Re: [PATCH v3 1/4] mm, memcontrol: avoid duplicated memcg enable check

Kairui Song
2024-12-22T14:52:14Z

On Sun, Dec 22, 2024 at 9:33 PM Huang, Ying
<ying.huang@linux.alibaba.com> wrote:
>
> Hi, Kairui,

Hi Ying,

>
> Sorry for jumping in so late.
>
> Kairui Song <ryncsn@gmail.com> writes:
>
> > From: Kairui Song <kasong@tencent.com>
> >
> > mem_cgroup_uncharge_swap() includes a mem_cgroup_disabled() check,
> > so the caller doesn't need to check that.
> >
> > Signed-off-by: Kairui Song <kasong@tencent.com>
> > Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
> > Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
> > Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
> > Acked-by: Chris Li <chrisl@kernel.org>
> > ---
> > mm/memcontrol.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 7b3503d12aaf..79900a486ed1 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -4609,7 +4609,7 @@ void mem_cgroup_swapin_uncharge_swap(swp_entry_t entry, unsigned int nr_pages)
> > * correspond 1:1 to page and swap slot lifetimes: we charge the
> > * page to memory here, and uncharge swap when the slot is freed.
> > */
> > - if (!mem_cgroup_disabled() && do_memsw_account()) {
> > + if (do_memsw_account()) {
> > /*
> > * The swap entry might not get freed for a long time,
> > * let's not wait for it. The page already received a
>
> I take a look at memcontrol.c, it appears that almost all extern
> functions check mem_cgroup_disabled() as the first step.

Hmm, just checked memcontrol.c and I saw quite a few extern functions
not doing that, so I think that's not a convention.

> that this is a convention of memcontrol.c? And the benefit of the
> change is minimal. In contrast, if someone makes more changes to
> mem_cgroup_swapin_uncharge_swap() in the future, he may forget to add
> this back. So, it may be unnecessary to make the change?

This change is minimal indeed, it only helps to remove a few unneeded
nop, still a gain though.

I think mem_cgroup_swapin_uncharge_swap should fade away in the future,
it's only for Cgroup V1, and it's a really simple function, just a
wrapper for mem_cgroup_uncharge_swap, so I think this is not a
problem?

If you are concerned about this, this patch can be dropped from this
series, rest of the patches still work the same.

>
> ---
> Best Regards,
> Huang, Ying