Re: s6-linux-init: Actions after unmounting filesystems

[Guillermo <gdiazhartusch@gmail.com>]

El dom., 18 ago. 2019 a las 15:36, Laurent Bercot escribió:
>
> >Simply excluding filesystems doesn't help when the root filesystem is on one of
> >these devices that needs teardown actions after being unmounted. In that case,
> >the only workable solution is to have PID1 pivot_root() to a tmpfs with the
> >teardown/reboot tools in it. That way you can actually fully unmount the former
> >root filesystem.
>
> Are there systems in the real world that actually work like that? That
> need pivoting into a "shutdownramfs" in order to be able to unmount the
> rootfs and perform teardown operations on it? This is doable, of course,
> but sounds too complex and too brittle. You'd need more than a fsck to
> recover after a power loss, for instance.

I know that there are people that have the rootfs on an LVM logical
volume or a LUKS encrypted volume, yes. However, those are specialized
setups. I don't use one myself, I think they are risky, and I don't
know the details, but I think they are handled with a (mandatory)
initramfs. Perhaps someone else knows this better. Our friends from
the systemd project have thought of that as well (the 'shutdownramfs'
would be the initramfs that is kept mounted at /run/initramfs):

* https://www.freedesktop.org/wiki/Software/systemd/InitrdInterface

But I think of this as a separate problem, that maybe also means
revisiting the decision of not having s6-svscan do its finish
procedure. I didn't want to mention all the complicated cases at once
:)

>   Now the fun part for me is to find a way for s6-l-i-umountall to
> leave the proper filesystems mounted. It's not as straightforward as
> it seems: if /dev is a symlink to, say, /mnt/tmpfs/dev, then you want
> to keep /mnt/tmpfs/dev, even if it means you have to keep /mnt/tmpfs.
> But if you have a second dev mount on /usr/local/mychroot/dev, then
> you want to unmount that one, so you can unmount /usr/local. I suppose
> I can count the number of devtmpfs, procfs and sysfs, and only keep
> one of each (the first that was mounted), but I have to think more
> about it to make sure it catches everything.

Why not just leaving all of them alone? s6-linux-init-umountall
already warns about, but then just ignores, umount(2) failures. What
happens if you try to unmount /mnt/tmpfs, and you have a devtmpfs
mounted on /mnt/tmpfs/dev that you have previously skipped?

G.