rbd: show the entire chain of parent images

Make /sys/bus/rbd/devices/<id>/parent show the entire chain of parent
images.  While at it, kernel sprintf() doesn't return negative values,
casting to unsigned long long is no longer necessary and there is no
good reason to split into multiple sprintf() calls.
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>

Documentation/ABI/testing/sysfs-bus-rbd

@@ -94,5 +94,5 @@ current_snap
 
 parent
 
-	Information identifying the pool, image, and snapshot id for
-	the parent image in a layered rbd image (format 2 only).
+	Information identifying the chain of parent images in a layered rbd
+	image.  Entries are separated by empty lines.

drivers/block/rbd.c

@@ -3685,46 +3685,36 @@ static ssize_t rbd_snap_show(struct device *dev,
 }
 
 /*
- * For an rbd v2 image, shows the pool id, image id, and snapshot id
- * for the parent image.  If there is no parent, simply shows
- * "(no parent image)".
+ * For a v2 image, shows the chain of parent images, separated by empty
+ * lines.  For v1 images or if there is no parent, shows "(no parent
+ * image)".
  */
 static ssize_t rbd_parent_show(struct device *dev,
 			       struct device_attribute *attr,
 			       char *buf)
 {
 	struct rbd_device *rbd_dev = dev_to_rbd_dev(dev);
-	struct rbd_spec *spec = rbd_dev->parent_spec;
-	int count;
-	char *bufp = buf;
+	ssize_t count = 0;
 
-	if (!spec)
+	if (!rbd_dev->parent)
 		return sprintf(buf, "(no parent image)\n");
 
-	count = sprintf(bufp, "pool_id %llu\npool_name %s\n",
-			(unsigned long long) spec->pool_id, spec->pool_name);
-	if (count < 0)
-		return count;
-	bufp += count;
-
-	count = sprintf(bufp, "image_id %s\nimage_name %s\n", spec->image_id,
-			spec->image_name ? spec->image_name : "(unknown)");
-	if (count < 0)
-		return count;
-	bufp += count;
+	for ( ; rbd_dev->parent; rbd_dev = rbd_dev->parent) {
+		struct rbd_spec *spec = rbd_dev->parent_spec;
 
-	count = sprintf(bufp, "snap_id %llu\nsnap_name %s\n",
-			(unsigned long long) spec->snap_id, spec->snap_name);
-	if (count < 0)
-		return count;
-	bufp += count;
+		count += sprintf(&buf[count], "%s"
+			    "pool_id %llu\npool_name %s\n"
+			    "image_id %s\nimage_name %s\n"
+			    "snap_id %llu\nsnap_name %s\n"
+			    "overlap %llu\n",
+			    !count ? "" : "\n", /* first? */
+			    spec->pool_id, spec->pool_name,
+			    spec->image_id, spec->image_name ?: "(unknown)",
+			    spec->snap_id, spec->snap_name,
+			    rbd_dev->parent_overlap);
+	}
 
-	count = sprintf(bufp, "overlap %llu\n", rbd_dev->parent_overlap);
-	if (count < 0)
 	return count;
-	bufp += count;
-
-	return (ssize_t) (bufp - buf);
 }
 
 static ssize_t rbd_image_refresh(struct device *dev,