XFS attr2 mount option removal may break system boot

[Oleksandr Natalenko <oleksandr@natalenko.name>]

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Hello.

In v6.18, the attr2 XFS mount option is removed. This may silently break system boot if the attr2 option is still present in /etc/fstab for rootfs.

Consider Arch Linux that is being set up from scratch with / being formatted as XFS. The genfstab command that is used to generate /etc/fstab produces something like this by default:

/dev/sda2 on / type xfs (rw,relatime,attr2,discard,inode64,logbufs=8,logbsize=32k,noquota)

Once the system is set up and rebooted, there's no deprecation warning seen in the kernel log:

# cat /proc/cmdline
root=UUID=77b42de2-397e-47ee-a1ef-4dfd430e47e9 rootflags=discard rd.luks.options=discard quiet

# dmesg | grep -i xfs
[    2.409818] SGI XFS with ACLs, security attributes, realtime, scrub, repair, quota, no debug enabled
[    2.415341] XFS (sda2): Mounting V5 Filesystem 77b42de2-397e-47ee-a1ef-4dfd430e47e9
[    2.442546] XFS (sda2): Ending clean mount

Although as per the deprecation intention, it should be there.

Vlastimil (in Cc) suggests this is because xfs_fs_warn_deprecated() doesn't produce any warning by design if the XFS FS is set to be rootfs and gets remounted read-write during boot. This imposes two problems:

1) a user doesn't see the deprecation warning; and
2) with v6.18 kernel, the read-write remount fails because of unknown attr2 option rendering system unusable:

systemd[1]: Switching root.
systemd-remount-fs[225]: /usr/bin/mount for / exited with exit status 32.

# mount -o rw /
mount: /: fsconfig() failed: xfs: Unknown parameter 'attr2'.

Thorsten (in Cc) suggested reporting this as a user-visible regression.

From my PoV, although the deprecation is in place for 5 years already, it may not be visible enough as the warning is not emitted for rootfs. Considering the amount of systems set up with XFS on /, this may impose a mass problem for users.

Vlastimil suggested making attr2 option a complete noop instead of removing it.

Please check.

Thank you.

-- 
Oleksandr Natalenko, MSE

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