Re: [PATCH v7 2/2] mm/memory_hotplug: optimize zone contiguous check when changing pfn range

[David Hildenbrand (Arm)]

On 8/19/26 16:56, Mike Rapoport wrote:
> On Tue, Aug 18, 2026 at 04:57:02AM -0400, Yuan Liu wrote:
>> When move_pfn_range_to_zone() or remove_pfn_range_from_zone() updates a
>> zone, set_zone_contiguous() rescans the entire zone pageblock-by-pageblock
>> to rebuild zone->contiguous. For large zones this is a significant cost
>> during memory hotplug and hot-unplug.
>>
>> Add a new zone member, pages_with_online_memmap, that tracks the
>> number of pages within the zone span that have an online memory map,
>> including present pages and memory holes whose memory map has been
>> initialized and for which pfn_to_online_page() succeeds.
>>
>> For early boot memory, pages_with_online_memmap is calculated in
>> memmap_init_zone_range(). PFNs initialized by memmap_init_range() are
>> included in pages_with_online_memmap, and hole PFNs for which
>> pfn_to_online_page() succeeds are also counted in
>> init_unavailable_range().
> 
> In what scenario it is possible that pfn_to_online_page() would fail at
> that point?
pfn_to_online_page()==false even though pfn_valid()==true in case we have a
subsection-sized hole in an early section.

-- 
Cheers,

David