netdev - Re: [PATCH net-next] net-memcg: pass in gfp_t mask to mem_cgroup_charge

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Message-ID: <CAEA6p_BUUzhHVAyaD3semV84M+TeZzmrkyjpwb-gs8e6sQRCWw@mail.gmail.com>
Date:   Wed, 12 Oct 2022 21:04:59 -0700
From:   Wei Wang <weiwan@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Roman Gushchin <roman.gushchin@...ux.dev>
Subject: Re: [PATCH net-next] net-memcg: pass in gfp_t mask to mem_cgroup_charge_skmem()

On Wed, Oct 12, 2022 at 8:49 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Wed, 12 Oct 2022 20:34:00 -0700 Wei Wang wrote:
> > > I pushed this little nugget to one affected machine via KLP:
> > >
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 03ffbb255e60..c1ca369a1b77 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -7121,6 +7121,10 @@ bool mem_cgroup_charge_skmem(struct mem_cgroup *memcg, unsigned int nr_pages,
> > >                 return true;
> > >         }
> > >
> > > +       if (gfp_mask == GFP_NOWAIT) {
> > > +               try_charge(memcg, gfp_mask|__GFP_NOFAIL, nr_pages);
> > > +               refill_stock(memcg, nr_pages);
> > > +       }
> > >         return false;
> > >  }
> > >
> > AFAICT, if you force charge by passing __GFP_NOFAIL to try_charge(),
> > you should return true to tell the caller that the nr_pages is
> > actually being charged.
>
> Ack - not sure what the best thing to do is, tho. Always pass NOFAIL
> in softirq?
>
> It's not clear to me yet why doing the charge/uncharge actually helps,
> perhaps try_to_free_mem_cgroup_pages() does more when NOFAIL is passed?
>
I am curious to know as well.

> I'll do more digging tomorrow.
>
> > Although I am not very sure what refill_stock() does. Does that
> > "uncharge" those pages?
>
> I think so, I copied it from mem_cgroup_uncharge_skmem().