Re: [PATCH RFC 01/14] mm/memory: drop highest_memmap_pfn sanity check in vm_normal_page()

[David Hildenbrand]

On 20.06.25 14:50, Oscar Salvador wrote:
> On Tue, Jun 17, 2025 at 05:43:32PM +0200, David Hildenbrand wrote:
> > In 2009, we converted a VM_BUG_ON(!pfn_valid(pfn)) to the current
> > highest_memmap_pfn sanity check in commit 22b31eec63e5 ("badpage:
> > vm_normal_page use print_bad_pte"), because highest_memmap_pfn was
> > readily available.
> >
> > Nowadays, this is the last remaining highest_memmap_pfn user, and this
> > sanity check is not really triggering ... frequently.
> >
> > Let's convert it to VM_WARN_ON_ONCE(!pfn_valid(pfn)), so we can
> > simplify and get rid of highest_memmap_pfn. Checking for
> > pfn_to_online_page() might be even better, but it would not handle
> > ZONE_DEVICE properly.
> >
> > Do the same in vm_normal_page_pmd(), where we don't even report a
> > problem at all ...
> >
> > What might be better in the future is having a runtime option like
> > page-table-check to enable such checks dynamically on-demand. Something
> > for the future.
> >
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Hi Oscar,

> I'm confused, I'm missing something here.
> Before this change we would return NULL if e.g: pfn > highest_memmap_pfn, but
> now we just print the warning and call pfn_to_page() anyway.
> AFAIK, pfn_to_page() doesn't return NULL?

You're missing that vm_normal_page_pmd() was created as a copy from 
vm_normal_page() [history of the sanity check above], but as we don't 
have (and shouldn't have ...) print_bad_pmd(), we made the code look 
like this would be something that can just happen.

"
Do the same in vm_normal_page_pmd(), where we don't even report a
problem at all ...
"

So we made something that should never happen a runtime sanity check 
without ever reporting a problem ...

--
Cheers,

David / dhildenb