[PATCH v2 0/4] 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 "zswap_writeback_only" key to memory.reclaim
cgroup interface, allowing users to proactively write back cold compressed
pages 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 100MB of pages from zswap to the backing swap
  echo "100M zswap_writeback_only" > memory.reclaim

Patch 1: Move the global zswap shrink cursor into struct mem_cgroup as a
  per-memcg zswap_wb_iter, so patch 2 can scope writeback to a given memcg
  and make forward progress across its subtree on repeated invocations.

Patch 2: Extend the memory.reclaim cgroup v2 interface with a new
  "zswap_writeback_only" key, allowing users to trigger proactive zswap
  writeback up to a requested budget.

Patch 3: Add a zswpwb_proactive counter to memory.stat and /proc/vmstat
  to track the number of writebacks triggered by proactive writeback.

Patch 4: Add tests for zswap proactive writeback.

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.

[v1] https://lore.kernel.org/all/20260511105149.75584-1-jiahao.kernel@gmail.com

Hao Jia (4):
  mm/zswap: Make shrink_worker writeback cursor per-memcg
  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                  |   3 +
 include/linux/vm_event_item.h               |   1 +
 include/linux/zswap.h                       |  16 ++
 mm/memcontrol.c                             |   4 +
 mm/vmscan.c                                 |  14 +
 mm/vmstat.c                                 |   1 +
 mm/zswap.c                                  | 292 +++++++++++++++++---
 tools/testing/selftests/cgroup/test_zswap.c | 161 ++++++++++-
 10 files changed, 470 insertions(+), 55 deletions(-)

-- 
2.34.1