[PATCH v5 0/2] Ignore non-LRU-based reclaim in memcg reclaim

[Yosry Ahmed]

Upon running some proactive reclaim tests using memory.reclaim, we
noticed some tests flaking where writing to memory.reclaim would be
successful even though we did not reclaim the requested amount fully.
Looking further into it, I discovered that *sometimes* we over-report
the number of reclaimed pages in memcg reclaim.

Reclaimed pages through other means than LRU-based reclaim are tracked
through reclaim_state in struct scan_control, which is stashed in
current task_struct. These pages are added to the number of reclaimed
pages through LRUs. For memcg reclaim, these pages generally cannot be
linked to the memcg under reclaim and can cause an overestimated count
of reclaimed pages. This short series tries to address that.

Patch 1 ignores pages reclaimed outside of LRU reclaim in memcg reclaim.
The pages are uncharged anyway, so even if we end up under-reporting
reclaimed pages we will still succeed in making progress during
charging.

Patch 2 is just refactoring, it adds helpers that wrap some
operations on current->reclaim_state, and rename
reclaim_state->reclaimed_slab to reclaim_state->reclaimed. It also adds
a huge comment explaining why we ignore pages reclaimed outside of LRU
reclaim in memcg reclaim.

The patches are divided as such so that patch 1 can be easily backported
without all the refactoring noise.

v4 -> v5:
- Separate the functional fix into its own patch, and squash all the
  refactoring into a single second patch for ease of backporting (Andrew
  Morton).

v4: https://lore.kernel.org/lkml/20230404001353.468224-1-yosryahmed@xxxxxxxxxx/

Yosry Ahmed (2):
  mm: vmscan: ignore non-LRU-based reclaim in memcg reclaim
  mm: vmscan: refactor reclaim_state helpers

 fs/inode.c           |  3 +-
 fs/xfs/xfs_buf.c     |  3 +-
 include/linux/swap.h | 17 ++++++++++-
 mm/slab.c            |  3 +-
 mm/slob.c            |  6 ++--
 mm/slub.c            |  5 ++-
 mm/vmscan.c          | 73 +++++++++++++++++++++++++++++++++-----------
 7 files changed, 78 insertions(+), 32 deletions(-)

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2.40.0.348.gf938b09366-goog