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

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

[Tianyou Li]

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: the fast path relies on the zone
contiguous state may skipped.

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: Michal Hocko <mhocko@suse.com>
Cc: stable@vger.kernel.org
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>
---
Separated from: https://lore.kernel.org/linux-mm/20260120143346.1427837-1-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);
 }
 
 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

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

[David Hildenbrand (Arm)]

On 1/30/26 17:09, Tianyou Li wrote:
> From: Yuan Liu <yuan1.liu@intel.com>

Nit: subject should start with "mm/memory_hotplug:"

> 
> 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: the fast path relies on the zone
> contiguous state may skipped.
> 
> 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: Michal Hocko <mhocko@suse.com>
> Cc: stable@vger.kernel.org
> 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>

How was Yuan Liu involved? If he is a co-author there is a

	Co-developed-by: Yuan Liu <yuan1.liu@intel.com>

missing.


Likely there is no resend required if you tell Andrew what to do, so he 
can just fixup when applying.

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David

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

[Li, Tianyou]

On 2/6/2026 9:03 PM, David Hildenbrand (Arm) wrote
> On 1/30/26 17:09, Tianyou Li wrote:
>> From: Yuan Liu <yuan1.liu@intel.com>
>
> Nit: subject should start with "mm/memory_hotplug:"
>

Got it. My bad, will pay attention next time. Thanks David.


>>
>> 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: the fast path relies on the zone
>> contiguous state may skipped.
>>
>> 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: Michal Hocko <mhocko@suse.com>
>> Cc: stable@vger.kernel.org
>> 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>
>
> How was Yuan Liu involved? If he is a co-author there is a
>
>     Co-developed-by: Yuan Liu <yuan1.liu@intel.com>
>
> missing.
>

Yes, Yuan Liu co-author the patch. Thanks and I will pay attention next 
time. Appreciated.


>
> Likely there is no resend required if you tell Andrew what to do, so 
> he can just fixup when applying.
>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
>

+ Andrew Morton for awareness.

Hi Andrew,

Please kindly let me know if resend required or you are willing to 
kindly fixup the titile and Co-developed-by tag for me. Appreciated in 
advance for any additional burden that costs.

Regards,

Tianyou