linux-kernel - Re: WARNING in try

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Date:   Tue, 7 Aug 2018 19:19:08 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     syzbot <syzbot+bab151e82a4e973fa325@...kaller.appspotmail.com>,
        cgroups@...r.kernel.org, dvyukov@...gle.com, hannes@...xchg.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com, vdavydov.dev@...il.com
Subject: Re: WARNING in try_charge

On 2018/08/07 6:50, Tetsuo Handa wrote:
>  	list_for_each_entry_safe(p, tmp, &oom_victim_list, oom_victim_list) {
>  		struct mm_struct *mm = p->signal->oom_mm;
>  
>  		if (oom_unkillable_task(p, oc->memcg, oc->nodemask))
>  			continue;
>  		ret = true;
> +		/*
> +		 * Since memcg OOM allows forced charge, we can safely wait
> +		 * until __mmput() completes.
> +		 */
> +		if (is_memcg_oom(oc))
> +			return true;

Oops. If this OOM victim was blocked on some lock which current thread is
holding, waiting forever unconditionally is not safe.

>  #ifdef CONFIG_MMU
>  		/*
>  		 * Since the OOM reaper exists, we can safely wait until
>  		 * MMF_OOM_SKIP is set.
>  		 */
>  		if (!test_bit(MMF_OOM_SKIP, &mm->flags)) {
>  			if (!oom_reap_target) {
>  				get_task_struct(p);
>  				oom_reap_target = p;
>  				trace_wake_reaper(p->pid);
>  				wake_up(&oom_reaper_wait);
>  			}
>  #endif
>  			continue;
>  		}
>  #endif
>  		/* We can wait as long as OOM score is decreasing over time. */
>  		if (!victim_mm_stalling(p, mm))
>  			continue;
>  		gaveup = true;
>  		list_del(&p->oom_victim_list);
>  		/* Drop a reference taken by mark_oom_victim(). */
>  		put_task_struct(p);
>  	}
>  	if (gaveup)
>  		debug_show_all_locks();
>  
>  	return ret;
>  }
>