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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Fri, 3 Apr 2020 13:48:51 +0200
Message-ID: <CAJfpegtYKhXB-HNddUeEMKupR5L=RRuydULrvm39eTung0=yRg@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 Fri, Apr 3, 2020 at 1:08 PM Lennart Poettering <mzxreary@0pointer.de> wr=
ote:
>
> On Do, 02.04.20 19:20, Miklos Szeredi (miklos@szeredi.hu) wrote:
>
> > 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 somethin=
g
> > > > > 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?
>
> When all mounts in the init mount namespace are unmounted and all
> remaining processes killed we switch root back to the initrd, so that
> even the root fs can be unmounted, and then we disassemble any backing
> complex storage if there is, i.e. lvm, luks, raid, =E2=80=A6

I think it could be done the other way round, much simpler:

 - switch back to initrd
 - umount root, keeping the tree intact (UMOUNT_DETACHED)
 - kill all remaining processes, wait for all to exit

I think that should guarantee that all super blocks have been shut down.  A=
l?

The advantage would be that there's no need to walk the mount tree
unmounting individual leafs, since it's all done automagically.

Thanks,
Miklos