[systemd-devel] Unmountable mounts and systemd-fsck at .service conflicting with shutdown.target

[Lennart Poettering]

On Do, 05.01.23 14:18, Valentin David (valentin.david at canonical.com) wrote:

> Hello,
>
> In Ubuntu Core, we have some mounts that cannot be unmounted until we have
> switched root.
>
> To simplify, this looks like that:
>
> / mounts a ro loop devices backed by /some/disk/some/path/image.img
> /some/disk mounts a block device (let's say /dev/some-block0p1)
>
> In this case, /some/disk cannot be unmounted.
>
> We do not want to lazily unmount, we cannot get errors if something fails.
> (Unless we had a lazy unmount that would only work when read-only)
>
> We do remount /some/disk read-only on shutdown. And in the shutdown
> intramfs, we unmount /oldroot/some/disk.
>
> However, we get an error message with systemd trying to unmount it. While
> functionally, it does not matter, it is still very problematic to have
> error messages.
>
> Using `DefaultDependencies=no` is not enough. I have tried to be clever and
> add some-disk.mount to shutdown.target.wants so it would not try to unmount
> it. But systemd got confused with conflicts and randomly kills stop jobs
> until there is no conflict.
>
> Debugging it, I have found that this is because some-disk.mount depends on
> systemd-fsck at some\x2dblock0p1.service. And systemd-fsck at .service conflicts
> with shutdown.target.
>
> I wonder if having conflict on shutdown.target really needed. Could we
> remove it? (And also add DefaultDepenencies=no to
> system-systemd\x2dfsck.slice) With this, mounts with DefaultDependencie=no
> do not get unmounted as part of shutdown.target. (They do during
> systemd-shutdown)

hmm, so we generally want system services to go away before
shutdown. This is a very special case though. I wonder if we can just
override systemd-fsck@….service for that specific case?

How are those mounts established? i.e. by which unit is the
systemd-fsck at .service instance pulled in? and how was that configured?
fstab? ubuntu-own code?

Lennart

--
Lennart Poettering, Berlin