[PATCH 00/29] fs: require filesystems to explicitly opt-in to nfsd export support

[Amir Goldstein]

On Thu, Jan 15, 2026 at 8:37 PM Chuck Lever <cel at kernel.org> wrote:
>
> On 1/15/26 2:14 PM, Amir Goldstein wrote:
> > On Thu, Jan 15, 2026 at 7:32 PM Chuck Lever <cel at kernel.org> wrote:
> >>
> >>
> >>
> >> On Thu, Jan 15, 2026, at 1:17 PM, Amir Goldstein wrote:
> >>> On Thu, Jan 15, 2026 at 6:48 PM Jeff Layton <jlayton at kernel.org> wrote:
> >>>>
> >>>> In recent years, a number of filesystems that can't present stable
> >>>> filehandles have grown struct export_operations. They've mostly done
> >>>> this for local use-cases (enabling open_by_handle_at() and the like).
> >>>> Unfortunately, having export_operations is generally sufficient to make
> >>>> a filesystem be considered exportable via nfsd, but that requires that
> >>>> the server present stable filehandles.
> >>>
> >>> Where does the term "stable file handles" come from? and what does it mean?
> >>> Why not "persistent handles", which is described in NFS and SMB specs?
> >>>
> >>> Not to mention that EXPORT_OP_PERSISTENT_HANDLES was Acked
> >>> by both Christoph and Christian:
> >>>
> >>> https://lore.kernel.org/linux-fsdevel/20260115-rundgang-leihgabe-12018e93c00c@brauner/
> >>>
> >>> Am I missing anything?
> >>
> >> PERSISTENT generally implies that the file handle is saved on
> >> persistent storage. This is not true of tmpfs.
> >
> > That's one way of interpreting "persistent".
> > Another way is "continuing to exist or occur over a prolonged period."
> > which works well for tmpfs that is mounted for a long time.
>
> I think we can be a lot more precise about the guarantee: The file
> handle does not change for the life of the inode it represents. It
> has nothing to do with whether the file system is mounted.
>
>
> > But I am confused, because I went looking for where Jeff said that
> > you suggested stable file handles and this is what I found that you wrote:
> >
> > "tmpfs filehandles align quite well with the traditional definition
> >  of persistent filehandles. tmpfs filehandles live as long as tmpfs files do,
> >  and that is all that is required to be considered "persistent".
>
> I changed my mind about the name, and I let Jeff know that privately
> when he asked me to look at these patches this morning.
>
>
> >> The use of "stable" means that the file handle is stable for
> >> the life of the file. This /is/ true of tmpfs.
> >
> > I can live with STABLE_HANDLES I don't mind as much,
> > I understand what it means, but the definition above is invented,
> > whereas the term persistent handles is well known and well defined.
>
> Another reason not to adopt the same terminology as NFS is that
> someone might come along and implement NFSv4's VOLATILE file
> handles in Linux, and then say "OK, /now/ can we export cgroupfs?"
> And then Linux will be stuck with overloaded terminology and we'll
> still want to say "NO, NFS doesn't support cgroupfs".
>
> Just a random thought.

Good argument. I'm fine with stable as well :)

Thanks,
Amir.