Re: [PATCH v2] NFSD: Implement NFSD_IO_DIRECT for NFS WRITE

[Chuck Lever <cel@kernel.org>]

On 10/8/25 2:59 PM, Mike Snitzer wrote:
> +
> +static noinline_for_stack int
> +nfsd_direct_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
> +		  struct nfsd_file *nf, loff_t offset, unsigned int nvecs,
> +		  unsigned long *cnt, struct kiocb *kiocb)
> +{
> +	struct nfsd_write_dio write_dio;
> +
> +	/* Any buffered IO issued here will be misaligned, use
> +	 * IOCB_SYNC to ensure it has completed before returning.
> +	 */
> +	kiocb->ki_flags |= IOCB_SYNC;
> +	/* Check if IOCB_DONTCACHE should be used when issuing buffered IO;
> +	 * if so, it will be ignored for any DIO issued here.
> +	 */
> +	if (nf->nf_file->f_op->fop_flags & FOP_DONTCACHE)
> +		kiocb->ki_flags |= IOCB_DONTCACHE;
> +
> +	if (nfsd_is_write_dio_possible(offset, *cnt, nf, &write_dio)) {
> +		trace_nfsd_write_direct(rqstp, fhp, offset, *cnt);
> +		return nfsd_issue_write_dio(rqstp, fhp, nf, offset, nvecs,
> +					    cnt, kiocb, &write_dio);
> +	}
> +
> +	return nfsd_buffered_write(rqstp, nf->nf_file, nvecs, cnt, kiocb);
> +}

Handful of initial comments:

The current NFSv3 code path, and perhaps NFSv4 too, doesn't support
changing a WRITE marked as UNSTABLE to FILE_SYNC. I'm not seeing that
rectified in this patch. I have a patch that enables changing the
"stable_how" setting, but it's on a system at home. I'll post it in
a couple of days and we can build on that.

In nfsd_direct_write we're setting IOCB_SYNC early. When falling back to
BUFFERED_IO, that setting is preserved, and the fallback buffered path
is now always FILE_SYNC. We should discuss whether the fallback in this
case should be always FILE_SYNC or should allow UNSTABLE.

Nit: I'd rather use the normal kernel comment style here, where an
initial "/*" appears on a line by itself.


-- 
Chuck Lever