Re: [PATCH] mm/filemap: let sequential faults restart mmap readahead

[Gopal Malaviya]

Hi Alireza,

I had sent a different fix for the same regression [1], reported by
Trace in [2]. Jan prefers yours, so I have dropped mine. I ran your
patch, also with the v2 change you describe to Jan, through the tests
I had: a KVM guest (x86_64, 2 vCPUs, 3 GiB), mm-hotfixes-unstable as
of 2026-11-08, ext4 and XFS with 16 KiB blocks on virtio-blk,
read_ahead_kb 128 unless noted.

The reproducer from [2] (2000 single-page faults 1 MiB apart, then a
64 MiB sequential read), major faults in the sequential part:

                       base   as posted   with v2 change
  ext4                16384           2                2
  XFS, 16 KiB blocks   4096        4096                2

Touching 1% of the pages of a cold 20 GiB file, larger than memory, in
random runs of 1, 2 or 4 adjacent pages, MiB read from disk with the
v2 change (first of two runs; the second was within 10%):

                       read_ahead_kb 128     read_ahead_kb 8192
                        base   with v2        base   with v2
  random                 259       234        1057      1070
  stride 2053 pages      271       236         992       992
  random 2-page runs     257       373        1135      2538
  random 4-page runs     240      1146        1200     57748

The 4-page runs at read_ahead_kb 8192, which is what this guest's
virtio-blk disk gets by default, read about 48 times more. As far as I
can tell, each run now has two misses and up to two hits on the new
window, so mmap_miss stays around MMAP_LOTSAMISS, and each time it is
back at the limit the next random miss gets the full 8 MiB
read-around. My patch ran into the same and kept mmap_miss at its cap
on that path; something like that may help here, but you and Jan know
this better.

On the Fixes tag: with the commits of the series built one by one,
commit 0b9c0aeba938 ("mm/filemap: count only the faulting address as
a mmap hit") alone does not show the problem (1 major fault with the
reproducer), and commit 9b0fcac3cfe7 ("mm/filemap: do not count
FAULT_FLAG_TRIED retries as mmap hits") does (16384). [2] is the same
regression, in case you want to add Reported-by and Closes tags for
regzbot.

I am happy to run these again on your v2. The tests were set up with
the help of an AI assistant.

[1] https://lore.kernel.org/all/20261109091444.128521-1-gopalmalaviya53@xxxxxxxxx/
[2] https://lore.kernel.org/all/20261105192517.228523-1-tracekotae@xxxxxxxxx/

Thanks,
Gopal