[PATCH mm-new v1 0/2] mm: vmscan: skip anon folios zswap would refuse when writeback is off

[Jiayuan Chen]

memory.zswap.writeback=0 is what we set for latency sensitive services:
their cold anon pages may go to zswap, but never to disk, a swapin
from disk is a latency spike they cannot afford. Such a memcg can only
swap into zswap. When zswap stops taking its pages, because the pool
hit max_pool_percent or the memcg hit memory.zswap.max, reclaim still
goes through the motions on its anon LRU: every folio it picks gets a
swap slot, is unmapped, is refused by zswap_store() and is activated
again. Nothing is freed, the folio now sits in the swap cache holding
a slot, and the owner takes a minor fault on the next access.

memory.reclaim below is just the easiest way to trigger it. The same
loop runs whenever the memcg is reclaimed: when it hits memory.max, or
when kswapd walks it under node pressure. Neither is something the
service can opt out of.

Patch 1 is a refactor so the memcg zswap check can be reused from
reclaim. Patch 2 adds the check to shrink_folio_list(). It cannot go
into can_reclaim_anon_pages(): zero-filled folios never reach zswap
and must stay reclaimable.

Numbers from a 16-CPU, 32G QEMU guest on mm-new, zswap with
max_pool_percent=2 (~640M), a memcg with memory.zswap.writeback=0
holding 3G of 2:1 compressible anon. The pool fills up after ~300k
pages. Then 5 x "echo 512M > memory.reclaim", and the task walks its
3G once:

                     minor faults on the walk
                     MGLRU        classic LRU
  before             786,454      786,452
  after              301,168      301,158

The ~300k after the patch are the pages that really are in zswap
(stored_pages was 301,133 / 301,165), swapped back in once each. The
other ~485k before the patch are pages zswap refused: unmapped, left
in the swap cache, mapped again on the next touch.

Jiayuan Chen (2):
  mm: memcontrol: split mem_cgroup_may_zswap() out of
    obj_cgroup_may_zswap()
  mm: vmscan: skip anon folios zswap would refuse when writeback is off

 include/linux/memcontrol.h |  5 +++++
 include/linux/zswap.h      |  5 +++++
 mm/memcontrol.c            | 26 ++++++++++++++++++--------
 mm/page_io.c               | 15 ++++++++++++++-
 mm/swap.h                  | 12 ++++++++++++
 mm/vmscan.c                | 17 +++++++++++++++++
 mm/zswap.c                 | 11 +++++++++++
 7 files changed, 82 insertions(+), 9 deletions(-)

-- 
2.43.0