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

[Matthew Wilcox <willy@infradead.org>]

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.