Re: [PATCH v11 2/3] NFSD: Implement NFSD_IO_DIRECT for NFS WRITE

[Mike Snitzer <snitzer@kernel.org>]

On Sat, Nov 08, 2025 at 08:58:56AM +1100, NeilBrown wrote:
> On Sat, 08 Nov 2025, Mike Snitzer wrote:
> > On Fri, Nov 07, 2025 at 03:08:11PM -0500, Chuck Lever wrote:
> > > On 11/7/25 3:05 PM, Mike Snitzer wrote:
> > > >> Agreed. The cover letter noted that this is still controversial.
> > > > Only see this, must be missing what you're referring to:
> > > > 
> > > >   Changes since v9:
> > > >   * Unaligned segments no longer use IOCB_DONTCACHE
> > > 
> > > From the v11 cover letter:
> > > 
> > > > One controversy remains: Whether to set DONTCACHE for the unaligned
> > > > segments.
> > 
> > Ha, blind as a bat...
> > 
> > hopefully the rest of my reply helps dispel the controversy
> > 
> 
> Unfortunately I don't think it does.  I don't think it even addresses
> it.
> 
> What Christoph said was:
> 
>    I'd like to sort out the discussion on why to set IOCB_DONTCACHE when
>    nothing is aligned, but not for the non-aligned parts as that is
>    extremely counter-intuitive.
> 
> You gave a lengthy (and probably valid) description on why "not for the
> non-aligned parts" but don't seem to address the "why to set
> IOCB_DONTCACHE when nothing is aligned" bit.

Because if the entire IO is so poorly formed that it cannot be issued
with DIO then at least we can provide the spirit of what was requested
from the admin having cared to enable NFSD_IO_DIRECT.

Those IOs could be quite large.. full WRITE payload.  I have a
reproducer if you'd like it!

Limiting extensive use of caching buffered IO is pretty important if
the admin enabled NFSD_IO_DIRECT.

A misaligned WRITE's non-aligned head and tail are _not_ extensive
amounts of mmeory.  They are at most 2 individial pages.

> I can see Christoph's point.
>
> Can you please explain why all the arguments you have for avoiding
> IOCB_DONTCACHE on the non-aligned head and tail do not equally apply
> when the whole body is assessed as non-aligned.  i.e.  the no_dio label
> in nfsd_write_dio_iters_init().

Sure, hopefully I just did above.