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

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Message-ID: <50309e8c-c046-4052-b112-8521d6141736@intel.com>
Date: Thu, 3 Jul 2025 17:24:09 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Andrew Morton <akpm@...ux-foundation.org>
CC: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	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>, 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

Hi Andrew,

On 7/3/2025 5:08 AM, Andrew Morton wrote:
> On Thu,  3 Jul 2025 00:32:47 +0800 Chen Yu <yu.c.chen@...el.com> wrote:
> 
>> It was reported that after Commit ad6b26b6a0a7
>> ("sched/numa: add statistics of numa balance task"),
>> a NULL pointer exception[1] occurs when accessing
>> p->mm. The following race condition was found to
>> trigger this bug: After a swap task candidate is
>> chosen during NUMA balancing, its mm_struct is
>> released due to task exit. Later, when the task
>> swapping is performed, p->mm is NULL, which causes
>> the problem:
>>
>> CPU0                                   CPU1
>> :
>> ...
>> task_numa_migrate
>>     task_numa_find_cpu
>>      task_numa_compare
>>        # a normal task p is chosen
>>        env->best_task = p
>>
>>                                          # p exit:
>>                                          exit_signals(p);
>>                                             p->flags |= PF_EXITING
>>                                          exit_mm
>>                                             p->mm = NULL;
>>
>>      migrate_swap_stop
>>        __migrate_swap_task((arg->src_task, arg->dst_cpu)
>>         count_memcg_event_mm(p->mm, NUMA_TASK_SWAP)# p->mm is NULL
>>
>> Fix this issue by checking if the task has the PF_EXITING
>> flag set in migrate_swap_stop(). If it does, skip updating
>> the memcg events. Additionally, log a warning if p->mm is
>> NULL to facilitate future debugging.
>>
>> ...
>>
>> --- 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);
>> +	}
> 
> I don't think we should warn on a condition which is known to occur and
> which we successfully handle.  What action can anyone take upon that
> warning?
> 
> Which means the change might as well become
> 
> +	/* comment goes here */
> +	if (p->mm)
> +		count_memcg_event_mm(p->mm, NUMA_TASK_SWAP);
> 
> But is that a real fix?  Can the other thread call exit(), set
> PF_EXITING and null its p->mm right between the above two lines?  After
> the p->mm test and before the count_memcg_event_mm() call?
> 

Thank you for the scenario description. Right, there is still a race
condition although the p->pi_lock is hold when reaching here, the
p->alloc_lock should be hold to access p->mm.

> IOW, there needs to be some locking in place to stabilize p->mm
> throughout the p->mm test and the count_memcg_event_mm() call?

Yes, Michal and Peter have given some feedback on how to do this.

thanks,
Chenyu