Re: [PATCH] mm: add stack trace when bad rss-counter state is detected

[Xuanye Liu <liuqiye2025@163.com>]

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在 2025/7/23 17:22, Lorenzo Stoakes 写道:
> On Wed, Jul 23, 2025 at 05:14:19PM +0800, Xuanye Liu wrote:
>> 在 2025/7/23 17:10, Xuanye Liu 写道:
>>> 在 2025/7/23 16:42, David Hildenbrand 写道:
>>>> On 23.07.25 10:05, David Hildenbrand wrote:
>>>>> On 23.07.25 09:45, Xuanye Liu wrote:
>>>>>> 在 2025/7/23 15:31, Kees Cook 写道:
>>>>>>> On Wed, Jul 23, 2025 at 03:23:49PM +0800, Xuanye Liu wrote:
>>>>>>>> The check_mm() function verifies the correctness of rss counters in
>>>>>>>> struct mm_struct. Currently, it only prints an alert when a bad
>>>>>>>> rss-counter state is detected, but lacks sufficient context for
>>>>>>>> debugging.
>>>>>>>>
>>>>>>>> This patch adds a dump_stack() call to provide a stack trace when
>>>>>>>> the rss-counter state is invalid. This helps developers identify
>>>>>>>> where the corrupted mm_struct is being checked and trace the
>>>>>>>> underlying cause of the inconsistency.
>>>>>>> Why not just convert the pr_alert to a WARN?
>>>>>> Good idea! I'll gather more feedback from others and then update to v2.
>>>>> Makes sense to me.
>>>> After discussion this with Lorenzo off-list, isn't the stack completely misleading/useless in that case?
>>>>
>>>> Whatever caused the RSS counter mismatch (e.g., unmapped the wrong pages, missed to unmap pages) quite possibly happened in different context, way way earlier.
>>>>
>>>> Why would you think the stack trace would be of any value when destroying an MM (__mmdrop)?
>>>>
>>>> Having that said, I really hate these "pr_*("BUG: ...") with passion. Probably we'd want to invoke the panic_on_warn machinery, because something unexpected happened.
>>>>
>>> The stack trace dumped here may indeed not reflect the root cause ——
>>> the actual error could have occurred much earlier, for example during a
>>> failed or missing page map/unmap operation.
>>> The current stack (e.g., in __mmdrop() or exit_mmap()) is merely part
>>> of the cleanup phase.
>> Dumping the stack still has some chance of helping identify the issue — at the very least, it
>> shows which task triggered the check.
> The stack will be actively misleading because it's highly likely to be totally
> unrelated.
>
> if you want to know the task, just output current->comm  :)
>
> I think it's not only of no value, it's _ACTIVELY_ misleading. So it's
> definitely a no to a dump_stack().
>
> I am also not in favour of a WARN_ON() for the same reason.
>
> Really we should be catching these elsewhere.
>
> If you want to send the patch just outputting thet ask then all good.

we can start by adding |current->comm| and |task_pid_nr(current)| to help identify the triggering task. 

As for possible detection or monitoring mechanisms, we can continue the discussion.


>
>>> Given that, how should we go about identifying the root cause when such an issue occurs?
>>>
>>> Is there any existing way to trace it more effectively, or could we introduce a new mechanism
>>> to monitor and detect these inconsistencies earlier?
>>>
>>> Let’s brainstorm possible solutions together.
>>>
>> --
>> Thanks,
>> Xuanye
>>
-- 
Thanks,
Xuanye

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