kernel/fork.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
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.
Signed-off-by: Xuanye Liu <liuqiye2025@163.com>
---
kernel/fork.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/kernel/fork.c b/kernel/fork.c
index cfe2f1df5f27..d38f1c5270ea 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -584,9 +584,11 @@ static void check_mm(struct mm_struct *mm)
for (i = 0; i < NR_MM_COUNTERS; i++) {
long x = percpu_counter_sum(&mm->rss_stat[i]);
- if (unlikely(x))
+ if (unlikely(x)) {
pr_alert("BUG: Bad rss-counter state mm:%p type:%s val:%ld\n",
mm, resident_page_types[i], x);
+ dump_stack();
+ }
}
if (mm_pgtables_bytes(mm))
--
2.43.0On 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?
-Kees
>
> Signed-off-by: Xuanye Liu <liuqiye2025@163.com>
> ---
> kernel/fork.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index cfe2f1df5f27..d38f1c5270ea 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -584,9 +584,11 @@ static void check_mm(struct mm_struct *mm)
> for (i = 0; i < NR_MM_COUNTERS; i++) {
> long x = percpu_counter_sum(&mm->rss_stat[i]);
>
> - if (unlikely(x))
> + if (unlikely(x)) {
> pr_alert("BUG: Bad rss-counter state mm:%p type:%s val:%ld\n",
> mm, resident_page_types[i], x);
> + dump_stack();
> + }
> }
>
> if (mm_pgtables_bytes(mm))
> --
> 2.43.0
>
--
Kees Cook
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.
>
> -Kees
>
>> Signed-off-by: Xuanye Liu <liuqiye2025@163.com>
>> ---
>> kernel/fork.c | 4 +++-
>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index cfe2f1df5f27..d38f1c5270ea 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -584,9 +584,11 @@ static void check_mm(struct mm_struct *mm)
>> for (i = 0; i < NR_MM_COUNTERS; i++) {
>> long x = percpu_counter_sum(&mm->rss_stat[i]);
>>
>> - if (unlikely(x))
>> + if (unlikely(x)) {
>> pr_alert("BUG: Bad rss-counter state mm:%p type:%s val:%ld\n",
>> mm, resident_page_types[i], x);
>> + dump_stack();
>> + }
>> }
>>
>> if (mm_pgtables_bytes(mm))
>> --
>> 2.43.0
>>
--
Thanks,
Xuanye
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. -- Cheers, David / dhildenb
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.
--
Cheers,
David / dhildenb
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.
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
On 7/23/25 11:10, Xuanye Liu wrote:
>
> 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.
>
> 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.
Excellent idea! How about we introduce a function that walks the whole page
tables and checks the numbers of individual pte types against the rss
counters. And if we invoke it before and after every single pte update, we
can pinpoint much sooner the moment it went wrong and the stack that lead to it?
On 23.07.25 11:17, Vlastimil Babka wrote:
> On 7/23/25 11:10, Xuanye Liu wrote:
>>
>> 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.
>>
>> 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.
>
> Excellent idea! How about we introduce a function that walks the whole page
> tables and checks the numbers of individual pte types against the rss
> counters. And if we invoke it before and after every single pte update, we
> can pinpoint much sooner the moment it went wrong and the stack that lead to it?
>
:)
On a more serious note, I ran into that usually after hitting a bunch of
print_bad_pte() statements: vm_normal_page() would return NULL where it
shouldn't, making us not adjust the RSS.
In which context would you run into this issue?
Usually it really indicates some fundamental page table handling flaw.
E.g., page table corruption leading to print_bad_pte() earlier.
--
Cheers,
David / dhildenb
On Wed, Jul 23, 2025 at 11:17:16AM +0200, Vlastimil Babka wrote:
> On 7/23/25 11:10, Xuanye Liu wrote:
> >
> > 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.
> >
> > 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.
>
> Excellent idea! How about we introduce a function that walks the whole page
> tables and checks the numbers of individual pte types against the rss
> counters. And if we invoke it before and after every single pte update, we
> can pinpoint much sooner the moment it went wrong and the stack that lead to it?
:)))))))
Haha well, I think this might be one of those reductio ad absurdum arguments :P
Yes the point is we can't figure out where this happened in any sane way if the
root cause doesn't have sufficient checks.
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
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.
>
> >
> > 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
>
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