[PATCH v1 0/3] mm: zswap: global shrinker fix and proactive shrink

[Takero Funaki <flintglass@gmail.com>]

This series addresses two issues and introduces a minor improvement in
zswap global shrinker:

1. Fix the memcg iteration logic that breaks iteration on offline memcgs.
2. Fix the error path that aborts on expected error codes.
3. Add proactive shrinking at 91% full, for 90% accept threshold.

These patches need to be applied in this order to avoid potential loops
caused by the first issue. Patch 3 can be applied independently, but the
two issues must be resolved to ensure the shrinker can evict pages.

Previously, the zswap pool could be filled with old pages that the
shrinker failed to evict, leading to zswap rejecting new pages. With
this series applied, the shrinker will continue to evict pages until the
pool reaches the accept_thr_percent threshold proactively, as
documented, and maintain the pool to keep recent pages.

As a side effect of changes in the hysteresis logic, zswap will no
longer reject pages under the max pool limit.

With this series, reclaims smaller than the proative shrinking amount
finish instantly and trigger background shrinking. Admins can check if
new pages are buffered by zswap by monitoring the pool_limit_hit
counter. 

Changes since v0:
mm: zswap: fix global shrinker memcg iteration
- Drop and reacquire spinlock before skipping a memcg.
- Add some comment to clarify the locking mechanism.
mm: zswap: proactive shrinking before pool size limit is hit
- Remove unneeded check before scheduling work.
- Change shrink start threshold to accept_thr_percent + 1%.

Now it starts shrinking at accept_thr_percent + 1%. Previously, the
threshold was at the midpoint of 100% to accept_threshold.

If a workload needs 10% space to buffer the average reclaim amount, with
the previous patch, it required setting the accept_thr_percent to 80%.
For 50%, it became 0%, which is not acceptable and unclear for admins.
We can use the accept percent as the shrink threshold directly but that
sounds shrinker is called too frequently around the accept threshold.  I
added 1% as a minimum gap to the shrink threshold.

----

Takero Funaki (3):
  mm: zswap: fix global shrinker memcg iteration
  mm: zswap: fix global shrinker error handling logic
  mm: zswap: proactive shrinking before pool size limit is hit

 Documentation/admin-guide/mm/zswap.rst |  17 ++-
 mm/zswap.c                             | 172 ++++++++++++++++++-------
 2 files changed, 136 insertions(+), 53 deletions(-)

-- 
2.43.0