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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

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.