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

[Dave Chinner <david@fromorbit.com>]

On Thu, Jan 15, 2026 at 02:37:09PM -0500, Chuck Lever wrote:
> On 1/15/26 2:14 PM, Amir Goldstein wrote:
> > On Thu, Jan 15, 2026 at 7:32 PM Chuck Lever <cel@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@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

<pedantic mode engaged>

File handles most definitely change over the life of a /physical/
inode. Unlinking a file does not require ending the life of the
physical object that provides the persistent data store for the
file.

e.g. XFS dynamically allocates physical inodes might in a life cycle
that looks somewhat life this:

	allocate physical inode
	insert record into allocated inode index
	mark inode as free

	while (don't need to free physical inode) {
		...
		allocate inode for a new file
		update persistent inode metadata to generate new filehandle
		mark inode in use
		...
		unlink file
		mark inode free
	}

	remove inode from allocated inode index
	free physical inode

i.e. a free inode is still an -allocated, indexed inode- in the
filesystem, and until we physically remove it from the filesystem
the inode life cycle has not ended.

IOWs, the physical (persistent) inode lifetime can span the lifetime
of -many- files. However, the filesystem guarantees that the handle
generated for that inode is different for each file it represents
over the whole inode life time.

Hence I think that file handle stability/persistence needs to be
defined in terms of -file lifetimes-, not the lifetimes of the
filesystem objects implement the file's persistent data store.

-Dave.
-- 
Dave Chinner
david@fromorbit.com