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

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Message-ID: <2bf497eb-47b0-4eb9-a66e-607c82c36c20@kernel.org>
Date: Fri, 6 Feb 2026 14:00:56 +0100
From: "David Hildenbrand (Arm)" <david@...nel.org>
To: Tianyou Li <tianyou.li@...el.com>, 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 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 :)

-- 
Cheers,

David