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

[Trond Myklebust <trondmy@kernel.org>]

On Tue, 2026-01-20 at 15:46 -0500, Olga Kornievskaia wrote:
> On Mon, Nov 24, 2025 at 8:01 PM Trond Myklebust <trondmy@kernel.org>
> 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.
> 
> Hi Trond,
> 
> Shouldn't open flags be something that's a generic concept for all
> filesystems? O_DELEGATION seems to be NFS specific (or at most
> network
> filesystem specific).
> 
> > That might also allow someone to add an LD_PRELOAD library to add
> > or
> > remove these flags from an existing application's open() calls.
> > 
> > It might also be useful for the directory delegation functionality
> > that
> > Anna and Jeff have been working on...
> 
> Being able to control directory delegations feature (might be) useful
> too? We used to be able to toggle features via mount options,
> wouldn't
> it be nice if we could again.... Toggling features via sysfs is
> useful
> for when mount is already in place though.
> > 

Isn't Jeff Layton in the process of exporting the NFSv4 delegation
functionality to userspace through the lease mechanisms?

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trondmy@kernel.org, trond.myklebust@hammerspace.com