On Tue, Jan 13, 2026 at 10:31 AM David Hildenbrand (Red Hat)
[off-list ref] wrote:

On 1/13/26 11:22, Mostafa Saleh wrote:

quoted

On Mon, Jan 12, 2026 at 7:12 PM David Hildenbrand (Red Hat)
[off-list ref] wrote:

quoted

On 1/12/26 15:58, Mostafa Saleh wrote:

quoted

On Mon, Jan 12, 2026 at 1:52 PM Jason Gunthorpe [off-list ref] wrote:

quoted

On Mon, Jan 12, 2026 at 01:43:41PM +0000, Mostafa Saleh wrote:

quoted

But I don’t see why not. from the documentation:
/**
   * pfn_valid - check if there is a valid memory map entry for a PFN
   * @pfn: the page frame number to check
   *
   * Check if there is a valid memory map entry aka struct page for the @pfn.
   * Note, that availability of the memory map entry does not imply that
   * there is actual usable memory at that @pfn. The struct page may
   * represent a hole or an unusable page frame.
…

That means that struct page exists, which is all what we need here.

A struct page that has never been initialize shouldn't ever be read. I
don't know how that relates to page_ext, but are you really sure that
is all you need?

AFAIU, if pfn_valid() returns true, it means the struct page is valid,
and lookup_page_ext() will check that a valid page_ext exists for this
entry.

Not always. Offline memory blocks have a memory map but no page ext. We
allocate the page ext at memory onlining time.

Also, I'm not sure about ZONE_DEVICE memory, very likely we never
allocate a page_ext for them?

I'd assume both cases are not relevant for your use case, though.

 From my understanding, in that case, page_ext_get() will return NULL.

So, as long as struct page exists, page_ext_get won't misbehave.

I am not sure about offline memory, but MMIO can be used. We use
pfn_valid() before getting the struct page that we pass to page_ext.
Would that be OK?

It's tricky. If you look at lookup_page_ext(), it relies on extracting
the pfn+nid from the "struct page".

If the "struct page" is uninitialized (e.g., offline memory) that cannot
possibly work, as it could just give you random garbage.

(note that there are two implementations of lookup_page_ext(), both
extracting the PFN but only one extracting the NID).

I see, thanks for the clarification.

I see that pfn_to_online_page() exits, which seems to handle online
memory and ZONE_DEVICE. Would that be a suitable alternative?

Would you have a problem if we added a new function in page_ext
"page_ext_from_phys()" as Jason suggested?

Thanks,
Mostafa


Thanks,
Mostafa

--
Cheers

David