dm: add support for get_unique_id

This adds support to obtain a device's unique id through dm, similar to the
existing ioctl and persistent resevation handling. We limit this to
single-target devices.

This enables knfsd to export pNFS SCSI luns that have been exported from
multipath devices.

Signed-off-by: Benjamin Coddington <[e-mail]>
Signed-off-by: Mikulas Patocka <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>

drivers/md/dm.c

···
 	kfree(pools);
 }
 
+struct dm_blkdev_id {
+	u8 *id;
+	enum blk_unique_id type;
+};
+
+static int __dm_get_unique_id(struct dm_target *ti, struct dm_dev *dev,
+				sector_t start, sector_t len, void *data)
+{
+	struct dm_blkdev_id *dm_id = data;
+	const struct block_device_operations *fops = dev->bdev->bd_disk->fops;
+
+	if (!fops->get_unique_id)
+		return 0;
+
+	return fops->get_unique_id(dev->bdev->bd_disk, dm_id->id, dm_id->type);
+}
+
+/*
+ * Allow access to get_unique_id() for the first device returning a
+ * non-zero result.  Reasonable use expects all devices to have the
+ * same unique id.
+ */
+static int dm_blk_get_unique_id(struct gendisk *disk, u8 *id,
+		enum blk_unique_id type)
+{
+	struct mapped_device *md = disk->private_data;
+	struct dm_table *table;
+	struct dm_target *ti;
+	int ret = 0, srcu_idx;
+
+	struct dm_blkdev_id dm_id = {
+		.id = id,
+		.type = type,
+	};
+
+	table = dm_get_live_table(md, &srcu_idx);
+	if (!table || !dm_table_get_size(table))
+		goto out;
+
+	/* We only support devices that have a single target */
+	if (table->num_targets != 1)
+		goto out;
+	ti = dm_table_get_target(table, 0);
+
+	if (!ti->type->iterate_devices)
+		goto out;
+
+	ret = ti->type->iterate_devices(ti, __dm_get_unique_id, &dm_id);
+out:
+	dm_put_live_table(md, srcu_idx);
+	return ret;
+}
+
 struct dm_pr {
 	u64	old_key;
 	u64	new_key;
···
 	.ioctl = dm_blk_ioctl,
 	.getgeo = dm_blk_getgeo,
 	.report_zones = dm_blk_report_zones,
+	.get_unique_id = dm_blk_get_unique_id,
 	.pr_ops = &dm_pr_ops,
 	.owner = THIS_MODULE
 };
···
 	.release = dm_blk_close,
 	.ioctl = dm_blk_ioctl,
 	.getgeo = dm_blk_getgeo,
+	.get_unique_id = dm_blk_get_unique_id,
 	.pr_ops = &dm_pr_ops,
 	.owner = THIS_MODULE
 };