Re: [PATCH v10 4/5] NFSD: Implement NFSD_IO_DIRECT for NFS WRITE

[Mike Snitzer <snitzer@kernel.org>]

On Thu, Nov 06, 2025 at 01:10:17PM -0500, Chuck Lever wrote:
> On 11/6/25 11:48 AM, Mike Snitzer wrote:
> >>  no_dio:
> >>  	/*
> >>  	 * No DIO alignment possible - pack into single non-DIO segment.
> >> -	 * IOCB_DONTCACHE preserves the intent of NFSD_IO_DIRECT.
> >>  	 */
> >> -	if (args->nf->nf_file->f_op->fop_flags & FOP_DONTCACHE)
> >> -		args->flags_buffered |= IOCB_DONTCACHE;
> >> -	nfsd_write_dio_seg_init(&args->segment[0], bvec, nvecs, total,
> >> -				0, total);
> >> -	args->nsegs = 1;
> >> +	nfsd_write_dio_seg_init(&segments[0], bvec, nvecs, total,
> >> +				0, total, iocb);
> >> +	return 1;
> >>  }
> > Selectively pushing the flag twiddling out to nfsd_direct_write()
> > ignores that we don't want to use DONTCACHE for the unaligned
> > prefix/suffix. Chuck and I discussed this a fair bit 1-2 days ago.
> 
> Agreed. I fixed this up after applying Christoph's patch.

OK, here is the incremental I just came up with, but sounds like
you're all set:

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 6fce4f08a4d4..db86b31e0c10 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1272,6 +1272,19 @@ static unsigned long iov_iter_bvec_offset(const struct iov_iter *iter)
 	return (unsigned long)(iter->bvec->bv_offset + iter->iov_offset);
 }
 
+/*
+ * Check if the bvec iterator is aligned for direct I/O.
+ *
+ * bvecs generated from RPC receive buffers are contiguous: After the first
+ * bvec, all subsequent bvecs start at bv_offset zero (page-aligned).
+ * Therefore, only the first bvec is checked.
+ */
+static bool
+nfsd_iov_iter_aligned_bvec(const struct iov_iter *iter, unsigned int addr_mask)
+{
+	return (iov_iter_bvec_offset(iter) & addr_mask) == 0;
+}
+
 static void
 nfsd_write_dio_seg_init(struct nfsd_write_dio_seg *segment,
 			struct bio_vec *bvec, unsigned int nvecs,
@@ -1324,16 +1337,8 @@ nfsd_write_dio_iters_init(struct nfsd_file *nf, struct bio_vec *bvec,
 
 	nfsd_write_dio_seg_init(&segments[nsegs], bvec, nvecs,
 				total, prefix, middle, iocb);
-
-	/*
-	 * Check if the bvec iterator is aligned for direct I/O.
-	 *
-	 * bvecs generated from RPC receive buffers are contiguous: After the
-	 * first bvec, all subsequent bvecs start at bv_offset zero
-	 * (page-aligned).
-	 * Therefore, only the first bvec is checked.
-	 */
-	if (iov_iter_bvec_offset(&segments[nsegs].iter) & (mem_align - 1))
+	if (!nfsd_iov_iter_aligned_bvec(&segments[nsegs].iter,
+					(mem_align - 1)))
 		goto no_dio;
 	segments[nsegs].flags |= IOCB_DIRECT;
 	nsegs++;
@@ -1348,9 +1353,12 @@ nfsd_write_dio_iters_init(struct nfsd_file *nf, struct bio_vec *bvec,
 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);
+	if (nf->nf_file->f_op->fop_flags & FOP_DONTCACHE)
+		segments[0].flags |= IOCB_DONTCACHE;
 	return 1;
 }
 
@@ -1370,16 +1378,9 @@ nfsd_direct_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	*cnt = 0;
 	for (i = 0; i < nsegs; i++) {
 		kiocb->ki_flags = segments[i].flags;
-		if (kiocb->ki_flags & IOCB_DIRECT) {
+		if (kiocb->ki_flags & IOCB_DIRECT)
 			trace_nfsd_write_direct(rqstp, fhp, kiocb->ki_pos,
 						segments[i].iter.count);
-		} else if (nf->nf_file->f_op->fop_flags & FOP_DONTCACHE) {
-			/*
-			 * IOCB_DONTCACHE preserves the intent of
-			 * NFSD_IO_DIRECT.
-			 */
-			kiocb->ki_flags |= IOCB_DONTCACHE;
-		}
 
 		host_err = vfs_iocb_iter_write(file, kiocb, &segments[i].iter);
 		if (host_err < 0)