Re: [PATCH memcg] memcg: prohibit unconditional exceeding the limit of dying tasks

From: Michal Hocko <mhocko@suse.com>
To: Vasily Averin <vvs@virtuozzo.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	cgroups@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH memcg] memcg: prohibit unconditional exceeding the limit of dying tasks
Date: Mon, 13 Sep 2021 10:39:42 +0200	[thread overview]
Message-ID: <YT8OTozT3FN9P2k7@dhcp22.suse.cz> (raw)
In-Reply-To: <8b98d44a-aeb2-5f5f-2545-ac2bd0c7049b@virtuozzo.com>

On Mon 13-09-21 10:51:37, Vasily Averin wrote:
> On 9/10/21 3:39 PM, Vasily Averin wrote:
> > The kernel currently allows dying tasks to exceed the memcg limits.
> > The allocation is expected to be the last one and the occupied memory
> > will be freed soon.
> > This is not always true because it can be part of the huge vmalloc
> > allocation. Allowed once, they will repeat over and over again.
> 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 389b5766e74f..67195fcfbddf 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2622,15 +2625,6 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
> >  	if (gfp_mask & __GFP_ATOMIC)
> >  		goto force;
> >  
> > -	/*
> > -	 * Unlike in global OOM situations, memcg is not in a physical
> > -	 * memory shortage.  Allow dying and OOM-killed tasks to
> > -	 * bypass the last charges so that they can exit quickly and
> > -	 * free their memory.
> > -	 */
> > -	if (unlikely(should_force_charge()))
> > -		goto force;
> > -
> 
> Should we keep current behaviour for (current->flags & PF_EXITING) case perhaps?

Why?

> It is set inside do_exit only and (I hope) cannot trigger huge vmalloc allocations.

Allocations in this code path should be rare but it is not like they are
non-existent. This is rather hard to review area spread at many places
so if we are deciding to make the existing model simpler (no bypassing)
then I would rather have no exceptions unless they are reaaly necessary
and document them if they are.
-- 
Michal Hocko
SUSE Labs