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

[Johannes Weiner]

On Fri, Sep 18, 2026 at 11:41:09AM +0200, Michal Hocko wrote:
> 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@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.
> > 
> > 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 had the same thought. It's kiiind of nice to see which members are
completely specific to memcg1.

> I am not convinced this is sufficient to justify the churn either.

IMO it helps that it's mostly in memcontrol-v1.c where the chances of
interfering with other work are pretty slim.

The patch 1 + patch 2 split is a bit bothersome: indenting everything
into a union, only to then move it out in the next patch.

Tao, if you folded these two patches to make it one clean extraction
and changed over the sites at the same time, I would probably lean
supportive on this.