linux-kernel - Re: [PATCH -mm v2 3/3] mm/oom_kill: fix the wrong task->mm == mm checks in oom_kill

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Date:	Thu, 1 Oct 2015 15:24:58 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Kyle Walker <kwalker@...hat.com>,
	Michal Hocko <mhocko@...nel.org>,
	Stanislav Kozina <skozina@...hat.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm v2 3/3] mm/oom_kill: fix the wrong task->mm == mm
 checks in oom_kill_process()

On Wed, 30 Sep 2015 20:24:11 +0200 Oleg Nesterov <oleg@...hat.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...

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