linux-kernel - Re: [PATCH] sched/numa: Fix NULL pointer access to mm

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Message-ID: <20250703072608.GS1613200@noisy.programming.kicks-ass.net>
Date: Thu, 3 Jul 2025 09:26:08 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Chen Yu <yu.c.chen@...el.com>
Cc: Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tim Chen <tim.c.chen@...el.com>, linux-kernel@...r.kernel.org,
	Jirka Hladky <jhladky@...hat.com>,
	Srikanth Aithal <Srikanth.Aithal@....com>,
	Suneeth D <Suneeth.D@....com>, Libo Chen <libo.chen@...cle.com>
Subject: Re: [PATCH] sched/numa: Fix NULL pointer access to mm_struct durng
 task swap

On Thu, Jul 03, 2025 at 12:32:47AM +0800, Chen Yu wrote:

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 8988d38d46a3..4e06bb955dad 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3364,7 +3364,14 @@ static void __migrate_swap_task(struct task_struct *p, int cpu)
>  {
>  	__schedstat_inc(p->stats.numa_task_swapped);
>  	count_vm_numa_event(NUMA_TASK_SWAP);
> -	count_memcg_event_mm(p->mm, NUMA_TASK_SWAP);
> +	/* exiting task has NULL mm */
> +	if (!(p->flags & PF_EXITING)) {
> +		WARN_ONCE(!p->mm, "swap task %d %s %x has no mm\n",
> +			  p->pid, p->comm, p->flags);
> +
> +		if (p->mm)
> +			count_memcg_event_mm(p->mm, NUMA_TASK_SWAP);
> +	}

Aside from the things already mentioned by Andrew and Michal; why not
simply do something like:

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 87b6688f124a..8396ebfab0d5 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -969,7 +969,7 @@ static inline void count_memcg_events_mm(struct mm_struct *mm,
 {
 	struct mem_cgroup *memcg;
 
-	if (mem_cgroup_disabled())
+	if (mem_cgroup_disabled() || !mm)
 		return;
 
 	rcu_read_lock();