[RFC PATCH 0/3] mm/lru_gen: add memory.lru_gen interface for cgroup v2

[Jiayuan Chen <jiayuan.chen@linux.dev>]

This patchset adds a memory.lru_gen interface to cgroup v2, allowing users
to interact with MGLRU directly on a specific cgroup without needing to
know the internal memcg_id.

Motivation
==========
Currently, the only way to perform aging or eviction on a specific memcg
is through the debugfs interface (/sys/kernel/debug/lru_gen), which
requires the memcg_id. However, there's no straightforward way to get the
memcg_id for a given cgroup path. This makes it difficult for users to
leverage MGLRU's proactive reclaim capabilities on specific cgroups.

Solution
========
The new memory.lru_gen interface operates directly on the cgroup:

  # Show lru_gen info for this cgroup
  cat /sys/fs/cgroup/mygroup/memory.lru_gen

  # Run aging on node 0
  echo '+ 0 <seq>' > /sys/fs/cgroup/mygroup/memory.lru_gen

  # Evict cold pages on node 0
  echo '- 0 <seq> <swappiness> <nr_to_reclaim>' > \
       /sys/fs/cgroup/mygroup/memory.lru_gen

This interface is available on all cgroups including root, providing the
same functionality as the debugfs lru_gen interface.

Testing
=======
Create 1GB page cache, loop access 200MB as hot pages.
After aging twice, 200MB hot pages are in young generation,
800MB cold pages remain in oldest generation.
Eviction with seq=min_seq only reclaims cold pages, hot pages preserved.

Patches
=======
Patch 1 refactors the existing debugfs code to extract helper functions.
Patch 2 adds the memory.lru_gen interface using these helpers.
Patch 3 adds documentation for the new interface.


Jiayuan Chen (3):
  mm/lru_gen: refactor to extract helper functions
  mm/lru_gen: add memory.lru_gen interface for cgroup v2
  docs/cgroup: document memory.lru_gen interface

 Documentation/admin-guide/cgroup-v2.rst |  17 +++
 include/linux/mmzone.h                  |  16 +++
 mm/memcontrol.c                         |  31 +++++
 mm/vmscan.c                             | 176 +++++++++++++++++++-----
 4 files changed, 206 insertions(+), 34 deletions(-)

-- 
2.43.0