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

[David Laight]

On Thu, 15 Jan 2026 16:37:27 -0500
"Chuck Lever" <cel at kernel.org> wrote:

> On Thu, Jan 15, 2026, at 4:09 PM, Dave Chinner wrote:
> > 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 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  
> >
> > <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.  
> 
> Fair enough, "inode" is the wrong term to use here.

Usually there is 'generation number' changes when the inode is used for
a new file.
IIRC the original nfs file handle was the major/minor for the disk partition,
the index into the 'on-disk inode table' (the inode number) and the
'generation number' (but I'm sure the length was a power of 2...).

It's not surprising Unix uses inode number and file handles.
K&R would have used RSM-11/M where 'file directory lookup' was a userspace
operation and the kernel only supported 'open by file handle'.
Although that got lost between there and ntfs.
(Windows IO is definitely based on RSM-11/M though.)

	David