linux-kernel - Re: [PATCH v9] mm/memory hotplug: Fix zone->contiguous always false when hotplug

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Message-ID: <a670fc3c-e298-4430-9c41-d51598d25e06@intel.com>
Date: Fri, 6 Feb 2026 21:56:29 +0800
From: "Li, Tianyou" <tianyou.li@...el.com>
To: "David Hildenbrand (Arm)" <david@...nel.org>, Oscar Salvador
	<osalvador@...e.de>, Mike Rapoport <rppt@...nel.org>, Wei Yang
	<richard.weiyang@...il.com>, Michal Hocko <mhocko@...e.com>
CC: <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 v9] mm/memory hotplug: Fix zone->contiguous always false
 when hotplug


On 2/6/2026 9:00 PM, David Hildenbrand (Arm) wrote:
> On 1/30/26 17:06, 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: 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@...e.com>
>> Cc: stable@...r.kernel.org
>> 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>
>>
>
>
> I assume this one is to be ignored :)
>

Thanks David for your review. Yes, this one should be ignored. I should 
clarify that ahead. Thanks.

Regards,

Tianyou