[PATCH v3 1/3] NFSD: avoid DONTCACHE for misaligned ends of misaligned DIO WRITE

[Mike Snitzer <snitzer@kernel.org>]

NFSD_IO_DIRECT can easily improve streaming misaligned WRITE
performance if it uses buffered IO (without DONTCACHE) for the
misaligned end segment(s) and O_DIRECT for the aligned middle
segment's IO.

On one capable testbed, this commit improved streaming 47008 byte
write performance from 0.3433 GB/s to 1.26 GB/s.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
---
 fs/nfsd/vfs.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

v3: drop unrelated change to avoid DONTCACHE if READ is less than 32K

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 701dd261c252..075d7162eb2e 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1347,6 +1347,14 @@ nfsd_write_dio_iters_init(struct bio_vec *bvec, unsigned int nvecs,
 		++args->nsegs;
 	}
 
+	/*
+	 * Don't use IOCB_DONTCACHE if misaligned DIO (args->nsegs > 1),
+	 * because IO for misaligned segments can benefit from the page
+	 * cache (e.g. when handling streaming misaligned IO).
+	 */
+	if (args->nsegs > 1 && (args->flags_buffered & IOCB_DONTCACHE))
+		args->flags_buffered &= ~IOCB_DONTCACHE;
+
 	return;
 
 no_dio:
@@ -1400,7 +1408,7 @@ nfsd_direct_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
 
 	/*
 	 * IOCB_DONTCACHE preserves the intent of NFSD_IO_DIRECT when
-	 * writing unaligned segments or handling fallback I/O.
+	 * falling back to buffered IO if entire WRITE is unaligned.
 	 */
 	args.flags_buffered = kiocb->ki_flags;
 	if (args.nf->nf_file->f_op->fop_flags & FOP_DONTCACHE)
-- 
2.44.0