Re: [PATCH v2 0/7] NFS DIRECT: handle misaligned READ and WRITE for LOCALIO

[Mike Snitzer <snitzer@kernel.org>]

On Wed, Jul 23, 2025 at 02:42:56PM -0400, Chuck Lever wrote:
> On 7/23/25 2:40 PM, Mike Snitzer wrote:
> > On Mon, Jul 21, 2025 at 10:49:17PM -0400, Mike Snitzer wrote:
> >> Hi,
> >>
> >> This "NFS DIRECT" series depends on the "NFSD DIRECT" series here:
> >> https://lore.kernel.org/linux-nfs/20250714224216.14329-1-snitzer@kernel.org/
> >> (for the benefit of nfsd_file_dio_alignment patch in this series)
> >>
> >> The first patch was posted as part of a LOCALIO revert series I posted
> >> a week or so ago, thankfully that series isn't needed thanks to Trond
> >> and Neil's efforts.  BUT the first patch is needed, has Reviewed-by
> >> from Jeff and Neil and is marked for stable@.
> >>
> >> The biggest change in v2 is the introduction of O_DIRECT misaligned
> >> READ and WRITE handling for the benefit of LOCALIO. Please see patches
> >> 6 and 7 for more details.
> > 
> > Turns out that these NFS client (fs/nfs/direct.c) changes that focused
> > on benefiting LOCALIO actually also benefit NFSD if/when it is
> > configured to use O_DIRECT [0].
> > 
> > Such that the NFS client's O_DIRECT code will split any misaligned
> > WRITE IO and NFSD will then happily issue the DIO-aligned "middle" of
> > a given misaligned WRITE IO down to the underlying filesystem.
> > 
> > Same goes for READ, NFS client will expand the front of any misaligned
> > READ such that the bulk of the IO becomes DIO-aligned (only the final
> > partial tail page is misaligned).
> > 
> > So with the NFS client changes in this series we actually don't _need_
> > NFSD to have the same type of misaligned IO analysis and handling to
> > expand READs or split WRITEs to be DIO-aligned.  Which reduces work
> > that NFSD needs to do by leaning on the NFS client having the
> > capability.
> 
> I'm not quite following. Does that apply to non-Linux NFS clients too?

No, old Linux clients without these changes or non-Linux clients
wouldn't have the capabilities offered (extending READs, splitting
WRITEs to be DIO-aligned).  The question is: do we care?  Long-term:
probably.

I'd be fine with the NFSD DIRECT's misaligned IO support (READ already
implemented/posted [0], WRITE partially implemented but not posted) to 
be land upstream so that we have the benefit of making the most of
NFSD's O_DIRECT support even if the client doesn't take steps to issue
IO that is DIO-aligned.

Whichever way we go, it is potentially convenient that the less
"involved" NFSD DIRECT patch 5 [0] could be withheld initially.  Let
the NFS client do the lifting for us assessing how well NFSD's
O_DIRECT works and yet have confidence that we can deploy support for
old Linux clients or non-Linux clients in the future by merging that
patch 5 (and NFSD misaligned WRITE support comparable to NFS's WRITE
splitting in this series [1]) in a secondary phase.

Good to have options.

Mike

[0]: https://lore.kernel.org/linux-nfs/20250723154351.59042-6-snitzer@kernel.org/
[1]: https://lore.kernel.org/linux-nfs/20250722024924.49877-8-snitzer@kernel.org/