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

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Message-ID: <CAMgjq7A17PKTcJ9dcTid4OkQ=xoj1x9WpzqeFCH235jQva88tg@mail.gmail.com>
Date: Sun, 22 Dec 2024 22:51:56 +0800
From: Kairui Song <ryncsn@...il.com>
To: "Huang, Ying" <ying.huang@...ux.alibaba.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Chris Li <chrisl@...nel.org>, Hugh Dickins <hughd@...gle.com>, 
	Yosry Ahmed <yosryahmed@...gle.com>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, Johannes Weiner <hannes@...xchg.org>, 
	Barry Song <baohua@...nel.org>, Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/4] mm, memcontrol: avoid duplicated memcg enable check

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

Hi Ying,

>
> Sorry for jumping in so late.
>
> Kairui Song <ryncsn@...il.com> writes:
>
> > From: Kairui Song <kasong@...cent.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@...cent.com>
> > Reviewed-by: Yosry Ahmed <yosryahmed@...gle.com>
> > Reviewed-by: Roman Gushchin <roman.gushchin@...ux.dev>
> > Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>
> > Acked-by: Chris Li <chrisl@...nel.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