On Mon, Jun 28, 2021 at 06:16:37PM +0800, Qu Wenruo wrote:

There is a report from the mail list that some subvolumes don't have any
ROOT_REF/BACKREF and has 0 ref.
But without an ORPHAN item.

Do you have link to the mail report?

Such ghost subvolumes can't be deleted by any ioctl but only rely on
btrfs-progs to add ORPHAN item.

Is there a way to list such subvolumes from progs?

Normally kernel only needs to gracefully abort/reject such corrupted
structure, but in this case we have all the needed infrastructures and
interface to allow BTRFS_IOC_SNAP_DESTROY_V2 to delete it.

So add the ability to delete such ghost subvolumes to
BTRFS_IOC_SNAP_DESTROY_V2.

So this is only extending the functionality and we don't need to handle
backward compatibility.