linux-kernel - Re: WARNING in try

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Message-ID: <20180809150735.GA15611@dhcp22.suse.cz>
Date:   Thu, 9 Aug 2018 17:07:35 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     Vladimir Davydov <vdavydov@...tuozzo.com>,
        Oleg Nesterov <oleg@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        syzbot <syzbot+bab151e82a4e973fa325@...kaller.appspotmail.com>,
        cgroups@...r.kernel.org, hannes@...xchg.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: WARNING in try_charge

On Thu 09-08-18 22:57:43, Tetsuo Handa wrote:
> >From b1f38168f14397c7af9c122cd8207663d96e02ec Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Date: Thu, 9 Aug 2018 22:49:40 +0900
> Subject: [PATCH] mm, oom: task_will_free_mem(current) should retry until
>  memory reserve fails
> 
> Commit 696453e66630ad45 ("mm, oom: task_will_free_mem should skip
> oom_reaped tasks") changed to select next OOM victim as soon as
> MMF_OOM_SKIP is set. But we don't need to select next OOM victim as
> long as ALLOC_OOM allocation can succeed. And syzbot is hitting WARN(1)
> caused by this race window [1].

It is not because the syzbot was exercising a completely different code
path (memcg charge rather than the page allocator).

> Since memcg OOM case uses forced charge if current thread is killed,
> out_of_memory() can return true without selecting next OOM victim.
> Therefore, this patch changes task_will_free_mem(current) to ignore
> MMF_OOM_SKIP unless ALLOC_OOM allocation failed.

And the patch is simply wrong for memcg.
-- 
Michal Hocko
SUSE Labs