[PATCH v5 0/6] mm/zswap: Implement per-cgroup proactive writeback

[Hao Jia <jiahao.kernel@gmail.com>]

From: Hao Jia <jiahao1@lixiang.com>

Zswap currently writes back pages to backing swap reactively, triggered
either by the shrinker or by the pool reaching its size limit. Although
proactive memory reclaim can automatically write back a portion of zswap
pages via the shrinker, it cannot explicitly control the amount of
writeback for a specific memory cgroup. Moreover, proactive memory reclaim
may not always be triggered during a steady state.

In certain scenarios, it is desirable to trigger writeback in advance to
free up memory. For example, users may want to prepare for an upcoming
memory-intensive workload by flushing cold memory to the backing storage
when the system is relatively idle.

This patch series introduces a "source=zswap" key to memory.reclaim
cgroup interface, allowing users to proactively write back cold compressed
data from zswap to the backing swap device. When specified, this key
bypasses standard memory reclaim and exclusively performs proactive zswap
writeback up to the requested budget. If omitted, the default reclaim
behavior remains unchanged.

Example usage:
  # Write back 10MB of compressed data from zswap to the backing swap
  echo "10M source=zswap" > memory.reclaim

Patch 1: Fix missing global shrinker when memory cgroup is disabled.
Patch 2: Extend shrink_memcg() to support batch writeback and update its
  return value semantics, thereby improving the writeback efficiency in
  the shrink_worker() path.
Patch 3: Extract a reusable writeback helper zswap_shrink_one_memcg() from
  shrink_worker().
Patch 4: Extend the memory.reclaim cgroup v2 interface with a new
  "source=zswap" key, allowing users to trigger proactive zswap
  writeback up to a requested budget.
Patch 5: Add the zswpwb_proactive_b stat to track the compressed bytes
  of proactive writeback for better monitoring and tuning.
Patch 6: Add tests for zswap proactive writeback.

v4->v5:
  - Add a new patch to fix missing global shrinker when memory cgroup is disabled.
  - Simplify batch writeback logic in shrink_memcg() and improve comments.
  - Refactor the writeback retry helper out of shrink_worker().
  - Replace the "zswap_writeback_only" memory.reclaim key with a more
    extensible "source=zswap" key, leaving room for selecting other
    reclaim sources in the future.

v3->v4:
  - Drop the per-memcg cursor and keep the root cgroup cursor
    (zswap_next_shrink) logic intact.
  - Stick to using the zswap_writeback_only key, and change the proactive
    writeback size to use the compressed size.
  - Consolidate and reuse the logic between shrink_worker() and
    shrink_memcg(). Enable batch writeback in the shrink_worker() path,
    while maintaining a low writeback budget in the zswap_store() path.

v2->v3:
    - Align the return value of zswap_proactive_writeback() with
      memory.reclaim and update the corresponding documentation accordingly.
    - Resolve conflicts in test_zswap.c on the mm-unstable branch.
    - Enhance the zswap proactive writeback selftests to guard against potential
      future regressions.

v1->v2:
    - As suggested by Yosry and Nhat, extend the memory.reclaim cgroup v2
      interface with a "zswap_writeback_only" key instead of adding a new
      dedicated cgroup interface.
    - Update the zswap documentation and add selftests for proactive writeback.

[v4] https://lore.kernel.org/all/20260618044857.69439-1-jiahao.kernel@gmail.com
[v3] https://lore.kernel.org/all/20260526114601.67041-1-jiahao.kernel@gmail.com
[v2] https://lore.kernel.org/all/20260525122242.36127-1-jiahao.kernel@gmail.com
[v1] https://lore.kernel.org/all/20260511105149.75584-1-jiahao.kernel@gmail.com

Hao Jia (6):
  mm/zswap: Fix global shrinker when memory cgroup is disabled
  mm/zswap: Support batch writeback in shrink_memcg()
  mm/zswap: Extract a reusable writeback helper from shrink_worker()
  mm/zswap: Implement proactive writeback
  mm/zswap: Add per-memcg stat for proactive writeback
  selftests/cgroup: Add tests for zswap proactive writeback

 Documentation/admin-guide/cgroup-v2.rst     |  22 +-
 Documentation/admin-guide/mm/zswap.rst      |  11 +-
 include/linux/memcontrol.h                  |   1 +
 include/linux/zswap.h                       |   7 +
 mm/memcontrol.c                             |   3 +
 mm/vmscan.c                                 |  24 +-
 mm/zswap.c                                  | 239 +++++++++++++++-----
 tools/testing/selftests/cgroup/test_zswap.c | 161 ++++++++++++-
 8 files changed, 410 insertions(+), 58 deletions(-)

-- 
2.34.1