rbd: add code to get the size of a v2 rbd image

The size of an rbd format 2 image is fetched from the server using a
"get_size" method.  The same method is used for getting the size of
a snapshot, so structure this addition with a generic helper routine
that we can get this information for either.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -2127,6 +2127,47 @@ static char *rbd_dev_v1_snap_info(struct rbd_device *rbd_dev, u32 which,
 	return snap_name;
 }
 
+/*
+ * Get the size and object order for an image snapshot, or if
+ * snap_id is CEPH_NOSNAP, gets this information for the base
+ * image.
+ */
+static int _rbd_dev_v2_snap_size(struct rbd_device *rbd_dev, u64 snap_id,
+				u8 *order, u64 *snap_size)
+{
+	__le64 snapid = cpu_to_le64(snap_id);
+	int ret;
+	struct {
+		u8 order;
+		__le64 size;
+	} __attribute__ ((packed)) size_buf = { 0 };
+
+	ret = rbd_req_sync_exec(rbd_dev, rbd_dev->header_name,
+				"rbd", "get_size",
+				(char *) &snapid, sizeof (snapid),
+				(char *) &size_buf, sizeof (size_buf),
+				CEPH_OSD_FLAG_READ, NULL);
+	dout("%s: rbd_req_sync_exec returned %d\n", __func__, ret);
+	if (ret < 0)
+		return ret;
+
+	*order = size_buf.order;
+	*snap_size = le64_to_cpu(size_buf.size);
+
+	dout("  snap_id 0x%016llx order = %u, snap_size = %llu\n",
+		(unsigned long long) snap_id, (unsigned int) *order,
+		(unsigned long long) *snap_size);
+
+	return 0;
+}
+
+static int rbd_dev_v2_image_size(struct rbd_device *rbd_dev)
+{
+	return _rbd_dev_v2_snap_size(rbd_dev, CEPH_NOSNAP,
+					&rbd_dev->header.obj_order,
+					&rbd_dev->header.image_size);
+}
+
 /*
  * Scan the rbd device's current snapshot list and compare it to the
  * newly-received snapshot context.  Remove any existing snapshots
@@ -2636,6 +2677,7 @@ out_err:
 static int rbd_dev_v2_probe(struct rbd_device *rbd_dev)
 {
 	size_t size;
+	int ret;
 
 	/*
 	 * Image id was filled in by the caller.  Record the header
@@ -2647,12 +2689,23 @@ static int rbd_dev_v2_probe(struct rbd_device *rbd_dev)
 		return -ENOMEM;
 	sprintf(rbd_dev->header_name, "%s%s",
 			RBD_HEADER_PREFIX, rbd_dev->image_id);
+
+	/* Get the size and object order for the image */
+
+	ret = rbd_dev_v2_image_size(rbd_dev);
+	if (ret < 0)
+		goto out_err;
 	rbd_dev->image_format = 2;
 
 	dout("discovered version 2 image, header name is %s\n",
 		rbd_dev->header_name);
 
 	return -ENOTSUPP;
+out_err:
+	kfree(rbd_dev->header_name);
+	rbd_dev->header_name = NULL;
+
+	return ret;
 }
 
 /*