Re: [PATCH v9 2/2] mm/memory_hotplug: optimize zone contiguous check when changing pfn range

[David Hildenbrand (Arm)]

On 9/15/26 08:03, Mike Rapoport wrote:
> Hi,
> 
> On Mon, Sep 14, 2026 at 03:29:29AM -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.
>>
>> diff --git a/mm/mm_init.c b/mm/mm_init.c
>> index 1533aebafb68..d40a8ff23370 100644
>> --- a/mm/mm_init.c
>> +++ b/mm/mm_init.c
>> @@ -817,22 +817,39 @@ void __meminit init_deferred_page(unsigned long pfn, int nid)
>>   *   zone/node above the hole except for the trailing pages in the last
>>   *   section that will be appended to the zone/node below.
>>   */
>> -static void __init init_unavailable_range(unsigned long spfn,
>> -					  unsigned long epfn,
>> -					  int zone, int node)
>> +static unsigned long __init init_unavailable_range(unsigned long spfn,
>> +						   unsigned long epfn,
>> +						   int zone, int node)
>>  {
>> +	unsigned long next_chunk_pfn __maybe_unused = spfn;
>>  	unsigned long pfn;
>> -	u64 pgcnt = 0;
>> +	u64 online_pgcnt = 0, pgcnt = 0;
>> +	bool is_online = true;
>>  
>>  	for_each_valid_pfn(pfn, spfn, epfn) {
>>  		__init_single_page(pfn_to_page(pfn), pfn, zone, node);
>>  		__SetPageReserved(pfn_to_page(pfn));
>>  		pgcnt++;
>> +
>> +		/*
>> +		 * With vmemmap, at this stage all pages in an early section
>> +		 * have a valid memmap and are marked as online. However, only
>> +		 * subsections in the subsection map are actually online.
>> +		 */
> 
> I'm having trouble parsing this comment. Shouldn't is say that some
> subsections in a section can be offline because of holes?


Maybe extending that a bit we could do

"With CONFIG_SPARSEMEM_VMEMMAP, early sections have a valid memmap for all PFNs
and the sections are marked online. However, we might have offline subsections
in such early sections, indicated by the subsection map. While we must
initialize the entire valid memmap, account only the online pages according to
the subsection map as online (see pfn_to_online_page())."


-- 
Cheers,

David