Re: [PATCH 1/2] memcg: move mem_cgroup_swappiness to memcontrol.h

[Ridong Chen]

On 7/13/2026 11:08 PM, Barry Song wrote:
> On Sat, Jul 11, 2026 at 5:12 PM Ridong Chen <ridong.chen@xxxxxxxxx> wrote:
> > From: Ridong Chen <chenridong@xxxxxxxxxx>
> >
> > The per-memcg swappiness knob is v1-only; v2 always uses global
> > vm_swappiness and ignores the per-cgroup field.
> >
> > Guard memcg->swappiness with CONFIG_MEMCG_V1, and move the helper
> > to memcontrol.h where it belongs.
> >
> > No functional change for v1; v2-only kernels drop the unused field.
> >
> > Signed-off-by: Ridong Chen <chenridong@xxxxxxxxxx>
> > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
>
> Reviewed-by: Barry Song <baohua@xxxxxxxxxx>
>
> With some nits.
>
> > ---
> [...]
> >          struct mem_cgroup_per_node *nodeinfo[];
> > @@ -365,6 +366,9 @@ enum objext_flags {
> >
> >   #define OBJEXTS_FLAGS_MASK (__NR_OBJEXTS_FLAGS - 1)
> >
> > +/* Defined in mm/vmscan.c; used by mem_cgroup_swappiness(). */
> > +extern int vm_swappiness;
>
> This is a bit unusual. I'm not sure whether mm/swap.h would be
> a more appropriate place for this.
Thank you for your reply.

The vm_swappiness variable is not utilized within mm/swap.c. 
Furthermore, since memcontrol.h does not include swap.h, retaining the 
extern int vm_swappiness declaration in mm/swap.h will result in a 
compilation failure.

--
Best regards
Ridong