Prompt error message "Failed to unmount /oldroot" when shutdown or reboot

[Bin Li]

After I installed the mdadm, it will show this error information, our OEM vendor request to fix this issue.

The root cause is that mdadm depend on finalrd, and finalrd will switch rootfs to clean disk, but it couldn't umount /oldroot, cause it's used by plymouth.

[Bin Li (binli)]

In systemd/mdadm.shutdown from mdadm, it also do "mdadm --wait-clean --scan". But in ubuntu we do not install mdadm-shutdown.service, and do ship finalrd hook.

$ cat ./debian/mdadm.finalrd
#!/bin/sh
# SPDX-License-Identifier: GPL-3.0-only
set -e

if [ "$1" = "setup" ]
then
    . /usr/share/initramfs-tools/hook-functions
    copy_exec /sbin/mdadm
    copy_exec /sbin/mdmon

    exit 0
fi

mdmon --all --takeover
mdadm -vv --wait-clean --scan
mdadm -vv --stop --scan

[Bin Li (binli)]

A solution from Daniel.

https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1928200/comments/11