Re: [PATCH v8 0/7] mm, iomap, NFSD: stop uncached writes reading back shared pages

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2026-10-22 at 22:45 -0400, Mike Snitzer wrote:
> Hi,
> 
> Please consider this v8 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 5/7:
> 
>   record  pwritev2() flags          4 KiB device       MiB/s
>   bytes                             reads per write
>                                     before   after     before  after
>   47008   RWF_DONTCACHE             0.97     0.0005    831     2281
>   47008   RWF_DONTCACHE|RWF_DSYNC   0.99     0.0005    601     1227
>   6000    RWF_DONTCACHE             0.97     0.0001    121      488
>   6000    RWF_DONTCACHE|RWF_DSYNC   0.99     0.0002     90      206
> 
> What is left after is metadata: 49152-byte records, which cover only
> whole pages, read 0.0005 per write either way.  The test program and
> script that produced this table, and how to run them, are in:
> https://lore.kernel.org/linux-nfs/atqJBS8MlYfeUU-t@kernel.org/
> 
> 5/7 has the page cache keep such a page for the write that completes
> it.  6/7 lets a writer that syncs its own write, as NFSD does for a
> stable WRITE, complete those pages itself once the sync is done.  7/7
> is the NFSD change that uses it.  3/7 reports the DONTCACHE dirty count
> in the bdi debugfs stats, and 4/7 adds folio_mark_dropbehind(), which
> 5/7 and 6/7 use.
> 
> 1/7 and 2/7 are fixes the review of v7 turned up in code that is
> already queued or upstream, and each stands on its own:
> 
> - 1/7 fixes 80728157d1bd ("NFSD: cut a misaligned direct-mode WRITE at
>   page boundaries") in cel/nfsd-testing: where the mapping's smallest
>   folio is a block larger than a page, NFSD cut a WRITE's misaligned
>   ends inside a block, so the direct middle and a buffered end shared
>   it.  Chuck, maybe best to fold into 80728157d1bd ?
> - 2/7 fixes 88d6f128d06d ("mm: track DONTCACHE dirty pages per
>   bdi_writeback"), in v7.3: a cached lookup that clears the dropbehind
>   flag could take a folio off WB_DONTCACHE_DIRTY that was never counted
>   there, or leave one counted with nothing to uncount it.  It applies
>   to v7.3 as it is, if the mm side would rather take it alone.
> 
> The prerequisite NFSD changes are staged in cel/nfsd-testing; this
> series applies on top of it at 6d13b7f74eec ("NFSD: Map every NFS_ACLv2
> reply status into aclstat2"), and each patch builds with W=1 with no
> warnings.
> 
> 2/7 to 6/7 are mm, with small iomap changes in 5/7 and 6/7, 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.  But whatever approach the maintainers prefer for this kind
> of cross-subsystem merge is obviously fine.
> 
> Changes since v7, from Jeff's review and the sashiko findings he
> relayed:
> - 1/7 and 2/7 are new (above).
> - 3/7 (v7 1/5): a comment dropped; Jeff's Reviewed-by.
> - 4/7 (v7 2/5): folio_mark_dropbehind() counted a folio as DONTCACHE
>   dirty when it found PG_dirty set, but PG_dirty is set before
>   folio_account_dirtied() counts the folio, without the folio lock, so
>   a folio marked in between was counted twice.  It now sets the flag
>   and tests the dirty tag, which is set in the same i_pages section that
>   counts the folio, under the i_pages lock.
> - 5/7 (v7 3/5): the edge is the smallest folio the mapping allows, a
>   page or a block where blocks are larger, both in iomap's test and in
>   what is marked: a larger folio found there, left by readahead or
>   another buffered write, is no longer marked whole.
> - 6/7 (v7 4/5): an edge already clean is dropped before the folio lock
>   is released, not after with a trylock that could miss it; and only
>   an edge-sized folio, as in 5/7.
> - 7/7 (v7 5/5): unchanged.
> 
> Tested on XFS on an emulated 4Kn NVMe, 4 KiB pages, 32 threads of
> interleaved 47008-byte records (12288 bytes where noted), 4 KiB device
> reads per write, v7 against v8:
> 
>                                              v7        v8
>   RWF_DONTCACHE over large folios
>     readahead left                          0.94      0.0026
>   RWF_DONTCACHE, 16 KiB blocks              1.62      0.0014
>     12288-byte records                      2.89      0.0004
>   NFS WRITEs through NFSD DIRECT,
>     16 KiB blocks                           3.46      0.0016
> 
> (v7 in 128 MiB runs, v8 in 1 GiB runs; on 16 KiB blocks one block read
> back per write is 4.0 here.)  In every run of v8, BdiDontcacheDirty
> (3/7) was back at 0 after the final sync, including runs mixing
> uncached writes, cached writes and mmap stores on one file, and every
> record verified.
> 
> v7: https://lore.kernel.org/linux-nfs/20261020220346.28020-1-snitzer@kernel.org/
> 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 (7):
>   NFSD: cut a misaligned direct-mode WRITE at the mapping's smallest folio
>   mm: take a folio off WB_DONTCACHE_DIRTY only if it was counted there
>   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                        |  20 +-
>  fs/iomap/trace.h                              |   3 +-
>  fs/nfsd/vfs.c                                 | 113 ++++++++--
>  include/linux/fs.h                            |   8 +-
>  include/linux/iomap.h                         |   1 +
>  include/linux/pagemap.h                       |  12 ++
>  mm/backing-dev.c                              |   6 +
>  mm/filemap.c                                  | 193 ++++++++++++++++--
>  9 files changed, 338 insertions(+), 35 deletions(-)
> 
> 
> base-commit: 6d13b7f74eec16e08d5e7f772ee119432a14ef2c

It might not hurt to have your LLM simplify the patch descriptions
here. I find that my appetite for reading claude-ish walls of text has
waned in recent weeks. This skill helps a lot:

    https://github.com/danyuchn/asd-ste100-skill

-- 
Jeff Layton <jlayton@kernel.org>