Re: [PATCH net-next] net-memcg: pass in gfp_t mask to mem_cgroup_charge_skmem()

From: Jakub Kicinski <kuba@kernel.org>
To: Wei Wang <weiwan@google.com>
Cc: Shakeel Butt <shakeelb@google.com>,
	Eric Dumazet <edumazet@google.com>,
	netdev@vger.kernel.org, "David S . Miller" <davem@davemloft.net>,
	cgroups@vger.kernel.org, linux-mm@kvack.org,
	Roman Gushchin <roman.gushchin@linux.dev>
Subject: Re: [PATCH net-next] net-memcg: pass in gfp_t mask to mem_cgroup_charge_skmem()
Date: Wed, 12 Oct 2022 20:49:41 -0700	[thread overview]
Message-ID: <20221012204941.3223d205@kernel.org> (raw)
In-Reply-To: <CAEA6p_CqqPtnWjr_yYr1oVF3UKe=6RqFLrg1OoANs2eg5_by0A@mail.gmail.com>

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'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().