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

[Libo Chen]

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?


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;