Re: overlayfs does not pin underlying layers

[Miklos Szeredi <miklos@szeredi.hu>]

On Wed, Dec 4, 2019 at 6:05 PM Fabian Vogt <fvogt@suse.de> wrote:
>
> Hi,
>
> Am Dienstag, 3. Dezember 2019, 15:19:28 CET schrieb Miklos Szeredi:
> > On Tue, Dec 3, 2019 at 2:49 PM Fabian Vogt <fvogt@suse.de> wrote:
> > >
> > > Hi,
> > >
> > > I noticed that you can still unmount the lower/upper/work layers, even if
> > > they're currently part of an active overlay mount. This is the case even when
> > > files in the overlay mount are currently open. After unmounting, the usual
> > > effects of a lazy umount can be observed, like still active loop devices.
> > >
> > > Is this intentional?
> >
> > It's a known feature.  Not sure how much thought was given to this,
> > but nobody took notice up till now.
> >
> > Do you have a good reason for wanting the underlying mounts pinned, or
> > you are just surprised by this behavior?  In the latter case we can
> > just add a paragraph to the documentation and be done with it.
>
> Both. It's obviously very inconsistent that it's possible to unmount something
> which you still have unrestricted access to.
>
> The specific issue we're facing here is system shutdown - if there's an active
> overlayfs mount, it's not guaranteed that the unmounts happen in the right
> order. Currently we work around that by adding the systemd specific
> "x-systemd.requires-mounts-for=foo-lower.mount" option in /etc/fstab.
> If for some reason the order is wrong, this behaviour of overlayfs might lead
> to the system shutting down without the actual unmount happening properly,
> as it's equivalent to "umount -l" on lower/upper FSs.
> I'm not sure whether there's a scenario in which this could even lead to data
> loss if something relies on umount succeeding to mean that the attached device
> is unused.

IDGI: what is the right order?  Why would it lead to corruption if the
shutdown of the underlying fs is delayed until the shutdown of
overlayfs?

Thanks,
Miklos