Re: [PATCH 1/6] NFSD: add the ability to enable use of RWF_DONTCACHE for all IO

Mike Snitzer
2025-06-16T16:10:40Z

On Mon, Jun 16, 2025 at 09:32:16AM -0400, Chuck Lever wrote:
> On 6/12/25 12:00 PM, Mike Snitzer wrote:
> > On Thu, Jun 12, 2025 at 09:21:35AM -0400, Chuck Lever wrote:
> >> On 6/11/25 3:18 PM, Mike Snitzer wrote:
> >>> On Wed, Jun 11, 2025 at 10:31:20AM -0400, Chuck Lever wrote:
> >>>> On 6/10/25 4:57 PM, Mike Snitzer wrote:
> >>>>> Add 'enable-dontcache' to NFSD's debugfs interface so that: Any data
> >>>>> read or written by NFSD will either not be cached (thanks to O_DIRECT)
> >>>>> or will be removed from the page cache upon completion (DONTCACHE).
> >>>>
> >>>> I thought we were going to do two switches: One for reads and one for
> >>>> writes? I could be misremembering.
> >>>
> >>> We did discuss the possibility of doing that. Still can-do if that's
> >>> what you'd prefer.
> >>
> >> For our experimental interface, I think having read and write enablement
> >> as separate settings is wise, so please do that.
> >>
> >> One quibble, though: The name "enable_dontcache" might be directly
> >> meaningful to you, but I think others might find "enable_dont" to be
> >> oxymoronic. And, it ties the setting to a specific kernel technology:
> >> RWF_DONTCACHE.
> >>
> >> So: Can we call these settings "io_cache_read" and "io_cache_write" ?
> >>
> >> They could each carry multiple settings:
> >>
> >> 0: Use page cache
> >> 1: Use RWF_DONTCACHE
> >> 2: Use O_DIRECT
> >>
> >> You can choose to implement any or all of the above three mechanisms.
> >
> > I like it, will do for v2. But will have O_DIRECT=1 and RWF_DONTCACHE=2.
>
> For io_cache_read, either settings 1 and 2 need to set
> disable_splice_read, or the io_cache_read setting has to be considered
> by nfsd_read_splice_ok() when deciding to use nfsd_iter_read() or
> splice read.

Yes, I understand.

> However, it would be slightly nicer if we could decide whether splice
> read can be removed /before/ this series is merged. Can you get NFSD
> tested with IOR with disable_splice_read both enabled and disabled (no
> direct I/O)? Then we can compare the results to ensure that there is no
> negative performance impact for removing the splice read code.

I can ask if we have a small window of opportunity to get this tested,
will let you know if so.