Re: [RFC PATCH 0/3] Initial NFS client support for RWF_DONTCACHE

[Mike Snitzer <snitzer@kernel.org>]

On Wed, Apr 23, 2025 at 11:30:21AM -0400, Chuck Lever wrote:
> On 4/23/25 11:22 AM, Matthew Wilcox wrote:
> > On Wed, Apr 23, 2025 at 10:38:37AM -0400, Chuck Lever wrote:
> >> On 4/23/25 12:25 AM, trondmy@kernel.org wrote:
> >>> From: Trond Myklebust <trond.myklebust@hammerspace.com>
> >>>
> >>> The following patch set attempts to add support for the RWF_DONTCACHE
> >>> flag in preadv2() and pwritev2() on NFS filesystems.
> >>
> >> Hi Trond-
> >>
> >> "RFC" in the subject field noted.
> >>
> >> The cover letter does not explain why one would want this facility, nor
> >> does it quantify the performance implications.
> >>
> >> I can understand not wanting to cache on an NFS server, but don't you
> >> want to maintain a data cache as close to applications as possible?
> > 
> > If you look at the original work for RWF_DONTCACHE, you'll see this is
> > the application providing the hint that it's doing a streaming access.
> > It's only applied to folios which are created as a result of this
> > access, and other accesses to these folios while the folios are in use
> > clear the flag.  So it's kind of like O_DIRECT access, except that it
> > does go through the page cache so there's none of this funky alignment
> > requirement on the userspace buffers.
> 
> OK, was wondering whether this behavior was opt-in; sounds like it is.
> Thanks for setting me straight.

Yes, its certainly opt-in (requires setting a flag for each use).
Jens added support in fio relatively recently, see:
https://git.kernel.dk/cgit/fio/commit/?id=43c67b9f3a8808274bc1e0a3b7b70c56bb8a007f

Looking ahead relative to NFSD, as you know we've discussed exposing
per-export config controls to enable use of DONTCACHE.  Finer controls
(e.g. only large sequential IO) would be more desirable but I'm not
aware of a simple means to detect such workloads with NFSD.

Could it be that we'd do well to carry through large folio support in
NFSD and expose a configurable threshold that if met or exceeded then
DONTCACHE used?

What is the status of large folio support in NFSD?  Is anyone actively
working on it?

Thanks,
Mike