[RFC PATCH 0/3] mm, memcg: isolate deprecated v1 state from struct mem_cgroup

[Tao Cui]

From: Tao Cui <cuitao@xxxxxxxxxx>

The legacy cgroup v1 memory controller has already been moved out of
the shared implementation at the file level (mm/memcontrol-v1.c) and at
the Kconfig level (CONFIG_MEMCG_V1, default n since 6.11).  Its
per-cgroup state, however, still sits as individual members inside
struct mem_cgroup, guarded by #ifdefs.

This series isolates the deprecated implementation from the shared hot
structure: all v1-only members are grouped into a dedicated
struct mem_cgroup_v1, and every access goes through memcg->v1.X.

With this in place the v1 implementation is self-contained: its
interface in mm/memcontrol-v1.c, its state in struct mem_cgroup_v1, and
its eventual removal becomes a localized deletion of this struct
together with mm/memcontrol-v1.c, instead of unwinding
ifdef-scattered members across the shared header.

Patch 1 introduces the substruct behind a transitional union whose
anonymous side preserves the historical layout, so the patch is
semantically empty.  Patch 2 converts the access sites and drops the
anonymous side.  Patch 3 documents the boundary.

This demonstrates one possible pattern for isolating deprecated cgroup
v1 implementation from shared structures.

As a validation detail, this does not regress the layout: the member
order is preserved, so offsetof(struct mem_cgroup, v1.swappiness) and
sizeof(struct mem_cgroup) are unchanged (verified with pahole), and the
generated code of hot paths such as mem_cgroup_swappiness() is
identical.  Build-tested with CONFIG_MEMCG_V1 both enabled and
disabled, and boot-tested with both a v1 and a v2 hierarchy.

Tao Cui (3):
  mm, memcg: introduce struct mem_cgroup_v1
  mm, memcg: move v1-only members into mem_cgroup_v1
  docs: cgroup-v1: note the v1 memory controller implementation boundary

 .../admin-guide/cgroup-v1/memory.rst          |   7 +
 include/linux/memcontrol.h                    |  74 +++++------
 include/net/sock.h                            |   2 +-
 mm/memcontrol-v1.c                            | 122 +++++++++---------
 mm/memcontrol-v1.h                            |   4 +-
 mm/memcontrol.c                               |  22 ++--
 mm/swap.h                                     |   2 +-
 7 files changed, 121 insertions(+), 112 deletions(-)

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2.43.0