Re: [PULL 10/12] virtiofsd: Announce sub-mount points

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Max Reitz (mreitz@redhat.com) wrote:
> On 02.11.20 20:56, Dr. David Alan Gilbert (git) wrote:
> > From: Max Reitz <mreitz@redhat.com>
> > 
> > Whenever we encounter a directory with an st_dev or mount ID that
> > differs from that of its parent, we set the FUSE_ATTR_SUBMOUNT flag so
> > the guest can create a submount for it.
> > 
> > We only need to do so in lo_do_lookup().  The following functions return
> > a fuse_attr object:
> > - lo_create(), though fuse_reply_create(): Calls lo_do_lookup().
> > - lo_lookup(), though fuse_reply_entry(): Calls lo_do_lookup().
> > - lo_mknod_symlink(), through fuse_reply_entry(): Calls lo_do_lookup().
> > - lo_link(), through fuse_reply_entry(): Creating a link cannot create a
> >   submount, so there is no need to check for it.
> > - lo_getattr(), through fuse_reply_attr(): Announcing submounts when the
> >   node is first detected (at lookup) is sufficient.  We do not need to
> >   return the submount attribute later.
> > - lo_do_readdir(), through fuse_add_direntry_plus(): Calls
> >   lo_do_lookup().
> > 
> > Make announcing submounts optional, so submounts are only announced to
> > the guest with the announce_submounts option.  Some users may prefer the
> > current behavior, so that the guest learns nothing about the host mount
> > structure.
> > 
> > (announce_submounts is force-disabled when the guest does not present
> > the FUSE_SUBMOUNTS capability, or when there is no statx().)
> > 
> > Signed-off-by: Max Reitz <mreitz@redhat.com>
> > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> > Message-Id: <20201102161859.156603-6-mreitz@redhat.com>
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  tools/virtiofsd/helper.c         |  1 +
> >  tools/virtiofsd/passthrough_ll.c | 22 ++++++++++++++++++++++
> >  2 files changed, 23 insertions(+)
> 
> > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> [...]
> 
> > index 34d107975f..ec1008bceb 100644
> > --- a/tools/virtiofsd/passthrough_ll.c
> > +++ b/tools/virtiofsd/passthrough_ll.c
> 
> [...]
> 
> > @@ -601,6 +604,20 @@ static void lo_init(void *userdata, struct fuse_conn_info *conn)
> 
> [...]
> 
> > +#ifndef CONFIG_STATX
> > +    if (lo->announce_submounts) {
> > +        fuse_log(FUSE_LOG_WARNING, "lo_init: Cannot announce submounts, there "
> > +                 "is no statx()\n");
> > +        lo->announce_submounts = false;
> > +    }
> > +#endif
> 
> When reviewing this series, Miklos noted today that this warning is
> wrong (we can still announce submounts even without statx()), and we
> concluded that we should probably drop it and the
> “lo->announce_submounts = false” assignment (i.e. this whole block).
> 
> I don’t think that needs to stop this pull request, though, we can
> easily fix it on top.

That's OK, send me a follow up patch to fix the warning; that's a minor.

Dave

> Max
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK