bcache: export backing_dev_uuid via sysfs

When there are multiple bcache devices, after a reboot the name of
bcache devices may change (e.g. current /dev/bcache1 was /dev/bcache0
before reboot). Therefore we need the backing device UUID (sb.uuid) to
identify each bcache device.

Backing device uuid can be found by program bcache-super-show, but
directly exporting backing_dev_uuid by sysfs file
/sys/block/bcache<?>/bcache/backing_dev_uuid is a much simpler method.

With backing_dev_uuid, and partition uuids from /dev/disk/by-partuuid/,
now we can identify each bcache device and its partitions conveniently.
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/sysfs.c

@@ -68,6 +68,7 @@ read_attribute(btree_written);
 read_attribute(metadata_written);
 read_attribute(active_journal_entries);
 read_attribute(backing_dev_name);
+read_attribute(backing_dev_uuid);
 
 sysfs_time_stats_attribute(btree_gc,	sec, ms);
 sysfs_time_stats_attribute(btree_split, sec, us);
@@ -250,6 +251,13 @@ SHOW(__bch_cached_dev)
 		return strlen(buf);
 	}
 
+	if (attr == &sysfs_backing_dev_uuid) {
+		/* convert binary uuid into 36-byte string plus '\0' */
+		snprintf(buf, 36+1, "%pU", dc->sb.uuid);
+		strcat(buf, "\n");
+		return strlen(buf);
+	}
+
 #undef var
 	return 0;
 }
@@ -460,6 +468,7 @@ static struct attribute *bch_cached_dev_files[] = {
 	&sysfs_bypass_torture_test,
 #endif
 	&sysfs_backing_dev_name,
+	&sysfs_backing_dev_uuid,
 	NULL
 };
 KTYPE(bch_cached_dev);