[systemd-devel] prevent unmount of single partitions on shutdown

[Tom Gundersen]

On Wed, Aug 21, 2013 at 5:56 PM, Colin Guthrie <gmane at colin.guthr.ie> wrote:
> 'Twas brillig, and Zbigniew Jędrzejewski-Szmek at 21/08/13 04:38 did
> gyre and gimble:
>> On Wed, Aug 21, 2013 at 05:21:59AM +0200, Stephan Raue wrote:
>>> Hi,
>>>
>>> i try to port systemd on a own embedded OS which is stored in a
>>> squashfs file. This file is on a fat partition (later mounted as
>>> /flash) on the drive.
>>>
>>> In our own initramfs (which dont uses systemd) /flash will be
>>> mounted and then the Squashfs file as /sysroot. later we do a
>>> switch_root and start systemd. On shutdown systemd trys now (5
>>> times) to unmount /flash and times out later with a error message.
>>> This delays the shutdown/reboot much. Systemd also trys to cleanup
>>> /dev/loop0. I need to prevent systemd to unmount /flash and clean
>>> /dev/loop0 (which is the / mount from the squashfs file).
>>>
>>> can i actually prevent this in some way and if nout could i request
>>> a feature to add a mount option which if avaible prevents systemd
>>> from unmounting single partitions and cleanup /dev/loopX if its
>>> still mounted as / ? I need this as mount option or a systemd unit
>>> file but for fstab based systems it would be usefull as a fstab
>>> option too (we dont use fstab)
>> I don't think it's possible currently with fstab. But with
>> a mount unit, I think
>>   DefaultDependencies=no
>>   RequiredBy=-.mount
>> should work. Have you tried something like that?
>
> Hmm, I thought the umount logic was such that it just tries to unmount
> everything from/proc/mounts rather than looking at units etc.
>
> I asked a similar question a while back and we figured at the time that
> adding a mount option might be a solution as there is already a fstab
> option to indicate a given filesystem should be mounted in the initrd,
> and it would make sense to honor that during shutdown (by not trying to
> umount it), but the problem was the umount loop doesn't read fstab or
> any mount options to check...

I have also been thinking of this problem recently. I have some
patches to not add a conflict with umount.target when a mount is
marked with x-initrd.mount, but as you point out we need to do
something to also cover the final umount loop. One option is to simply
jump to the shutdown ramfs straight away if it exists and let that do
all the final unmounting/killing, which should always work.

-t