[Virtio-fs] [fuse-devel] 'FORGET' ordering semantics (vs unlink & NFS)

[Miklos Szeredi]

On Mon, Jan 4, 2021 at 7:57 PM Dr. David Alan Gilbert
<dgilbert at redhat.com> wrote:
>
> * Vivek Goyal (vgoyal at redhat.com) wrote:
> > On Mon, Jan 04, 2021 at 04:00:13PM +0000, Dr. David Alan Gilbert wrote:
> > > Hi,
> > >   On virtio-fs we're hitting a problem with NFS, where
> > > unlinking a file in a directory and then rmdir'ing that
> > > directory fails complaining about the directory not being empty.
> > >
> > > The problem here is that if a file has an open fd, NFS doesn't
> > > actually delete the file on unlink, it just renames it to
> > > a hidden file (e.g. .nfs*******).  That open file is there because
> > > the 'FORGET' hasn't completed yet by the time the rmdir is issued.
> > >
> > > Question:
> > >   a) In the FUSE protocol, are requests assumed to complete in order;
> > > i.e.  unlink, forget, rmdir   is it required that 'forget' completes
> > > before the rmdir is processed?
> > >      (In virtiofs we've been processing requests, in parallel, and
> > > have sent forgets down a separate queue to keep them out of the way).
> > >
> > >   b) 'forget' doesn't send a reply - so the kernel can't wait for the
> > > client to have finished it;  do we need a synchronous forget here?
> >
> > Even if we introduce a synchronous forget, will that really fix the
> > issue. For example, this could also happen if file has been unlinked
> > but it is still open and directory is being removed.
> >
> > fd = open(foo/bar.txt)
> > unlink foo/bar.txt
> > rmdir foo
> > close(fd).
> >
> > In this case, final forget should go after fd has been closed. Its
> > not a forget race.
> >
> > I wrote a test case for this and it works on regular file systems.
> >
> > https://github.com/rhvgoyal/misc/blob/master/virtiofs-tests/rmdir.c
> >
> > I suspect it will fail on nfs because I am assuming that temporary
> > file will be there till final close(fd) happens. If that's the
> > case this is a NFS specific issue because its behavior is different
> > from other file systems.
>
> That's true; but that's NFS just being NFS; in our case we're keeping
> an fd open even though the guest has been smart enough not to; so we're
> causing the NFS oddity when it wouldn't normally happen.

Can't think of anything better than a synchronous forget.   Compile
only tested patch attached.

Thanks,
Miklos
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