btrfs: add comment about metadata_uuid in btrfs_fs_devices

Add comment about metadata_uuid in btrfs_fs_devices.
No functional change.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 236ae696c9841285db8c8a9d66b5511b63ceebab..24f30542ef7c8d9e3014fbe7891e9a38f3d4a4c4 100644 (file)
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -280,7 +280,19 @@ enum btrfs_read_policy {
 
 struct btrfs_fs_devices {
        u8 fsid[BTRFS_FSID_SIZE]; /* FS specific uuid */
+
+       /*
+        * UUID written into the btree blocks:
+        *
+        * - If metadata_uuid != fsid then super block must have
+        *   BTRFS_FEATURE_INCOMPAT_METADATA_UUID flag set.
+        *
+        * - Following shall be true at all times:
+        *   - metadata_uuid == btrfs_header::fsid
+        *   - metadata_uuid == btrfs_dev_item::fsid
+        */
        u8 metadata_uuid[BTRFS_FSID_SIZE];
+
        struct list_head fs_list;
 
        /*