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Date: Thu, 1 Oct 2015 15:24:58 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: David Rientjes <rientjes@google.com>, Kyle Walker <kwalker@redhat.com>,
        Michal Hocko <mhocko@kernel.org>,
        Stanislav Kozina <skozina@redhat.com>,
        Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH -mm v2 3/3] mm/oom_kill: fix the wrong task->mm == mm
 checks in oom_kill_process()
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	<20150930182411.GA15250@redhat.com>
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On Wed, 30 Sep 2015 20:24:11 +0200 Oleg Nesterov <oleg@redhat.com> wrote:

> Both "child->mm == mm" and "p->mm != mm" checks in oom_kill_process()
> are wrong. task->mm can be NULL if the task is the exited group leader.
> This means in particular that "kill sharing same memory" loop can miss
> a process with a zombie leader which uses the same ->mm.
> 
> Note: the process_has_mm(child, p->mm) check is still not 100% correct,
> p->mm can be NULL too. This is minor, but probably deserves a fix or a
> comment anyway.
> 
> ...
>
> +static bool process_shares_mm(struct task_struct *p, struct mm_struct *mm)
> +{
> +	struct task_struct *t;
> +
> +	for_each_thread(p, t) {
> +		struct mm_struct *t_mm = READ_ONCE(t->mm);
> +		if (t_mm)
> +			return t_mm == mm;
> +	}
> +	return false;
> +}

Guys, please don't write write-only code.  This function is deeply
unobvious and I don't think a typical reader will have a hope of
understanding why things are this way.

I had a lame attempt:

--- a/mm/oom_kill.c~mm-oom_kill-fix-the-wrong-task-mm-==-mm-checks-in-oom_kill_process-fix
+++ a/mm/oom_kill.c
@@ -483,6 +483,12 @@ void oom_killer_enable(void)
 	oom_killer_disabled = false;
 }
 
+/*
+ * task->mm can be NULL if the task is the exited group leader.  So to
+ * determine whether the task is using a particular mm, we examine all the
+ * task's threads: if one of those is using this mm then this task was also
+ * using it.
+ */
 static bool process_shares_mm(struct task_struct *p, struct mm_struct *mm)
 {
 	struct task_struct *t;
_

Which makes me wonder if "process_shared_mm" or even
"process_used_to_share_mm" would be better names...