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

[Xuanye Liu <liuqiye2025@163.com>]

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.                                                       

>
> 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