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

[Ridong Chen]

On 7/14/2026 9:48 AM, Barry Song wrote:
> On Tue, Jul 14, 2026 at 9:43 AM Barry Song <baohua@xxxxxxxxxx> wrote:
> > On Tue, Jul 14, 2026 at 9:20 AM Ridong Chen <ridong.chen@xxxxxxxxx> wrote:
> > > 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.
> >
> > If this is the case, it still seems better to keep
> > extern int vm_swappiness in include/linux/swap.h.
> >
> > Then we don't need the comment:
> > /* Defined in mm/vmscan.c; used by mem_cgroup_swappiness(). */
> >
> > It also makes it clearer that vm_swappiness is an extern variable
> > belonging to the swap module, rather than the memcontrol module.
>
> BTW, if mem_c_group_swappiness() and vm_swappiness are only used
> within mm/, could all of these be moved to mm/swap.h and
> mm/internal.h instead?
>
> We are making a big effort to move many unrelated things out of
> include/linux/swap.h recently. Could you check?
>
> https://lore.kernel.org/linux-mm/20260708-ch-swap-series-plus-folio-lru-cleanup-v9-0-2bc72b4f8730@xxxxxxxxx/

Good suggestion. Moving them to mm/internal.h makes sense. Will update 
in the next version.

--
Best regards
Ridong