Re: [PATCH v7 1/2] mm/memory_hotplug: make shrink_zone_span() more robust

[David Hildenbrand (Arm)]

On 8/21/26 05:14, Wei Yang wrote:
> On Tue, Aug 18, 2026 at 04:57:01AM -0400, Yuan Liu wrote:
>> From: "David Hildenbrand (Arm)" <david@xxxxxxxxxx>
>>
>> Let's make shrink_zone_span() more robust by checking in
>> find_smallest_section_pfn() / find_biggest_section_pfn() that the
>> start and end PFNs of the subsection are within the zone.
>>
>> While at it, clean up the function by factoring the core check out
>> into subsection_overlaps_zone().
>>
>> There likely is no need to check the nid first. We require
>> SPARSEMEM_VMEMMAP_ENABLE, where pfn_to_page() is cheap, and
>> pfn_to_nid() on CONFIG_NUMA would call pfn_to_page() either way.
>> So let's just drop that for now.
>>
>> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>> Tested-by: Yuan Liu <yuan1.liu@xxxxxxxxx>
>> Signed-off-by: Yuan Liu <yuan1.liu@xxxxxxxxx>
>> ---
>> mm/memory_hotplug.c | 59 ++++++++++++++++++---------------------------
>> 1 file changed, 24 insertions(+), 35 deletions(-)
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 7ac19fab2263..cd82e79f0782 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -422,49 +422,39 @@ int __add_pages(int nid, unsigned long pfn, unsigned long nr_pages,
>> 	return err;
>> }
>>
>> -/* find the smallest valid pfn in the range [start_pfn, end_pfn) */
>> -static unsigned long find_smallest_section_pfn(int nid, struct zone *zone,
>> -				     unsigned long start_pfn,
>> -				     unsigned long end_pfn)
>> +static bool subsection_overlaps_zone(unsigned long pfn, struct zone *zone)
>> {
>> -	for (; start_pfn < end_pfn; start_pfn += PAGES_PER_SUBSECTION) {
>> -		if (unlikely(!pfn_to_online_page(start_pfn)))
>> -			continue;
>> +	const unsigned long start_pfn = ALIGN_DOWN(pfn, PAGES_PER_SUBSECTION);
>> +	const unsigned long end_pfn = start_pfn + PAGES_PER_SUBSECTION - 1;
>>
>> -		if (unlikely(pfn_to_nid(start_pfn) != nid))
>> -			continue;
> 
> Would we have a memory layout with node interleave?

I think core-mm is not prepared for actual overlapping zone ranges during boot.

-- 
Cheers,

David