linux-kernel - Re: [PATCH 0/2] oom, memcg: do not report racy no-eligible OOM

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Date:   Sat, 12 Jan 2019 00:37:05 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, Johannes Weiner <hannes@...xchg.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] oom, memcg: do not report racy no-eligible OOM

On 2019/01/12 0:07, Michal Hocko wrote:
> On Fri 11-01-19 23:31:18, Tetsuo Handa wrote:
>> The OOM killer invoked by [ T9694] called printk() but didn't kill anything.
>> Instead, SIGINT from Ctrl-C killed all thread groups sharing current->mm.
> 
> I still do not get it. Those other processes are not sharing signals.
> Or is it due to injecting the signal too all of them with the proper
> timing?

Pressing Ctrl-C between after task_will_free_mem(p) in oom_kill_process() and
before __oom_kill_process() (e.g. dump_header()) made fatal_signal_pending() = T
for all of them.

> Anyway, could you update your patch and abstract 
> 	if (unlikely(tsk_is_oom_victim(current) ||
> 		     fatal_signal_pending(current) ||
> 		     current->flags & PF_EXITING))
> 
> in try_charge and reuse it in mem_cgroup_out_of_memory under the
> oom_lock with an explanation please?

I don't think doing so makes sense, for

  tsk_is_oom_victim(current) = T && fatal_signal_pending(current) == F

can't happen for mem_cgroup_out_of_memory() under the oom_lock, and
current->flags cannot get PF_EXITING when current is inside
mem_cgroup_out_of_memory(). fatal_signal_pending(current) alone is
appropriate for mem_cgroup_out_of_memory() under the oom_lock because

  tsk_is_oom_victim(current) = F && fatal_signal_pending(current) == T

can happen there.

Also, doing so might become wrong in future, for mem_cgroup_out_of_memory()
is also called from memory_max_write() which does not bail out upon
PF_EXITING. I don't think we can call memory_max_write() after current
thread got PF_EXITING, but nobody knows what change will happen in future.