Re: [PATCH] sched/numa: Fix NULL pointer access to mm_struct durng task swap

["Chen, Yu C" <yu.c.chen@intel.com>]

Hi Michal,

On 7/3/2025 3:18 PM, Michal Hocko wrote:
> On Thu 03-07-25 00:32:47, Chen Yu 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.
>>
>> Fixes: ad6b26b6a0a7 ("sched/numa: add statistics of numa balance task")
>> Reported-by: Jirka Hladky <jhladky@redhat.com>
>> Closes: https://lore.kernel.org/all/CAE4VaGBLJxpd=NeRJXpSCuw=REhC5LWJpC29kDy-Zh2ZDyzQZA@mail.gmail.com/
>> Reported-by: Srikanth Aithal <Srikanth.Aithal@amd.com>
>> Reported-by: Suneeth D <Suneeth.D@amd.com>
>> Suggested-by: Libo Chen <libo.chen@oracle.com>
>> Signed-off-by: Chen Yu <yu.c.chen@intel.com>
>> ---
>>   kernel/sched/core.c | 9 ++++++++-
>>   1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> 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);
> 
> As Andrew already said this is not really acceptable because this is
> very likely too easy to trigger and a) you do not want logs flooded with
> warnings and also there are setups with panic_on_warn configured and for
> those this would be a fatal situation without any good reason.
> 

OK, got it, thanks for pointing it out.

>> +
>> +		if (p->mm)
>> +			count_memcg_event_mm(p->mm, NUMA_TASK_SWAP);
>> +	}
> 
> Why are you testing for p->mm here? Isn't PF_EXITING test sufficient?
> A robust way to guarantee non-NULL mm against races when a task is
> exiting is find_lock_task_mm. Probably too heavy weight for this path.

I suppose we might only need to grab task_lock(p), check if its mm
pointer is NULL. If yes, we skip the update of memcg event without
scanning for a non-NULL mm within the process(as find_lock_task_mm()
does)? If the mm is non-NULL, we update the memcg event with task_lock(p)
hold and releases it later.

thanks,
Chenyu

>>   
>>   	if (task_on_rq_queued(p)) {
>>   		struct rq *src_rq, *dst_rq;
>> -- 
>> 2.25.1
>>
>