PI 5 kernel update to 6.18.4 fails booting

[s-pouya-sh]

Describe the bug

I have a PI 5 with linux-rpi kernel installed, I am using btrfs for root partition and vfat for boot.
after updating kernel to linux-rpi 6.18.4, the system can not boot

Failed to pivot root to new rootfs '/run/systemd/mount-rootfs': Invalid argument
Will try to use NS_MOVE instead: Invalid argument

and for btrfs:

lsinitcpio initramfs-linux.img | grep btrfs
usr/bin/fsck.btrfs
usr/lib/udev/rules.d/64-btrfs.rules

Steps to reproduce the behaviour

pacman -Syu linux-rpi

Device (s)

Raspberry Pi 5

System

2025/01/27 13:08:59
Copyright (c) 2012 Broadcom
version 76eda9d7 (release) (embedded)

previous kernel: 6.12.61-1-rpi

Logs

No response

Additional context

No response

[pelwell]

How did you update? btrfs is included in Raspberry Pi kernel builds, but as a module. That means that if your root filesystem is btrfs, you must provide the btrfs module as part of an initramfs. If your kernel changes, the initramfs modules should be updated to match.

The rpi-update utility, a common way to install new kernels for testing, gives this warning before every installation:

Updating a system with initramfs configured is not supported by rpi-update.
If your system relies on drivers provided by the initramfs (e.g. custom filesystem options)
it may not boot without regenerating the initramfs.
If you are unsure, test if your system boots with initramfs options disabled from config.txt

This is also covered in the rpi-update README: https://github.com/raspberrypi/rpi-update?tab=readme-ov-file#caveats

[s-pouya-sh]

I used pacman -Syu

[pelwell]

We don't use or support pacman. If you are using Arch Linux (you don't say), you should ask on the Arch forums, at least until the Arch devs have established that this is a Raspberry Pi kernel bug.

Going back to your report:

lsinitcpio initramfs-linux.img | grep btrfs
usr/bin/fsck.btrfs
usr/lib/udev/rules.d/64-btrfs.rules

The Raspberry Pi initramfs files are not in cpio format, so the equivalent command is lsinitramfs, but I would have expected the lsinitcpio output to include the btrfs module, e.g.:

$ lsinitramfs initramfs8 | grep btrfs
usr/lib/modules/6.6.62+rpt-rpi-v8/kernel/fs/btrfs
usr/lib/modules/6.6.62+rpt-rpi-v8/kernel/fs/btrfs/btrfs.ko
``
Are there _any_ modules in there?

[s-pouya-sh]

Yes, I am using Arch Linux, and the kernel is installed from the pacman repo.
No btrfs module here:

lsinitramfs initramfs-linux.img | grep btrfs
bash: lsinitramfs: command not found
[root@stellarmate boot]# lsinitcpio initramfs-linux.img | grep btrfs
usr/bin/fsck.btrfs
usr/lib/udev/rules.d/64-btrfs.rules

[pelwell]

Ah - Arch's RPi support includes a file of configuration changes from the standard RPi configuration. This includes making BTRFS a built-in. This shows how different Arch is, and why you should take it up with the Arch devs.