mm: zswap: add per-memcg stat for incompressible pages

In containerized environments, knowing which cgroup is contributing incompressible pages to zswap is essential for effective resource management. This series adds a new per-memcg stat 'zswap_incomp' to track incompressible pages, along with a selftest.

Patch 1: Add the per-memcg zswap_incomp stat and documentation Patch 2: Add selftest for the new stat

Changes v2 -> v3:
https://lore.kernel.org/linux-mm/[email protected]/

- Remove inline comments for incompressibility check (Yosry Ahmed)
- Use PAGE_SIZE instead of hardcoded 4096 in selftest (SeongJae Park)
- Use pipe for parent-child synchronization instead of usleep (JP Kobryn, SeongJae Park)
- Use MADV_PAGEOUT instead of memory.reclaim to push pages into zswap, and use mmap instead of malloc accordingly (Nhat Pham)
- Collect Reviewed-by and Acked-by tags

Changes v1 -> v2:
https://lore.kernel.org/linux-mm/[email protected]/

- Rename zswpraw/MEMCG_ZSWAP_RAW to zswap_incomp/MEMCG_ZSWAP_INCOMP (Shakeel Butt, Yosry Ahmed)
- Drop zswap_is_incomp() helper, keep opencode (size == PAGE_SIZE) with comments explaining the incompressibility check (Yosry Ahmed)
- Add documentation in cgroup-v2.rst (Nhat Pham, SeongJae Park)
- Add selftest as a separate patch (Nhat Pham)
- Add reference link to Chris Li's discussion on the need for per-memcg incompressible page tracking (Nhat Pham)

Jiayuan Chen (2):
mm: zswap: add per-memcg stat for incompressible pages selftests/cgroup: add test for zswap incompressible pages

 Documentation/admin-guide/cgroup-v2.rst     |   5 +
 include/linux/memcontrol.h                  |   1 +
 mm/memcontrol.c                             |   6 +
 tools/testing/selftests/cgroup/test_zswap.c | 136 ++++++++++++++++++++
 4 files changed, 148 insertions(+)

2.43.0