[PATCH v7 0/5] mm/page_counter: move stock from mem_cgroup to page_counter

[Joshua Hahn <joshua.hahnjy@gmail.com>]

v6 --> v7
=========
- Preserve two cgroup v1 proprerties that previously followed implicitly
  from sharing a stock:
  - memory.usage_in_bytes <= memory.memsw.usage_in_bytes
  - Reclaim does not swap when memory and memsw have equal limits.
- Wordsmithing commit messages for 4/5 and 5/5 to document the
  cgroupv1 divergence (bounded) between memory and memsw caused by
  independent stock.

INTRODUCTION
============
Memcg keeps a per-CPU stock of precharged pages so that small, frequent
allocations do not walk the page_counter hierarchy every time.
Today, the stock implementation is within memcontrol code, even though
the operation it caches is a page_counter charge. This makes it
difficult to add new page_counters to a memcg and preserve the fast
path behavior.

This matters for future work like my tiered memcg limits series [1]
which introduces multiple new page_counters to memcg. Without making
stock a page_counter-level property, it means that every memcg charge
now goes through multiple page_counter hierarchy walks, instead of
being able to cache these charges.

To make future page_counters scalable and performant, move stock from
mem_cgroup to page_counter so that each page_counter can opt into its
own per-CPU cache of pre-charged pages.

We get an added benefit of simplifying try_charge_memcg(), which now
has all the stock management handled transparently within the
page_counter layer.

EFFECT ON MEMCG V2 USERS
========================
This series has no functional changes intended for memcg v2 users. We
preserve all draining, refilling, and (un)charging behavior, including
the uncharge path's refills / direct uncharges.

EFFECT ON MEMCG V1 USERS
========================
For memcg v1 users, the decoupling of the memsw and memory stock means
that each of them now manage their own independent stocks and can lead
to a different size of precharged cache for each.

Cgroup v1 has an invariant that memory.memsw.usage_in_bytes is larger
than or equal to memory.usage_in_bytes, because memsw is a superset of
memory. With separate stocking, this could have been broken in scenarios
where the memory stock is bigger than the memsw stock, leading to
memory usage appearing to be inflated and greater than memsw usage,
even though the real usage preserves the invariant.

Preserve the user-visible invariant by reporting memory.usage_in_bytes
as the smaller of the raw memory and memsw values. Both counters are
upper bounds on charged memory, so this remains a bounded stock-related
overestimate and importantly does not affect limit enforcement.

Based on latest mm-new as of 10/21/26:
fcec4f4bcc110 "riscv: mm: fix concurrency in mark_new_valid_map()"

[1] https://lore.kernel.org/all/20260807202059.2620949-1-joshua.hahnjy@gmail.com/

Joshua Hahn (5):
  mm/memcontrol: flatten try_charge_memcg control flow
  mm/page_counter: introduce per-CPU stock
  mm/page_counter: make page_counter_try_charge() stock-aware
  mm/memcontrol: move memory stock to page counters
  mm/memcontrol: add stock to the memsw page counter

 include/linux/page_counter.h |  35 +++-
 kernel/cgroup/dmem.c         |   2 +-
 mm/hugetlb_cgroup.c          |   2 +-
 mm/memcontrol-v1.c           |  17 +-
 mm/memcontrol.c              | 352 ++++++++++++-----------------------
 mm/page_counter.c            | 209 ++++++++++++++++++++-
 6 files changed, 361 insertions(+), 256 deletions(-)

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2.53.0-Meta