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

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Message-ID: <aXIHG8MQhMRt7M5d@kernel.org>
Date: Thu, 22 Jan 2026 13:16:43 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Tianyou Li <tianyou.li@...el.com>
Cc: David Hildenbrand <david@...nel.org>,
	Oscar Salvador <osalvador@...e.de>,
	Wei Yang <richard.weiyang@...il.com>,
	Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
	Yong Hu <yong.hu@...el.com>, Nanhai Zou <nanhai.zou@...el.com>,
	Yuan Liu <yuan1.liu@...el.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Qiuxu Zhuo <qiuxu.zhuo@...el.com>, Yu C Chen <yu.c.chen@...el.com>,
	Pan Deng <pan.deng@...el.com>, Chen Zhang <zhangchen.kidd@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 1/3] mm/memory hotplug: Fix zone->contiguous always
 false when hotplug

Hi,

On Tue, Jan 20, 2026 at 10:33:44PM +0800, Tianyou Li wrote:
> From: Yuan Liu <yuan1.liu@...el.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?

> Cc: Michal Hocko <mhocko@...e.com>
> Reviewed-by: Nanhai Zou <nanhai.zou@...el.com>
> Signed-off-by: Yuan Liu <yuan1.liu@...el.com>
> Signed-off-by: Tianyou Li <tianyou.li@...el.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?

>  }
>  
>  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
> 

-- 
Sincerely yours,
Mike.