The id 0 of the default subvolume is an internal alias for the toplevel
fs tree, kernel does that conversion. Until 2116398b1d ("btrfs-progs:
use libbtrfsutil for set-default") there was no manual conversion and
the value was passed to kernel as-is. With the switch to the
libbtrfsutil API this got broken (4.19).
$ btrfs subvol set-default 0 /path
In this case the default subvolume would be containing subvolume of
/path instead of the toplevel one.
Fix it by manually switching the 0 to 5 in case user specifies that to
avoid the difference in the API, that we can't change.
Issue: #327
Reported-by: Chris Murphy
Signed-off-by: David Sterba <dsterba@suse.com>
3c49295e8a