[PATCH 00/10] NFSD: keep direct-mode I/O out of the page cache and elide COMMITs

[Mike Snitzer <snitzer@hammerspace.com>]

Hi,

This series builds on NFSD_IO_DIRECT.  When a READ or WRITE in a direct
mode cannot be direct I/O it now falls back to DONTCACHE rather than to
cached buffered I/O, a direct-mode WRITE can report the stability it
actually has so the client skips its COMMIT, and new tracepoints show
which path each request took.

Patch 1 interlocks io_cache_read and io_cache_write so READ and WRITE
are never left on opposite sides of the buffered/direct divide.

Patches 2-4 cover what cannot or should not be direct: the direct
middle of a split WRITE is also marked IOCB_DONTCACHE, for when XFS
falls back to buffered I/O (-ENOTBLK); a WRITE is split only when that
buys a worthwhile direct middle (direct_misaligned_num_pages, default
2); and a READ smaller than its alignment no longer costs a full
aligned device read.

Patch 5 is Chuck's "Enable return of an updated stable_how to NFS
clients", reworked onto the @iocb_flags argument nfsd_write() now takes
in nfsd-next.  The Reviewed-by tags from its first posting are dropped
because the argument changed.

Patch 6 adds io_cache_write modes 3 and 4, which issue direct I/O like
NFSD_IO_DIRECT and raise the reply's stable_how to at least DATA_SYNC
or FILE_SYNC.  On a pNFS flexfiles share where every write is split
across two data servers, the COMMITs alone cost NFSD_IO_DIRECT 31% more
server CPU and 42% more client CPU for the same bytes.

Patch 7 persists a synchronous direct-mode WRITE once, after all of its
segments, instead of up to three fsyncs per WRITE.

Patch 8 keeps the page that two misaligned WRITEs share in the page
cache until both have written it, so the second one no longer has to
read it back from disk: with 32 interleaved writers, 704 device reads
for 42895 WRITEs where there were 45144 for 45664.

Patch 9 (Jonathan) adds the direct_misaligned_dontcache debugfs knob,
default Y; set to N, the parts of a direct-mode WRITE that cannot be
direct use cached buffered I/O instead of DONTCACHE.

Patch 10 adds tracepoints for how each direct-mode READ and WRITE was
serviced, including why a WRITE was not direct.

Documentation/filesystems/nfs/nfsd-io-modes.rst is updated throughout.
The series applies to cel/nfsd-next (ac04dab23b5f) and each patch
builds cleanly with W=1.

All review appreciated, thanks.
Mike

Chuck Lever (1):
  NFSD: Enable return of an updated stable_how to NFS clients

Jonathan Flynn (1):
  NFSD: add direct_misaligned_dontcache debugfs knob

Mike Snitzer (8):
  NFSD: interlock the use of NFSD_IO_DIRECT for NFS READ and WRITE
  NFSD: mark the direct middle of a split WRITE IOCB_DONTCACHE as well
  NFSD: only split a direct-mode WRITE for a worthwhile direct middle
  NFSD: do not use direct I/O for a READ smaller than its alignment
  NFSD: let a direct-mode WRITE raise stable_how and elide the client's COMMIT
  NFSD: persist a synchronous direct-mode WRITE once, after all of its segments
  NFSD: keep boundary page of a split direct-mode WRITE until both writers complete
  NFSD: add tracing for how direct-mode READ and WRITE are serviced

 .../filesystems/nfs/nfsd-io-modes.rst         | 164 ++++++++-
 fs/nfsd/debugfs.c                             |  94 +++++-
 fs/nfsd/nfs3proc.c                            |  16 +-
 fs/nfsd/nfs4proc.c                            |  15 +-
 fs/nfsd/nfsd.h                                |   4 +
 fs/nfsd/nfsproc.c                             |   3 +-
 fs/nfsd/trace.h                               |  90 +++++
 fs/nfsd/vfs.c                                 | 318 +++++++++++++++---
 fs/nfsd/vfs.h                                 |  26 +-
 fs/nfsd/xdr3.h                                |   2 +-
 10 files changed, 665 insertions(+), 67 deletions(-)

-- 
2.52.0