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

[Barry Song]

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/