Re: [PATCH v8 1/3] mm/memory hotplug: Fix zone->contiguous always false when hotplug

["Li, Tianyou" <tianyou.li@intel.com>]

Appreciated for your review, Mike.

On 1/22/2026 7:16 PM, Mike Rapoport wrote:
> Hi,
>
> On Tue, Jan 20, 2026 at 10:33:44PM +0800, Tianyou Li wrote:
>> From: Yuan Liu <yuan1.liu@intel.com>
>>
>> set_zone_contiguous() uses __pageblock_pfn_to_page() to detect
>> pageblocks that either do not exist (hole) or that do not belong
>> to the same zone.
>>
>> __pageblock_pfn_to_page(), however, relies on pfn_to_online_page(),
>> effectively always returning NULL for memory ranges that were not
>> onlined yet. So when called on a range-to-be-onlined, it indicates
>> a memory hole to set_zone_contiguous().
>>
>> Consequently, the set_zone_contiguous() call in move_pfn_range_to_zone(),
>> which happens early during memory onlining, will never detect a
>> zone as being contiguous. Bad.
>>
>> To fix the issue, move the set_zone_contiguous() call to a later
>> stage in memory onlining, where pfn_to_online_page() will succeed:
>> after we mark the memory sections to be online.
>>
>> Fixes: 2d070eab2e82 ("mm: consider zone which is not fully populated to have holes")
> cc stable@ perhaps?

Yes, will do. Thanks.

>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Reviewed-by: Nanhai Zou <nanhai.zou@intel.com>
>> Signed-off-by: Yuan Liu <yuan1.liu@intel.com>
>> Signed-off-by: Tianyou Li <tianyou.li@intel.com>
>> ---
>>   mm/memory_hotplug.c | 9 +++++++--
>>   1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index a63ec679d861..c8f492b5daf0 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -782,8 +782,6 @@ void move_pfn_range_to_zone(struct zone *zone, unsigned long start_pfn,
>>   	memmap_init_range(nr_pages, nid, zone_idx(zone), start_pfn, 0,
>>   			 MEMINIT_HOTPLUG, altmap, migratetype,
>>   			 isolate_pageblock);
>> -
>> -	set_zone_contiguous(zone);
> move_pfn_range_to_zone() is also called from memremap::pagemap_range().
> Shouldn't we add set_zone_contiguous() there as well?

I did not find the place where the online_pages was invoked along path 
of the memremap:pagemap_range() function. Would there be other functions 
to online the pages remapped? Much appreciated for the guidance.

I leave the zone contiguous state remains the same as the optimization 
not took place in case any unexpected behavior.

>>   }
>>   
>>   struct auto_movable_stats {
>> @@ -1205,6 +1203,13 @@ int online_pages(unsigned long pfn, unsigned long nr_pages,
>>   	}
>>   
>>   	online_pages_range(pfn, nr_pages);
>> +
>> +	/*
>> +	 * Now that the ranges are indicated as online, check whether the whole
>> +	 * zone is contiguous.
>> +	 */
>> +	set_zone_contiguous(zone);
>> +
>>   	adjust_present_page_count(pfn_to_page(pfn), group, nr_pages);
>>   
>>   	if (node_arg.nid >= 0)
>> -- 
>> 2.47.1
>>