[PATCH v2 01/31] Documentation: document EXPORT_OP_NOLOCKS

[NeilBrown]

On Wed, 21 Jan 2026, Jeff Layton wrote:
> On Wed, 2026-01-21 at 20:58 +1100, NeilBrown wrote:
> > On Wed, 21 Jan 2026, Jeff Layton wrote:
> > > On Tue, 2026-01-20 at 09:12 -0500, Jeff Layton wrote:
> > > > On Tue, 2026-01-20 at 08:20 -0500, Jeff Layton wrote:
> > > > > On Mon, 2026-01-19 at 23:44 -0800, Christoph Hellwig wrote:
> > > > > > On Mon, Jan 19, 2026 at 11:26:18AM -0500, Jeff Layton wrote:
> > > > > > > +  EXPORT_OP_NOLOCKS - Disable file locking on this filesystem. Some
> > > > > > > +    filesystems cannot properly support file locking as implemented by
> > > > > > > +    nfsd. A case in point is reexport of NFS itself, which can't be done
> > > > > > > +    safely without coordinating the grace period handling. Other clustered
> > > > > > > +    and networked filesystems can be problematic here as well.
> > > > > > 
> > > > > > I'm not sure this is very useful.  It really needs to document what
> > > > > > locking semantics nfs expects, because otherwise no reader will know
> > > > > > if they set this or not.
> > > > > 
> > > > > Fair point. I'll see if I can draft something better. Suggestions
> > > > > welcome.
> > > > 
> > > > How about this?
> > > > 
> > > > +  EXPORT_OP_NOLOCKS - Disable file locking on this filesystem. Filesystems
> > > > +    that want to support locking over NFS must support POSIX file locking
> > > > +    semantics and must handle lock recovery requests from clients after a
> > > > +    reboot. Most local disk, RAM, or pseudo-filesystems use the generic POSIX
> > > > +    locking support in the kernel and naturally provide this capability. Network
> > > > +    or clustered filesystems usually need special handling to do this properly.
> > > 
> > > Even better, I think?
> > > 
> > > +
> > > +  EXPORT_OP_NOLOCKS - Disable file locking on this filesystem. Filesystems
> > > +    that want to support locking over NFS must support POSIX file locking
> > > +    semantics. When the server reboots, the clients will issue requests to
> > > +    recover their locks, which nfsd will issue to the filesystem as new lock
> > > +    requests. Those must succeed in order for lock recovery to work. Most
> > > +    local disk, RAM, or pseudo-filesystems use the generic POSIX locking
> > > +    support in the kernel and naturally provide this capability. Network or
> > > +    clustered filesystems usually need special handling to do this properly.
> > > +    Set this flag on filesystems that can't guarantee the proper semantics
> > > +    (e.g. reexported NFS).
> > 
> > I think this is quite thorough, which it good ...  maybe too good :-) It
> > reminds me that for true NFS compatibility the fs shouldn't allow local
> > locks (or file opens!) until the grace period has passed.  I don't think
> > any local filesystems enforce that - it would have to be locks.c that
> > does I expect.  I doubt there would be much appetite for doing that
> > though.
> > 
> 
> Yeah, I don't see us ever doing that. It'd be a tricky chicken-and-egg
> problem, given the demand-driven way that the mountd upcalls work
> today. We don't even know that anything is exported until something
> asks for it.

statd keeps state in /var/lib/nfs/sm, and nfsd keeps v4 state elsewhere
in /var/lib/nfs.  This state effectively records if any NFS client might
try to recover a lock.
I think the v4 state is granular enough to identify the filesystem.
lockd could be enhanced to use the same state I suspect.

We would need to generalise that state and load it at mount time and
block new state creation accordingly.

i.e. this would have to be a vfs-level thing which nfsd makes use of.

Possibly, but there are other things better worth our time.

NeilBrown