
+		 * image now being probed is itself a parent.  We
+		 * can tell at this point because we won't know its
+		 * pool name yet (just its pool id).
+		 */
+		warn = rbd_dev->parent_spec && rbd_dev->spec->pool_name;
+		if (first_time && warn)
+			rbd_warn(rbd_dev, "WARNING: kernel layering "
+					"is EXPERIMENTAL!");
+	}
+
 	ret = rbd_dev_v2_image_size(rbd_dev);
 	if (ret)
 		goto out;
+
 	if (rbd_dev->spec->snap_id == CEPH_NOSNAP)
 		if (rbd_dev->mapping.size != rbd_dev->header.image_size)
 			rbd_dev->mapping.size = rbd_dev->header.image_size;
 
 	ret = rbd_dev_v2_snap_context(rbd_dev);
 	dout("rbd_dev_v2_snap_context returned %d\n", ret);
-	if (ret)
-		goto out;
 out:
 	up_write(&rbd_dev->header_rwsem);
 
@@ -4498,24 +4527,6 @@ static int rbd_dev_v2_header_onetime(struct rbd_device *rbd_dev)
 	if (ret)
 		goto out_err;
 
-	/* If the image supports layering, get the parent info */
-
-	if (rbd_dev->header.features & RBD_FEATURE_LAYERING) {
-		ret = rbd_dev_v2_parent_info(rbd_dev);
-		if (ret)
-			goto out_err;
-		/*
-		 * Print a warning if this image has a parent.
-		 * Don't print it if the image now being probed
-		 * is itself a parent.  We can tell at this point
-		 * because we won't know its pool name yet (just its
-		 * pool id).
-		 */
-		if (rbd_dev->parent_spec && rbd_dev->spec->pool_name)
-			rbd_warn(rbd_dev, "WARNING: kernel layering "
-					"is EXPERIMENTAL!");
-	}
-
 	/* If the image supports fancy striping, get its parameters */
 
 	if (rbd_dev->header.features & RBD_FEATURE_STRIPINGV2) {
@@ -4527,11 +4538,7 @@ static int rbd_dev_v2_header_onetime(struct rbd_device *rbd_dev)
 
 	return 0;
 out_err:
-	rbd_dev->parent_overlap = 0;
-	rbd_spec_put(rbd_dev->parent_spec);
-	rbd_dev->parent_spec = NULL;
-	kfree(rbd_dev->header_name);
-	rbd_dev->header_name = NULL;
+	rbd_dev->header.features = 0;
 	kfree(rbd_dev->header.object_prefix);
 	rbd_dev->header.object_prefix = NULL;