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Thu, 8 Jan 2026 07:35:37 +0000 Message-ID: <4b9b29aa-b643-439e-b71a-be9f9241b384@intel.com> Date: Thu, 8 Jan 2026 15:35:28 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 1/2] mm/memory hotplug: fix zone->contiguous always false when hotplug To: "David Hildenbrand (Red Hat)" , Oscar Salvador , Mike Rapoport , Wei Yang , Michal Hocko CC: , Yong Hu , Nanhai Zou , Yuan Liu , Tim Chen , Qiuxu Zhuo , Yu C Chen , Pan Deng , Chen Zhang , References: <20251222145807.11351-1-tianyou.li@intel.com> <20251222145807.11351-2-tianyou.li@intel.com> <857be078-1464-4e29-979d-0459cad8508b@kernel.org> Content-Language: en-US From: "Li, Tianyou" In-Reply-To: <857be078-1464-4e29-979d-0459cad8508b@kernel.org> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: SI2PR02CA0050.apcprd02.prod.outlook.com (2603:1096:4:196::11) To MW4PR11MB8289.namprd11.prod.outlook.com (2603:10b6:303:1e8::9) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: MW4PR11MB8289:EE_|IA1PR11MB6395:EE_ X-MS-Office365-Filtering-Correlation-Id: c6ec0bc8-12d7-4341-3827-08de4e8883ed X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|366016; 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The suggestions you made are clear and the code/comments you posted are well formatted, I can even copy/pasted without modification. Thanks. Regards, Tianyou On 1/7/2026 4:03 AM, David Hildenbrand (Red Hat) wrote: > On 12/22/25 15:58, Tianyou Li wrote: >> Function set_zone_contiguous used __pageblock_pfn_to_page to >> check the whole pageblock is in the same zone. One assumption is >> the memory section must online, otherwise the __pageblock_pfn_to_page >> will return NULL, then the set_zone_contiguous will be false. >> When move_pfn_range_to_zone invoked set_zone_contiguous, since the >> memory section did not online, the return value will always be false. >> >> To fix this issue, we removed the set_zone_contiguous from the >> move_pfn_range_to_zone, and place it after memory section onlined. >> >> Function remove_pfn_range_from_zone did not have this issue because >> memory section remains online at the time set_zone_contiguous invoked. > > The description is a bit hard to follow. Let me try: > > > "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" > Will change accordingly.  Thanks. > > Now, there is no need to add the handling to > mhp_init_memmap_on_memory(). Note how mhp_init_memmap_on_memory() in > memory_block_online() is always followed by online_pages(). > Plus there is no dependencies of previous zone contiguous state for the set_zone_contiguous now, it totally makes sense to remove the set_zone_contiguous in mhp_init_memmap_on_memory() as you suggested. > So, it's sufficient to move it after the online_pages_range(). I would > also add a comment there saying something like: > > /* >  * Now that the ranges are indicated as online, check whether the whole >  * zone is contiguous. >  */ > Will change accordingly. Thanks. > > Can we find some Fixes: tag (which commit introduced the regression)? > Likely we want to CC stable. > Yes, probably we can add the tags as below, where the pfn_to_page() changed to pfn_to_online_page() in __pageblock_pfn_to_page(). Fixes: 2d070eab2e82 (mm: consider zone which is not fully populated to have holes) Cc: Michal Hocko