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

[Tao Cui <cui.tao@linux.dev>]

Hi Shakeel, Michal,

在 2026/9/18 17:41, Michal Hocko 写道:
> On Thu 17-09-26 13:26:40, Shakeel Butt wrote:
>> On Wed, Sep 16, 2026 at 08:57:34PM +0800, Tao Cui wrote:
>>> From: Tao Cui <cuitao@kylinos.cn>
>>>
>>> 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.
>>
>> Sorry I don't see any benefit of this code churn. The code is already behind
>> config. What exactly this code churn is giving us?
> 
> The only arguable upside is that this would make it ever so slightly
> easier to track v1 specific stuff (once that s@v1@memcg1@ or similar).
> I am not convinced this is sufficient to justify the churn either.

Thanks for taking a look.

You are right that the file and Kconfig layers already isolate the v1
implementation at build time, and =n builds get nothing from this
series. The benefit is only for =y builds.

In a =y build today, the deprecated controller's state is still
embedded directly in struct mem_cgroup and common code accesses it
directly. This series moves that state behind a single
struct mem_cgroup_v1, complementing the existing file- and
Kconfig-level separation at the data structure level. The layout is
unchanged, so there is no runtime cost.

My intention was to explore whether this could serve as a general
pattern for isolating the remaining cgroup v1-only state, rather than
as a standalone memcg cleanup. I should have made that context clearer
in the cover letter.

If that isn't sufficient to justify the churn, I understand.

Thanks,
Tao