Re: [PATCH 4/4] ovl: alllow remote upper

From: Amir Goldstein <amir73il@gmail.com>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: Vivek Goyal <vgoyal@redhat.com>,
	Miklos Szeredi <mszeredi@redhat.com>,
	overlayfs <linux-unionfs@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 4/4] ovl: alllow remote upper
Date: Tue, 4 Feb 2020 19:02:05 +0200	[thread overview]
Message-ID: <CAOQ4uxj_pVp9-EN2Gmq9j6G3xozzpK_zQiRO-brx6PZ9VpgD0Q@mail.gmail.com> (raw)
In-Reply-To: <CAJfpegtq4A-m9vOPwUftiotC_Xv6w-dnhCi9=E0t-b1ZPJXPGw@mail.gmail.com>

On Tue, Feb 4, 2020 at 6:17 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> On Tue, Feb 4, 2020 at 3:59 PM Vivek Goyal <vgoyal@redhat.com> wrote:
> >
> > On Fri, Jan 31, 2020 at 12:50:04PM +0100, Miklos Szeredi wrote:
> > > No reason to prevent upper layer being a remote filesystem.  Do the
> > > revalidation in that case, just as we already do for lower layers.
> > >
> > > This lets virtiofs be used as upper layer, which appears to be a real use
> > > case.
> >
> > Hi Miklos,
> >
> > I have couple of very basic questions.
> >
> > - So with this change, we will allow NFS to be upper layer also?
>
> I haven't tested, but I think it will fail on the d_type test.

But we do not fail mount on no d_type support...
Besides, I though you were going to add the RENAME_WHITEOUT
test to avert untested network fs as upper.

>
> > - What does revalidation on lower/upper mean? Does that mean that
> >   lower/upper can now change underneath overlayfs and overlayfs will
> >   cope with it.
>
> No, that's a more complicated thing.  Especially with redirected
> layers (i.e. revalidating a redirect actually means revalidating all
> the path components of that redirect).
>
> > If we still expect underlying layers not to change, then
> >   what's the point of calling ->revalidate().
>
> That's a good question; I guess because that's what the filesystem
> expects.  OTOH, it's probably unnecessary in most cases, since the
> path could come from an open file descriptor, in which case the vfs
> will not do any revalidation on that path.
>

Note that ovl_dentry_revalidate() never returns 0 and therefore, vfs
will never actually redo the lookup, but instead return -ESTALE
to userspace. Right? This makes some sense considering that underlying
layers are not expected to change.

The question is, with virtiofs, can we know that the server/host will not
invalidate entries? And if it does, should it cause a permanent error
in overlayfs or a transient error? If we do not want a permanent error,
then ->revalidate() needs to be called to invalidate the overlay dentry. No?

Thanks,
Amir.