Re: [BUG] Kernel panic in __migrate_swap_task() on 6.16-rc2 (NULL pointer dereference)

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

Hi Libo,

On 7/1/2025 3:32 PM, Libo Chen wrote:
> Hi Chenyu,
> 
> On 6/27/25 00:33, Chen, Yu C wrote:
>> On 6/27/2025 3:16 PM, Chen, Yu C wrote:
>>> Hi Jirka,
>>>
>>> On 6/27/2025 5:46 AM, Jirka Hladky wrote:
>>>> Hi Chen and all,
>>>>
>>>> we have now verified that the following commit causes a kernel panic
>>>> discussed in this thread:
>>>>
>>>> ad6b26b6a0a79 sched/numa: add statistics of numa balance task
>>>>
>>>> Reverting this commit fixes the issue.
>>>>
>>>> I'm happy to help debug this further or test a proposed fix.
>>>>
>>>
>>> Thanks very much for your report, it seems that there is a
>>> race condition that when the swap task candidate was chosen,
>>> but its mm_struct get released due to task exit, then later
>>> when doing the task swaping, the p->mm is NULL which caused
>>> 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
>>>
>>> Could you please help check if the following debug patch works,
>>
>> Attached the patch:
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 8988d38d46a3..82fc966b390c 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -3364,7 +3364,12 @@ 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);
>> +    if (unlikely(!p->mm)) {
> 
> I am starting to wonder if we should keep this check and add a big fat warning
> like  "there is a bug here, please report it!" rather than brick the kernel.
> A kernel panic, for sure, helps catch bugs like this more than a line of dmesg,
> so it's a tradeoff I guess. What do you think?
> 

I agree with you that adding something like WARN_ON_ONCE()
could help, and it seems that the kernel panic is not expected
in this scenario because this feature is a statistic calculation
rather than the critical logic. Maybe waiting for Jirka's feedback
to decide the next step.

thanks,
Chenyu



> 
> Thanks,
> Libo
> 
>> +        trace_printk("!! (%d %s) flags=%lx\n", p->pid, p->comm,
>> +                p->flags);
>> +    } else {
>> +        count_memcg_event_mm(p->mm, NUMA_TASK_SWAP);
>> +    }
>>
>>       if (task_on_rq_queued(p)) {
>>           struct rq *src_rq, *dst_rq;
>