rbd: lay out header probe infrastructure

This defines a new function rbd_dev_probe() as a top-level
function for populating detailed information about an rbd device.

It first checks for the existence of a format 2 rbd image id object.
If it exists, the image is assumed to be a format 2 rbd image, and
another function rbd_dev_v2() is called to finish populating
header data for that image.  If it does not exist, it is assumed to
be an old (format 1) rbd image, and calls a similar function
rbd_dev_v1() to populate its header information.

A new field, rbd_dev->format, is defined to record which version
of the rbd image format the device represents.  For a valid mapped
rbd device it will have one of two values, 1 or 2.

So far, the format 2 images are not really supported; this is
laying out the infrastructure for fleshing out that support.

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

drivers/block/rbd.c

@@ -170,6 +170,7 @@ struct rbd_device {
 	int			major;		/* blkdev assigned major */
 	struct gendisk		*disk;		/* blkdev's gendisk and rq */
 
+	u32			image_format;	/* Either 1 or 2 */
 	struct rbd_options	rbd_opts;
 	struct rbd_client	*rbd_client;
 
@@ -507,6 +508,11 @@ static void rbd_coll_release(struct kref *kref)
 	kfree(coll);
 }
 
+static bool rbd_image_format_valid(u32 image_format)
+{
+	return image_format == 1 || image_format == 2;
+}
+
 static bool rbd_dev_ondisk_valid(struct rbd_image_header_ondisk *ondisk)
 {
 	size_t size;
@@ -2584,6 +2590,96 @@ out:
 	return ret;
 }
 
+static int rbd_dev_v1_probe(struct rbd_device *rbd_dev)
+{
+	int ret;
+	size_t size;
+
+	/* Version 1 images have no id; empty string is used */
+
+	rbd_dev->image_id = kstrdup("", GFP_KERNEL);
+	if (!rbd_dev->image_id)
+		return -ENOMEM;
+	rbd_dev->image_id_len = 0;
+
+	/* Record the header object name for this rbd image. */
+
+	size = rbd_dev->image_name_len + sizeof (RBD_SUFFIX);
+	rbd_dev->header_name = kmalloc(size, GFP_KERNEL);
+	if (!rbd_dev->header_name) {
+		ret = -ENOMEM;
+		goto out_err;
+	}
+	sprintf(rbd_dev->header_name, "%s%s", rbd_dev->image_name, RBD_SUFFIX);
+
+	/* Populate rbd image metadata */
+
+	ret = rbd_read_header(rbd_dev, &rbd_dev->header);
+	if (ret < 0)
+		goto out_err;
+	rbd_dev->image_format = 1;
+
+	dout("discovered version 1 image, header name is %s\n",
+		rbd_dev->header_name);
+
+	return 0;
+
+out_err:
+	kfree(rbd_dev->header_name);
+	rbd_dev->header_name = NULL;
+	kfree(rbd_dev->image_id);
+	rbd_dev->image_id = NULL;
+
+	return ret;
+}
+
+static int rbd_dev_v2_probe(struct rbd_device *rbd_dev)
+{
+	size_t size;
+
+	/*
+	 * Image id was filled in by the caller.  Record the header
+	 * object name for this rbd image.
+	 */
+	size = sizeof (RBD_HEADER_PREFIX) + rbd_dev->image_id_len;
+	rbd_dev->header_name = kmalloc(size, GFP_KERNEL);
+	if (!rbd_dev->header_name)
+		return -ENOMEM;
+	sprintf(rbd_dev->header_name, "%s%s",
+			RBD_HEADER_PREFIX, rbd_dev->image_id);
+	rbd_dev->image_format = 2;
+
+	dout("discovered version 2 image, header name is %s\n",
+		rbd_dev->header_name);
+
+	return -ENOTSUPP;
+}
+
+/*
+ * Probe for the existence of the header object for the given rbd
+ * device.  For format 2 images this includes determining the image
+ * id.
+ */
+static int rbd_dev_probe(struct rbd_device *rbd_dev)
+{
+	int ret;
+
+	/*
+	 * Get the id from the image id object.  If it's not a
+	 * format 2 image, we'll get ENOENT back, and we'll assume
+	 * it's a format 1 image.
+	 */
+	ret = rbd_dev_image_id(rbd_dev);
+	if (ret)
+		ret = rbd_dev_v1_probe(rbd_dev);
+	else
+		ret = rbd_dev_v2_probe(rbd_dev);
+	if (ret)
+		dout("probe failed, returning %d\n", ret);
+
+	return ret;
+}
+
 static ssize_t rbd_add(struct bus_type *bus,
 		       const char *buf,
 		       size_t count)
@@ -2631,35 +2727,10 @@ static ssize_t rbd_add(struct bus_type *bus,
 		goto err_out_client;
 	rbd_dev->pool_id = rc;
 
-	rc = rbd_dev_image_id(rbd_dev);
+	rc = rbd_dev_probe(rbd_dev);
-	if (!rc) {
+	if (rc < 0)
-		rc = -ENOTSUPP;	/* Not actually supporting format 2 yet */
-		goto err_out_client;
-	}
-
-	/* Version 1 images have no id; empty string is used */
-
-	rbd_dev->image_id = kstrdup("", GFP_KERNEL);
-	if (!rbd_dev->image_id) {
-		rc = -ENOMEM;
-		goto err_out_client;
-	}
-	rbd_dev->image_id_len = 0;
-
-	/* Create the name of the header object */
-
-	rbd_dev->header_name = kmalloc(rbd_dev->image_name_len
-						+ sizeof (RBD_SUFFIX),
-					GFP_KERNEL);
-	if (!rbd_dev->header_name)
-		goto err_out_client;
-	sprintf(rbd_dev->header_name, "%s%s", rbd_dev->image_name, RBD_SUFFIX);
-
-	/* Get information about the image being mapped */
-
-	rc = rbd_read_header(rbd_dev, &rbd_dev->header);
-	if (rc)
 		goto err_out_client;
+	rbd_assert(rbd_image_format_valid(rbd_dev->image_format));
 
 	/* no need to lock here, as rbd_dev is not registered yet */
 	rc = rbd_dev_snaps_update(rbd_dev);