Re: [PATCH RFC] NFS: Add some knobs for disabling delegations in sysfs

[Scott Mayhew <smayhew@redhat.com>]

On Mon, 24 Nov 2025, Trond Myklebust wrote:

> Hi Scott,
> 
> On Mon, 2025-11-24 at 19:15 -0500, Scott Mayhew wrote:
> > There's occasionally a need to disable delegations, whether it be due
> > to
> > known bugs or simply to give support staff some breathing room to
> > troubleshoot issues.  Currently the only real method for disabling
> > delegations in Linux NFS is via /proc/sys/fs/leases-enable, which has
> > some major drawbacks in that 1) it's only applicable to knfsd, and 2)
> > it
> > affects all clients using that server.
> > 
> > Technically it's not really possible to disable delegations from the
> > client side since it's ultimately up to the server whether grants a
> > delegation or not, but we can achieve a similar affect in NFSv4.1+ by
> > manipulating the OPEN4_SHARE_ACCESS_WANT* flags.
> > 
> > Rather than proliferating a bunch of new mount options, add some
> > sysfs
> > knobs to allow some of the nfs_server->caps flags related to
> > delegations
> > to be adjusted.
> > 
> 
> Shouldn't we rather be allowing the application to select whether it
> wants to request a delegation or not?
> 
> IOW: while there may or may not be a place for a 'big hammer' solution
> like you propose, should we not rather first try to enable a solution
> in which someone could add a O_DELEGATION or O_NODELEGATION flag to
> open() in order to specify what they want.
> 
> That might also allow someone to add an LD_PRELOAD library to add or
> remove these flags from an existing application's open() calls.

Sure, allowing the application to specify whether it wants a delegation
or not would be better... but with open() being governed by POSIX and
with delegations being an NFS-specific feature, I figured adding open
flags related to delegations wasn't feasible.  I discussed it with Olga,
and she had the same opinion.  Are we mistaken here?

> 
> It might also be useful for the directory delegation functionality that
> Anna and Jeff have been working on...
> 
> Just some food for thought while you're digesting on Thursday :-)...
> 
> -- 
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trondmy@kernel.org, trond.myklebust@hammerspace.com
>