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

[Trond Myklebust <trondmy@kernel.org>]

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.

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