[systemd-devel] shutdown umount order

[Dave Reisner]

On Sat, Jan 05, 2013 at 02:22:11AM +0400, Ilya Basin wrote:
> Hi list.
> On shutdown I have
>    Buffer I/O error on device dm-...
> because the truecrypt disk is unmounted later than the usb disk
> containing the truecrypt image.
> I think this happens, because
> 1) systemd can't figure out the correct mount dependencies
> 2) umount is called with the force flag
> ---------
> There are workarounds specially for truecrypt, but why not solve the
> problem entirely?
> 
> 1) I think the mount dependencies can't be made 100% reliable. systemd
> should use the combination of them and /proc/mounts, but only as a HINT
> In my case /proc/mounts contains the right order.

No, /proc/mounts does not contain this information. /proc/self/mountinfo
contains sufficient data to reconstruct the mount hierarchy, but still
falls short when you introduce loop devices -- backing files are not
considered anywhere, as you're seeing. To fix this properly requires
fixing the kernel, not systemd. Feel free to override your own mount
unit and add the extra dependency on the USB disk. Then, systemd will do
the right thing.

> 2) Instead of force umount, systemd should first try to umount
> everything gracefully, and if at least one mount point was unmounted
> successfully, then retry.
> Only if all mounts are busy, systemd should force umount the first
> candidate and then repeat the graceful umount for others.

MNT_FORCE is only effective with NFS mounts. It has no effect otherwise.

Cheers,
Dave