linux-kernel - Re: [PATCH memcg 0/1] false global OOM triggered by memcg-limited task

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Date:   Tue, 19 Oct 2021 14:04:22 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Vasily Averin <vvs@...tuozzo.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Uladzislau Rezki <urezki@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Shakeel Butt <shakeelb@...gle.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel@...nvz.org
Subject: Re: [PATCH memcg 0/1] false global OOM triggered by memcg-limited
 task

On Tue 19-10-21 13:54:42, Michal Hocko wrote:
> On Tue 19-10-21 13:30:06, Vasily Averin wrote:
> > On 19.10.2021 11:49, Michal Hocko wrote:
> > > On Tue 19-10-21 09:30:18, Vasily Averin wrote:
> > > [...]
> > >> With my patch ("memcg: prohibit unconditional exceeding the limit of dying tasks") try_charge_memcg() can fail:
> > >> a) due to fatal signal
> > >> b) when mem_cgroup_oom -> mem_cgroup_out_of_memory -> out_of_memory() returns false (when select_bad_process() found nothing)
> > >>
> > >> To handle a) we can follow to your suggestion and skip excution of out_of_memory() in pagefault_out_of memory()
> > >> To handle b) we can go to retry: if mem_cgroup_oom() return OOM_FAILED.
> > 
> > > How is b) possible without current being killed? Do we allow remote
> > > charging?
> > 
> > out_of_memory for memcg_oom
> >  select_bad_process
> >   mem_cgroup_scan_tasks
> >    oom_evaluate_task
> >     oom_badness
> > 
> >         /*
> >          * Do not even consider tasks which are explicitly marked oom
> >          * unkillable or have been already oom reaped or the are in
> >          * the middle of vfork
> >          */
> >         adj = (long)p->signal->oom_score_adj;
> >         if (adj == OOM_SCORE_ADJ_MIN ||
> >                         test_bit(MMF_OOM_SKIP, &p->mm->flags) ||
> >                         in_vfork(p)) {
> >                 task_unlock(p);
> >                 return LONG_MIN;
> >         }
> > 
> > This time we handle userspace page fault, so we cannot be kenrel thread,
> > and cannot be in_vfork().
> > However task can be marked as oom unkillable, 
> > i.e. have p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN
> 
> You are right. I am not sure there is a way out of this though. The task
> can only retry for ever in this case. There is nothing actionable here.
> We cannot kill the task and there is no other way to release the memory.

Btw. don't we force the charge in that case?
-- 
Michal Hocko
SUSE Labs