On 28/07/2021 08:01, NeilBrown wrote:

On Wed, 28 Jul 2021, Wang Yugui wrote:

quoted

Hi,

This patchset works well in 5.14-rc3.

Thanks for testing.

quoted

1, fixed dummy inode(255, BTRFS_FIRST_FREE_OBJECTID - 1 )  is changed to
dynamic dummy inode(18446744073709551358, or 18446744073709551359, ...)

The BTRFS_FIRST_FREE_OBJECTID-1 was a just a hack, I never wanted it to
be permanent.
The new number is ULONG_MAX - subvol_id (where subvol_id starts at 257 I
think).
This is a bit less of a hack.  It is an easily available number that is
fairly unique.

quoted

2, btrfs subvol mount info is shown in /proc/mounts, even if nfsd/nfs is
not used.
/dev/sdc                btrfs   94G  3.5M   93G   1% /mnt/test
/dev/sdc                btrfs   94G  3.5M   93G   1% /mnt/test/sub1
/dev/sdc                btrfs   94G  3.5M   93G   1% /mnt/test/sub2

This is a visiual feature change for btrfs user.

Hopefully it is an improvement.  But it is certainly a change that needs
to be carefully considered.

Would this change the behaviour of findmnt? I have several scripts that
depend on findmnt to select btrfs filesystems. Just to take a couple of
examples (using the example shown above): my scripts would depend on
'findmnt --target /mnt/test/sub1 -o target' providing /mnt/test, not the
subvolume; and another script would depend on 'findmnt -t btrfs
--mountpoint /mnt/test/sub1' providing no output as the directory is not
an /etc/fstab mount point for a btrfs filesystem.

Maybe findmnt isn't affected? Or maybe the change is worth making
anyway? But it needs to be carefully considered if it breaks existing
user interfaces.