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

[David Hildenbrand <david@xxxxxxxxxx>]

On 18.07.25 09:59, Demi Marie Obenour wrote:
> On 7/18/25 03:44, David Hildenbrand wrote:
> > 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@xxxxxxxxxx>
> > > 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.
> Which one?  I'm only subscribed to xen-devel and might have missed it.
This one here:

https://lkml.kernel.org/r/4e1b7d2d-ed54-4e0a-a0a4-906b14d9cd41@p183

--
Cheers,

David / dhildenb