On 2021/6/30 下午9:16, David Sterba wrote:

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

quoted

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?

Here it is:
https://lore.kernel.org/linux-btrfs/CAJ9tZB-G7KZkxGfrADbmHruuEfwyhV1bihUvRZrJ_ypt_iUVKg@mail.gmail.com/T/#t (local)

quoted

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?

No, root with 0 ref will not show up in "btrfs subv list".

In fact unless we pass @check_ref = false, btrfs_get_fs_root() won't 
return such ghost root at all.

quoted

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.

Exactly.

In fact, I hope we never need to bother such case in the future...

Thanks,
Qu