Root mount options are ignored in Fedora Atomic Desktops 42

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Problem

In Fedora Atomic Desktops 42, mount options set in /etc/fstab for the following mount points are ignored:

  • root (/) mount point
  • /proc & /sys mount points, if manually specified (they are not created by default) and listed after the entry for the root mount point

All other mount points are processed normally.

For the default installation with btrfs, this means that on-demand zstd compression is not enabled anymore. This might mean slower disk access times or larger disk usage, depending on the data and usage.

If the user set other special mount options for their root partition then they are also not taken into account.

Cause

This is due to the transition to composefs for the Fedora Atomic Desktops 42.

You can detect this problem be seeing the systemd-remount-fs.service service fail to start with this error:

mount: /: fsconfig system call failed: overlay: No changes allowed in reconfigure.

Related Issues

Bugzilla report: #2348934
Upstream issue: root mount options from /etc/fstab are not taken into account anymore with composefs (symptom: `systemd-remount-fs.service` fails to start) (#72) · Issues · fedora / Fedora Atomic Desktops / SIG Issue Tracker · GitLab
Anaconda bug report: 2332319 – Switch from /etc/fstab to root= and rootflags= (at least for bootc installs)

Workarounds

If you want to keep using or enable on-demand zstd compression for your main btrfs partition, update your kernel arguments to match what’s in your /etc/fstab. Here’s an example (adjust to your case):

$ grep 'root' /etc/fstab
UUID=... / btrfs subvol=root,compress=zstd:1,ro 0 0

$ cat /proc/cmdline
... rootflags=subvol=root ...

$ sudo rpm-ostree kargs --delete=rootflags=subvol=root --append=rootflags=subvol=root,compress=zstd:1

Then comment out the entry for / in your /etc/fstab (which means add # at the very beginning of the line). Make sure that any future changes to the root mount point options are added to the kernel command line.

Reboot for the changes to apply.

Use the same approach also for any other mount options for the root partition that you want to have applied.

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