[PATCH v7 0/5] mm, iomap, NFSD: stop uncached writes reading back shared pages

[Mike Snitzer <snitzer@hammerspace.com>]

Hi,

Please consider this v7 for the v7.5 development cycle, so that it can
go upstream in the v7.5 merge window.

An uncached (RWF_DONTCACHE) buffered write drops each page it writes
once that page is written back.  A page the write covers only partly is
shared with the neighbouring write, and once the first of the two has
been written back the second has to read it back from disk before it
can write its part.  32 threads writing interleaved records to one file
on XFS (emulated 4Kn NVMe, 4 KiB pages), before and after 3/5:

  record  pwritev2() flags          4 KiB device       MiB/s
  bytes                             reads per write
                                    before   after     before  after
  47008   RWF_DONTCACHE             0.97     0.000     831     2281
  47008   RWF_DONTCACHE|RWF_DSYNC   0.99     0.000     601     1227
  6000    RWF_DONTCACHE             0.97     0.000     121      488
  6000    RWF_DONTCACHE|RWF_DSYNC   0.99     0.000      90      206

3/5 has the page cache keep such a page for the write that completes
it.  4/5 lets a writer that syncs its own write, as NFSD does for a
stable WRITE, complete those pages itself once the sync is done.  5/5
is the NFSD change that uses it.  1/5 reports the DONTCACHE dirty count
in the bdi debugfs stats, and 2/5 adds folio_mark_dropbehind(), which
3/5 and 4/5 use.

The prerequisite NFSD changes are staged in cel/nfsd-testing, through
ad42983b887e ("NFSD: add tracing for how direct-mode READ and WRITE
are serviced"), which was v6's 1/6.  This series applies on top of it,
and each patch builds with W=1 with no warnings.

1/5 to 4/5 are mm, with small iomap changes in 3/5 and 4/5, and are Cc'd
to linux-mm and linux-fsdevel.  If the mm and iomap maintainers are
agreeable, their acks would let Chuck take the series through the nfsd
tree.

Changes since v6:
- v6 1/6 dropped: Chuck applied it to cel/nfsd-testing as ad42983b887e.
- 1/5 to 4/5 (v6 2/6 to 5/6) are unchanged; 5/5 (v6 6/6) only differs
  in context, from its rebase.

v6: https://lore.kernel.org/linux-nfs/atbfEbgMU6UGkn2X@kernel.org/
v5: https://lore.kernel.org/linux-nfs/20261019150725.1507-1-snitzer@kernel.org/
v4: https://lore.kernel.org/linux-nfs/20261015233309.42999-1-snitzer@kernel.org/
v3: https://lore.kernel.org/linux-nfs/20261001045502.48381-1-snitzer@kernel.org/
v2: https://lore.kernel.org/linux-nfs/20260929231329.22018-1-snitzer@kernel.org/
v1: https://lore.kernel.org/linux-nfs/20260929173423.16149-1-snitzer@kernel.org/

Thanks,
Mike

Mike Snitzer (5):
  mm: expose WB_DONTCACHE_DIRTY in bdi debugfs stats
  mm: add folio_mark_dropbehind() to mark a folio dropbehind and account it
  mm, iomap: keep a partly covered page until an uncached write completes it
  mm, iomap: let an uncached writer complete its shared pages itself
  NFSD: drop a shared boundary page only once both WRITEs have written it

 .../filesystems/nfs/nfsd-io-modes.rst         |  17 +++
 fs/iomap/buffered-io.c                        |  16 ++-
 fs/iomap/trace.h                              |   3 +-
 fs/nfsd/vfs.c                                 |  99 ++++++++++++++--
 include/linux/fs.h                            |   8 +-
 include/linux/iomap.h                         |   1 +
 include/linux/pagemap.h                       |  12 ++
 mm/backing-dev.c                              |  11 ++
 mm/filemap.c                                  | 106 +++++++++++++++++-
 9 files changed, 257 insertions(+), 16 deletions(-)

-- 
2.52.0