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

NeilBrown
2026-01-19T07:23:08Z

On Mon, 19 Jan 2026, Christoph Hellwig wrote:
> On Mon, Jan 19, 2026 at 10:23:13AM +1100, NeilBrown wrote:
> > > This was Chuck's suggested name. His point was that STABLE means that
> > > the FH's don't change during the lifetime of the file.
> > >
> > > I don't much care about the flag name, so if everyone likes PERSISTENT
> > > better I'll roll with that.
> >
> > I don't like PERSISTENT.
> > I'd rather call a spade a spade.
> >
> > EXPORT_OP_SUPPORTS_NFS_EXPORT
> > or
> > EXPORT_OP_NOT_NFS_COMPATIBLE
> >
> > The issue here is NFS export and indirection doesn't bring any benefits.
>
> No, it absolutely is not. And the whole concept of calling something
> after the initial or main use is a recipe for a mess.

We are calling it for it's only use. If there was ever another use, we
could change the name if that made sense. It is not a public name, it
is easy to change.

>
> Pick a name that conveys what the flag is about, and document those
> semantics well. This flag is about the fact that for a given file,
> as long as that file exists in the file system the handle is stable.
> Both stable and persistent are suitable for that, nfs is everything
> but.

My understanding is that kernfs would not get the flag.
kernfs filehandles do not change as long as the file exist.
But this is not sufficient for the files to be usefully exported.

I suspect kernfs does re-use filehandles relatively soon after the
file/object has been destroyed. Maybe that is the real problem here:
filehandle reuse, not filehandle stability.

Jeff: could you please give details (and preserve them in future cover
letters) of which filesystems are known to have problems and what
exactly those problems are?

>
> Remember nfs also support volatile file handles, and other applications
> might rely on this (I know of quite a few user space applications that
> do, but they are kinda hardwired to xfs anyway).

The NFS protocol supports volatile file handles. knfsd does not.
So maybe
EXPORT_OP_NOT_NFSD_COMPATIBLE
might be better. or EXPORT_OP_NOT_LINUX_NFSD_COMPATIBLE.
(I prefer opt-out rather than opt-in because nfsd export was the
original purpose of export_operations, but it isn't something
I would fight for)

NeilBrown