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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Alistair Popple , linuxppc-dev@lists.ozlabs.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, amd-gfx@lists.freedesktop.org, nouveau@lists.freedesktop.org, linux-pci@vger.kernel.org, linux-mm@kvack.org, linux-cxl@vger.kernel.org Subject: Re: [PATCH v4 1/7] mm/zone_device: Add order argument to folio_free callback Date: Mon, 12 Jan 2026 12:46:57 -0500 X-Mailer: MailMate (2.0r6290) Message-ID: <86D91C8B-C3EA-4836-8DC2-829499477618@nvidia.com> In-Reply-To: <20260112165001.GG745888@ziepe.ca> References: <20260111205820.830410-1-francois.dugast@intel.com> <20260111205820.830410-2-francois.dugast@intel.com> <874d29da-2008-47e6-9c27-6c00abbf404a@nvidia.com> <0D532F80-6C4D-4800-9473-485B828B55EC@nvidia.com> <20260112134510.GC745888@ziepe.ca> <218D42B0-3E08-4ABC-9FB4-1203BB31E547@nvidia.com> <20260112165001.GG745888@ziepe.ca> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: SJ0PR03CA0195.namprd03.prod.outlook.com (2603:10b6:a03:2ef::20) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB9473:EE_|DM4PR12MB6012:EE_ X-MS-Office365-Filtering-Correlation-Id: 0ed69ae2-ee15-4670-2fff-08de52029ac4 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|1800799024|366016; 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It knows how much memory to put back because folio->order >>> is valid from #2 >>> 6) #1 happens again, then #2 happens again and the folio is in the >>> right state for use. The successor #2 fully undoes the work of the >>> predecessor #2. >> >> But how can a successor #2 undo the work if the second #1 only allocates >> half of the original folio? For example, an order-9 at PFN 0 is >> allocated and freed, then an order-8 at PFN 0 is allocated and another >> order-8 at PFN 256 is allocated. How can two #2s undo the same order-9 >> without corrupting each other’s data? > > What do you mean? The fundamental rule is you can't read the folio or > the order outside folio_free once it's refcount reaches 0. There is no such a rule. In core MM, folio_split(), which splits a high order folio to low order ones, freezes the folio (turning refcount to 0) and manipulates the folio order and all tail pages compound_head to restructure the folio. Your fundamental rule breaks this. Allowing compound information to stay after a folio is freed means you cannot tell whether a folio is under split or freed. > > So the successor #2 will write updated heads and order to the order 8 > pages at PFN 0 and the ones starting at PFN 256 will remain with > garbage. > > This is OK because nothing is allowed to read them as their refcount > is 0. > > If later PFN256 is allocated then it will get updated head and order > at the same time it's refcount becomes 1. > > There is corruption and they don't corrupt each other's data. > >>> If the allocator is using the struct page memory then step #5 should >>> also clean up the struct page with the allocator data before returning >>> it to the allocator. >> >> Do you mean ->folio_free() callback should undo prep_compound_page() >> instead? > > I wouldn't say undo, I was very careful to say it needs to get the > struct page memory into a state that the allocator algorithm expects, > whatever that means. > > Jason Best Regards, Yan, Zi