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

[Christoph Hellwig <hch@infradead.org>]

On Fri, Nov 07, 2025 at 10:34:21AM -0500, Chuck Lever wrote:
> +no_dio:
> +	/*
> +	 * No DIO alignment possible - pack into single non-DIO segment.
> +	 * IOCB_DONTCACHE preserves the intent of NFSD_IO_DIRECT.
> +	 */
> +	nfsd_write_dio_seg_init(&segments[0], bvec, nvecs, total, 0,
> +				total, iocb);

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.  From the other thread it might be because
the test case used to justify it is very unaligned and caching partial
pages is helpful, but if that is indeed the case the right check would
be for writes that are file offset unaligned vs the page or max folio
size and not about being able to do parts of it as direct I/O.

Either way a tweak to suddenly use cached buffered I/O when the mode
asks for direct should have a comment explaining the justification
and explain the rationale instead of rushing it in.

The rest of the patch looks good to me.