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

[David Hildenbrand]

On 03.07.25 14:34, Lance Yang wrote:
> On Mon, Jun 23, 2025 at 10:04 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > 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 ...
>
> IIUC, the reasoning is that because this case should never happen, we can
> change the behavior from returning NULL to a "warn and continue" model?

Well, yes. Point is, that check should have never been copy-pasted that 
way, while dropping the actual warning :)

It's a sanity check for something that should never happen, turned into 
something that looks like it must be handled and would be valid to 
encounter.

--
Cheers,

David / dhildenb