[PATCH v4 0/5] NFSD: add "NFSD DIRECT" and "NFSD DONTCACHE" IO modes

Mike Snitzer
2025-07-23T15:43:53Z

Hi,

Some workloads benefit from NFSD avoiding the page cache, particularly
those with a working set that is significantly larger than available
system memory. This patchset introduces _optional_ support to
configure the use of O_DIRECT or DONTCACHE for NFSD's READ and WRITE
support. The NFSD default to use page cache is left unchanged.

The performance win associated with using NFSD DIRECT was previously
summarized here:
https://lore.kernel.org/linux-nfs/aEslwqa9iMeZjjlV@kernel.org/
This picture offers a nice summary of performance gains:
https://original.art/NFSD_direct_vs_buffered_IO.jpg

Very minor changes for this v4.

Thanks,
Mike

Changes since v3:
- Fix a copy-n-paste bug in nfsd_vfs_write:
s/nf_dio_read_offset_align/nf_dio_offset_align/
- Rename struct nfsd_dio_io to nfsd_read_dio.

Changes since v2:
- explored suggestion to use string based interface (e.g. "direct"
instead of 3) but debugfs seems to only supports numeric values.
- shifted numeric values for debugfs interface from 0-2 to 1-3 and
made 0 UNSPECIFIED (which is the default)
- if user specifies io_cache_read or io_cache_write mode other than 1,
2 or 3 (via debugfs) they will get an error message
- pass a data structure to nfsd_analyze_read_dio rather than so many
in/out params
- improved comments as requested (e.g. "Must remove first
start_extra_page from rqstp->rq_bvec" was reworked)
- use memmove instead of opencoded shift in
nfsd_complete_misaligned_read_dio
- dropped the still very important "lib/iov_iter: remove piecewise
bvec length checking in iov_iter_aligned_bvec" patch because it
needs to be handled separately.
- various other changes to improve code

Mike Snitzer (5):
NFSD: filecache: add STATX_DIOALIGN and STATX_DIO_READ_ALIGN support
NFSD: pass nfsd_file to nfsd_iter_read()
NFSD: add io_cache_read controls to debugfs interface
NFSD: add io_cache_write controls to debugfs interface
NFSD: issue READs using O_DIRECT even if IO is misaligned

fs/nfsd/debugfs.c | 102 +++++++++++++++++++
fs/nfsd/filecache.c | 32 ++++++
fs/nfsd/filecache.h | 4 +
fs/nfsd/nfs4xdr.c | 8 +-
fs/nfsd/nfsd.h | 10 ++
fs/nfsd/nfsfh.c | 4 +
fs/nfsd/trace.h | 37 +++++++
fs/nfsd/vfs.c | 197 ++++++++++++++++++++++++++++++++++---
fs/nfsd/vfs.h | 2 +-
include/linux/sunrpc/svc.h | 5 +-
10 files changed, 383 insertions(+), 18 deletions(-)

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2.44.0