[PATCH 0/4] mm: improve write performance with RWF_DONTCACHE

Jeff Layton
2026-04-01T19:11:17Z

Recently, we've added controls that allow nfsd to use different IO modes
for reads and writes. There are currently 3 different settings for each:

- buffered: traditional buffered reads and writes (this is the default)
- dontcache: set the RWF_DONTCACHE flag on the read or write
- direct: use direct I/O

One of my goals for this half of the year was to do some benchmarking of
these different modes with different workloads to see if we can come
up with some guidance about what should be used and when.

I had Claude cook up a set of benchmarks that used fio's libnfs backend
and started testing the different modes. The initial results weren't
terribly surprising, but one thing that really stood out was how badly
RWF_DONTCACHE performed with write-heavy workloads. This turned out to
be the case on a local xfs with io_uring as well as with nfsd.

The nice thing about these new debugfs controls for nfsd is that it
makes it easy to experiement with different IO modes for nfsd. After
testing several different approaches, I think this patchset represents a
fairly clear improvement. The first two patches alleviate the flush
contention when RWF_DONTCACHE is used with heavy write activity.

The last two patches add the performance benchmarking scripts. I don't
expect us to merge those, but I wanted to include them to make it clear
how this was tested. The results of my testing with all 4 modes
(buffered, direct, patched and unpatched dontcache) along with Claude's
analysis are at the links below:

nfsd results: https://markdownpastebin.com/?id=0eaf694bd54046b584a8572895abcec2
xfs results: https://markdownpastebin.com/?id=96249deb897a401ba32acbce05312dcc

I can also send them inline if people don't want to chase links.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
Jeff Layton (4):
mm: fix IOCB_DONTCACHE write performance with rate-limited writeback
mm: add atomic flush guard for IOCB_DONTCACHE writeback
testing: add nfsd-io-bench NFS server benchmark suite
testing: add dontcache-bench local filesystem benchmark suite

include/linux/fs.h | 7 +-
include/linux/pagemap.h | 1 +
mm/filemap.c | 51 ++
.../dontcache-bench/fio-jobs/lat-reader.fio | 12 +
.../dontcache-bench/fio-jobs/multi-write.fio | 9 +
.../dontcache-bench/fio-jobs/noisy-writer.fio | 12 +
.../testing/dontcache-bench/fio-jobs/rand-read.fio | 13 +
.../dontcache-bench/fio-jobs/rand-write.fio | 13 +
.../testing/dontcache-bench/fio-jobs/seq-read.fio | 13 +
.../testing/dontcache-bench/fio-jobs/seq-write.fio | 13 +
.../dontcache-bench/scripts/parse-results.sh | 238 +++++++++
.../dontcache-bench/scripts/run-benchmarks.sh | 518 ++++++++++++++++++++
.../testing/nfsd-io-bench/fio-jobs/lat-reader.fio | 15 +
.../testing/nfsd-io-bench/fio-jobs/multi-write.fio | 14 +
.../nfsd-io-bench/fio-jobs/noisy-writer.fio | 14 +
tools/testing/nfsd-io-bench/fio-jobs/rand-read.fio | 15 +
.../testing/nfsd-io-bench/fio-jobs/rand-write.fio | 15 +
tools/testing/nfsd-io-bench/fio-jobs/seq-read.fio | 14 +
tools/testing/nfsd-io-bench/fio-jobs/seq-write.fio | 14 +
.../testing/nfsd-io-bench/scripts/parse-results.sh | 238 +++++++++
.../nfsd-io-bench/scripts/run-benchmarks.sh | 543 +++++++++++++++++++++
.../testing/nfsd-io-bench/scripts/setup-server.sh | 94 ++++
22 files changed, 1874 insertions(+), 2 deletions(-)
---
base-commit: 9147566d801602c9e7fc7f85e989735735bf38ba
change-id: 20260401-dontcache-5811efd7eaf3

Best regards,
--
Jeff Layton <jlayton@kernel.org>