linux-kernel - Re: [PATCH 2/2] mm: replace TIF_MEMDIE checks by tsk_is_oom

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Message-ID: <20170727144544.GC31031@dhcp22.suse.cz>
Date:   Thu, 27 Jul 2017 16:45:44 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     akpm@...ux-foundation.org, rientjes@...gle.com, hannes@...xchg.org,
        guro@...com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: replace TIF_MEMDIE checks by tsk_is_oom_victim

On Thu 27-07-17 23:01:05, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 544d47e5cbbd..86a48affb938 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1896,7 +1896,7 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> >  	 * bypass the last charges so that they can exit quickly and
> >  	 * free their memory.
> >  	 */
> > -	if (unlikely(test_thread_flag(TIF_MEMDIE) ||
> > +	if (unlikely(tsk_is_oom_victim(current) ||
> >  		     fatal_signal_pending(current) ||
> >  		     current->flags & PF_EXITING))
> >  		goto force;
> 
> Did we check http://lkml.kernel.org/r/20160909140508.GO4844@dhcp22.suse.cz ?

OK, so your concern was

> Does this test_thread_flag(TIF_MEMDIE) (or tsk_is_oom_victim(current)) make sense?
> 
> If current thread is OOM-killed, SIGKILL must be pending before arriving at
> do_exit() and PF_EXITING must be set after arriving at do_exit().

> But I can't find locations which do memory allocation between clearing
> SIGKILL and setting PF_EXITING.

I can't find them either and maybe there are none. But why do we care
in this particular patch which merely replaces TIF_MEMDIE check by
tsk_is_oom_victim? The code will surely not become less valid. If
you believe this check is redundant then send a patch with the clear
justification. But I would say, at least from the robustness point of
view I would just keep it there. We do not really have any control on
what happens between clearing signals and setting PF_EXITING.
-- 
Michal Hocko
SUSE Labs