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Content-Language: en-US From: "Yin, Fengwei" To: Matthew Wilcox , Zi Yan CC: , Andrew Morton , Yang Shi , Huang Ying , "Kirill A . Shutemov" , Ryan Roberts , References: <20240311195848.135067-1-zi.yan@sent.com> <74AF044A-A14A-4C66-A019-70F8F138A9AB@nvidia.com> <76BBDB5D-9D40-44EB-A996-767404B85BE0@nvidia.com> In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: SI2PR04CA0014.apcprd04.prod.outlook.com (2603:1096:4:197::18) To CO1PR11MB4820.namprd11.prod.outlook.com (2603:10b6:303:6f::8) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CO1PR11MB4820:EE_|SA2PR11MB5065:EE_ X-MS-Office365-Filtering-Correlation-Id: 23d47aff-104d-4225-4c0b-08dc43060ae6 X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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9r1GabjQuESyz3KXlNhyaFZWetsxgJVsErHn3NNTLKXxj2zYnkyI93Ti3qWZS3TVpEM9MRaQhVfR5H2eYFOn3rwzqLXSxkJ+ZsJ6TweOywd3zP2/9Czxz1BUKvFomMWlRlQiq3jDOHi4GdM9dLl18CU+1MkqwSJFQy3IL/5qK8cN74a+cemHQ/1wx98Tz0b6p2vGyoijZn+tk6FMZ9oydpdF8IB1fIAH0RpUsbL99EskSlTGYf0uUt+g4HIBzD3xyUmfPFBMH/QCz5ZQInBxC3/J2SAqzcHpJuiyWfAD4K+RcREoE4JSiKQn9/x/pDGki4oYmgVr/L6i5fQRIZUBwebAfZQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/13/2024 10:07 AM, Yin, Fengwei wrote: > > > On 3/13/2024 2:46 AM, Matthew Wilcox wrote: >> On Tue, Mar 12, 2024 at 02:32:43PM -0400, Zi Yan wrote: >>> On 12 Mar 2024, at 12:38, Matthew Wilcox wrote: >>>> Folios with a positive refcount are >>>> removed from the per-node or per-cgroup list _at which point there is >>>> an undocumented assumption_ that they will not be removed from the >>>> local list because they have a positive refcount. >>> >>> But that sounds very subtle if not broken. As an outsider of >> >> I merely deduced this requirement; I didn't come up with it ... > My understanding is that this requirement is because of just local > list in deferred_split_scan(). > > Using fbatch instead of local list here as your created for that > issue debugging can eliminate this subtlety? May not good idea because it's possible the folios in fbatch can be removed from deferred_list by migration. > > > Regards > Yin, Fengwei > >> >>> deferred_split_scan(), only !list_empty(folio->_deferred_list) is >>> checked. >>> The condition can be true if the folio is on split_queue or >>> local list of deferred_split_scan() with elevated refcount. In that >>> case, >>> the folio cannot be removed from the list (either split_queue or >>> local list) >>> even if split_queue_lock is held, since local list manipulation is >>> not under >>> split_queue_lock. This makes _deferred_list a one-way train to anyone >>> except deferred_split_scan(), namely folios can only be added into >>> _deferred_list until they are freed or split by deferred_split_scan(). >>> >>> Is that intended? If yes, maybe we should document it. If not, using >>> split_queue_lock to protect local list, or more explicitly >>> folio->_deferred_list >>> might be better? >> >> To be fair, the folio can be split by anybody as >> split_huge_page_to_list_to_order() is careful to only manipulate the >> deferred list while the refcount is frozen at 0.  I'm still trying to >> figure out where to document this behaviour of the deferred list that >> someone (for example, your good self) would actually see it. >> >> >> >