Re: [PATCH v2 6/9] mm/memory: convert print_bad_pte() to print_bad_page_map()

Demi Marie Obenour
2025-07-17T22:07:08Z

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On 7/17/25 07:52, David Hildenbrand wrote:
> print_bad_pte() looks like something that should actually be a WARN
> or similar, but historically it apparently has proven to be useful to
> detect corruption of page tables even on production systems -- report
> the issue and keep the system running to make it easier to actually detect
> what is going wrong (e.g., multiple such messages might shed a light).
>
> As we want to unify vm_normal_page_*() handling for PTE/PMD/PUD, we'll have
> to take care of print_bad_pte() as well.
>
> Let's prepare for using print_bad_pte() also for non-PTEs by adjusting the
> implementation and renaming the function -- we'll rename it to what
> we actually print: bad (page) mappings. Maybe it should be called
> "print_bad_table_entry()"? We'll just call it "print_bad_page_map()"
> because the assumption is that we are dealing with some (previously)
> present page table entry that got corrupted in weird ways.
>
> Whether it is a PTE or something else will usually become obvious from the
> page table dump or from the dumped stack. If ever required in the future,
> we could pass the entry level type similar to "enum rmap_level". For now,
> let's keep it simple.
>
> To make the function a bit more readable, factor out the ratelimit check
> into is_bad_page_map_ratelimited() and place the dumping of page
> table content into __dump_bad_page_map_pgtable(). We'll now dump
> information from each level in a single line, and just stop the table
> walk once we hit something that is not a present page table.
>
> Use print_bad_page_map() in vm_normal_page_pmd() similar to how we do it
> for vm_normal_page(), now that we have a function that can handle it.
>
> The report will now look something like (dumping pgd to pmd values):
>
> [ 77.943408] BUG: Bad page map in process XXX entry:80000001233f5867
> [ 77.944077] addr:00007fd84bb1c000 vm_flags:08100071 anon_vma: ...
> [ 77.945186] pgd:10a89f067 p4d:10a89f067 pud:10e5a2067 pmd:105327067
>
> Not using pgdp_get(), because that does not work properly on some arm
> configs where pgd_t is an array. Note that we are dumping all levels
> even when levels are folded for simplicity.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Should this still use a WARN? If the admin sets panic-on-warn they
have asked for "crash if anything goes wrong" and so that is what
they should get. Otherwise the system will still stay up.
--
Sincerely,
Demi Marie Obenour (she/her/hers)

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