Re: [PATCH 02/13] filelock: add a lm_may_setlease lease_manager callback

On Wed, 2025-10-15 at 09:10 +1100, NeilBrown wrote:
> On Tue, 14 Oct 2025, Jeff Layton wrote:
> > On Tue, 2025-10-14 at 16:34 +1100, NeilBrown wrote:
> > > On Tue, 14 Oct 2025, Jeff Layton wrote:
> > > > The NFSv4.1 protocol adds support for directory delegations, but it
> > > > specifies that if you already have a delegation and try to request a new
> > > > one on the same filehandle, the server must reply that the delegation is
> > > > unavailable.
> > > > 
> > > > Add a new lease manager callback to allow the lease manager (nfsd in
> > > > this case) to impose this extra check when performing a setlease.
> > > > 
> > > > Signed-off-by: Jeff Layton <[email protected]>
> > > > ---
> > > >  fs/locks.c               |  5 +++++
> > > >  include/linux/filelock.h | 14 ++++++++++++++
> > > >  2 files changed, 19 insertions(+)
> > > > 
> > > > diff --git a/fs/locks.c b/fs/locks.c
> > > > index 0b16921fb52e602ea2e0c3de39d9d772af98ba7d..9e366b13674538dbf482ffdeee92fc717733ee20 100644
> > > > --- a/fs/locks.c
> > > > +++ b/fs/locks.c
> > > > @@ -1826,6 +1826,11 @@ generic_add_lease(struct file *filp, int arg, struct file_lease **flp, void **pr
> > > >  			continue;
> > > >  		}
> > > >  
> > > > +		/* Allow the lease manager to veto the setlease */
> > > > +		if (lease->fl_lmops->lm_may_setlease &&
> > > > +		    !lease->fl_lmops->lm_may_setlease(lease, fl))
> > > > +			goto out;
> > > > +
> > > 
> > > I don't see any locking around this.  What if the condition which
> > > triggers a veto happens after this check, and before the lm_change
> > > below?
> > > Should lm_change implement the veto?  Return -EAGAIN?
> > > 
> > > 
> > 
> > The flc_lock is held over this check and any subsequent lease addition.
> > Is that not sufficient?
> 
> Ah - I didn't see that - sorry.
> 
> But I still wonder why ->lm_change cannot do the veto.
> 
> I also wonder if the current code can work.  If that loop finds an
> existing lease with the same file and the same owner the it invokes
> "continue" before the code that you added.
> So unless I'm misunderstanding (again) in the case that you are
> interested in, the new code doesn't run.
> 

I wrote this a couple of years ago and had to go back and refresh my
memory as to why I did it this way...

The "same owner" check doesn't quite work here. The fl_owner points to
the struct nfs4_delegation, not the nfs4_client, so that test will
never return true. That means that lm_change never happens in this
scenario.

One approach to fix this might be to turn that check into a new
"lm_compare" operation that would return true if the delegations were
both owned by the same client. That could have other effects on the
lease handling code that I haven't considered yet however so I'm
hesitant to go that route.

I'm actually leaning now toward dropping this patch and doing this all
inside of nfsd. I think that's possible and that means less complexity
at the VFS layer. I'll take a stab at respinning this part of the
patchset and see if I can make that work instead.

Thanks for the review!
-- 
Jeff Layton <[email protected]>