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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Thu, 2 Apr 2020 19:20:26 +0200
Message-ID: <CAJfpeguM__+S6DiD4MWFv5GCf_EUWvGFT0mzuUCCrfQwggqtDQ@mail.gmail.com>
Subject: Re: Upcoming: Notifications, FS notifications and fsinfo()
To:     Lennart Poettering <mzxreary@0pointer.de>
Cc:     Ian Kent <raven@themaw.net>, David Howells <dhowells@redhat.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>, dray@redhat.com,
        Karel Zak <kzak@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Jeff Layton <jlayton@redhat.com>, andres@anarazel.de,
        keyrings@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, Aleksa Sarai <cyphar@cyphar.com>
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On Thu, Apr 2, 2020 at 5:50 PM Lennart Poettering <mzxreary@0pointer.de> wrote:
>
> On Do, 02.04.20 17:35, Miklos Szeredi (miklos@szeredi.hu) wrote:
>
> > > systemd cares about all mount points in PID1's mount namespace.
> > >
> > > The fact that mount tables can grow large is why we want something
> > > better than constantly reparsing the whole /proc/self/mountinfo. But
> > > filtering subsets of that is something we don't really care about.
> >
> > I can accept that, but you haven't given a reason why that's so.
> >
> > What does it do with the fact that an automount point was crossed, for
> > example?  How does that affect the operation of systemd?
>
> We don't care how a mount point came to be. If it's autofs or
> something else, we don't care. We don't access these mount points
> ourselves ever, we just watch their existance.
>
> I mean, it's not just about startup it's also about shutdown. At
> shutdown we need to unmount everything from the leaves towards the
> root so that all file systems are in a clean state.

Unfortunately that's not guaranteed by umounting all filesystems from
the init namespace.  A filesystem is shut down when all references to
it are gone.  Perhaps you instead want to lazy unmount root (yeah,
that may not actually be allowed, but anyway, lazy unmounting the top
level ones should do) and watch for super block shutdown events
instead.

Does that make any sense?

Thanks,
Miklos