mm: vmscan: apply proportional reclaim pressure for memcg when MGLRU is enabled

mm: vmscan: apply proportional reclaim pressure for memcg when MGLRU is enabled

The scan implementation for MGLRU was missing proportional reclaim
pressure for memcg, which contradicts the description in
Documentation/admin-guide/cgroup-v2.rst (memory.{low,min} section).

This issue can be observed in kselftest cgroup:test_memcontrol
(specifically test_memcg_min and test_memcg_low).  The following table
shows the actual values observed in my local test env (on xfs) and the
error "e", which is the symmetric absolute percentage error from the ideal
values of 29M for c[0] and 21M for c[1].

  test_memcg_min

         | MGLRU enabled   | MGLRU enabled   | MGLRU disabled
         | Without patch   | With patch      |
    -----|-----------------|-----------------|---------------
    c[0] | 25964544 (e=8%) | 28770304 (e=3%) | 27820032 (e=4%)
    c[1] | 26214400 (e=9%) | 23998464 (e=4%) | 24776704 (e=6%)

  test_memcg_low

         | MGLRU enabled   | MGLRU enabled   | MGLRU disabled
         | Without patch   | With patch      |
    -----|-----------------|-----------------|---------------
    c[0] | 26214400 (e=7%) | 27930624 (e=4%) | 27688960 (e=5%)
    c[1] | 26214400 (e=9%) | 24764416 (e=6%) | 24920064 (e=6%)

Factor out the proportioning logic to a new function and have MGLRU reuse
it.  While at it, update the eviction behavior via debugfs 'lru_gen'
interface ('-' command with an explicit 'nr_to_reclaim' parameter) to
ensure eviction is limited to the specified number.

Link: https://lkml.kernel.org/r/20250530162353.541882-1-den@valinux.co.jp
Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Reviewed-by: Yuanchu Xie <yuanchu@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>