Re: [PATCH v6 0/7] NFSD: add "NFSD DIRECT" and "NFSD DONTCACHE" IO modes

[Mike Snitzer <snitzer@kernel.org>]

On Sat, Aug 09, 2025 at 01:02:50AM -0400, Mike Snitzer wrote:
> Hi,
> 
> Some workloads benefit from NFSD avoiding the page cache, particularly
> those with a working set that is significantly larger than available
> system memory.  This patchset introduces _optional_ support to
> configure the use of O_DIRECT or DONTCACHE for NFSD's READ and WRITE
> support.  The NFSD default to use page cache is left unchanged.
> 
> The performance win associated with using NFSD DIRECT was previously
> summarized here:
> https://lore.kernel.org/linux-nfs/aEslwqa9iMeZjjlV@kernel.org/
> This picture offers a nice summary of performance gains:
> https://original.art/NFSD_direct_vs_buffered_IO.jpg
> 
> This series builds on what has been staged in the nfsd-testing branch.
> 
> This code has proven to work well during my testing.  Any suggestions
> for further refinement are welcome.
> 
> Thanks,
> Mike
> 
> Changes since v5:
> - #define NFSD_READ_DIO_MIN_KB (32 << 10)
> - switch to using pre-increment from post-increment.
> - always get DIO alignment when opening regular nfsd_file (dropped
>   patch that only did if NFSD_IO_DIRECT).
> - fixed nfsd_io_cache_{read,write}_set to not set NFSD_IO_UNSPECIFIED
>   if returning -EINVAL due to unrecognized value. 
> - use a switch statement in nfsd_iter_read like nfsd_iter_write
> - Optimize nfsd_iter_read for default being buffered IO, like was done
>   for nfsd_iter_write in v5.

FYI, I've found that in general we _do_ need to verify DIO-alignment
more comprehensively. Which means I've needed to reinstate the use of
a modified iov_iter_is_aligned.

Otherwise the piece-wise checking that Keith suggested, by adding
checks to existing loops and such, results in the underlying
filesystem (XFS) returning -EINVAL due to alignment issues.

So I'm now focusing on correctness and will hopefully be posting v7 of
this patchset by the end of the week.

Mike