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

David Hildenbrand
2025-07-18T07:44:38Z

On 18.07.25 00:06, Demi Marie Obenour wrote:
> 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.

I assume you're comment is in context of the other proposal regarding
panicking.

It's a good question whether we should WARN: likely we should convert
the "BUG:" ... message into a WARN. On panic-on-warn you'd panic
immediately without being able to observe any other such messages (and
as discussed in the RFC, apparently that can be valuable for debugging,
because a single such report is often insufficient)

But as panic-on-warn is "panic on the first sight of a problem", that
sounds right.

That change should not be part of this patch, though.

--
Cheers,

David / dhildenb