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Howlett" , Vlastimil Babka , Mike Rapoport , "Suren Baghdasaryan" , Michal Hocko , "Balbir Singh" , , , , , , , Subject: Re: [PATCH v5 1/5] mm/zone_device: Reinitialize large zone device private folios Message-ID: References: <20260114192111.1267147-1-francois.dugast@intel.com> <20260114192111.1267147-2-francois.dugast@intel.com> <20260114134825.8bf1cb3e897c8e41c73dc8ae@linux-foundation.org> Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: BY3PR05CA0037.namprd05.prod.outlook.com (2603:10b6:a03:39b::12) To PH7PR11MB6522.namprd11.prod.outlook.com (2603:10b6:510:212::12) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH7PR11MB6522:EE_|SA2PR11MB4796:EE_ X-MS-Office365-Filtering-Correlation-Id: 82b5003e-13df-436e-8938-08de53c7d069 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|1800799024|7416014; X-Microsoft-Antispam-Message-Info: =?utf-8?B?QmlEWVc2M25wbGhwZFU4U3kzYW9Bc3gwZGhhRVdWSEhVVWcwN3c2R0Z6MlFi?= =?utf-8?B?NTh2dkFnT0s3b0JXWm43S3FsVFZQYS8zN1k2TC9Ndng0a0t1OTUrUnVmUGJQ?= =?utf-8?B?b1hBeGtVcUF1S2hXeEpRVkJHYk1ZdWtMWTkxWWZiM1J2NytudWFSQW5CY1E0?= =?utf-8?B?bFQ1UmVheHVoUXJTcFhvYWk1UnJqdTEyZzdrSGRhc2Y0QzZpcXVESHlvbVgz?= =?utf-8?B?Y0NETnNxdFk0Vzg4NXBFMEcreHBpV3NodXhMSXg3SEpLS3EzY01Edi9OZFE0?= =?utf-8?B?eVNiTTRnR3hTZUhzTk01YVF6QWtOdmQxNUVBazNtT2h6SzVScEJDVmJYOEZw?= =?utf-8?B?c2pvTmg2Uk1mT1ppS1dWS2RlZUJPVSszdENPeE5ZSThuWFRXYmNOODdBN1V1?= =?utf-8?B?dmt2aldTTDdSdXdvcTI1YVVlZDZSZGhUSHlhYXkyU2U1dkszM1RsenA0K2Zx?= =?utf-8?B?ZlBuODN3QVlxeDFDNC9oRk5pNmNiMnZ2Ykh6RGhsQVM0VnRrOXI4eFllb2Rj?= =?utf-8?B?OVlHL21kMEVjR3FtaW5Lc0xFNUE2cnFvVkY0bkFxcGhLUHhXOUl5MisySmdk?= =?utf-8?B?TEpQVVdjVGlPMHZjdUd0NU84MUJJZzI0MUdjcHlsSVMxWWE2b0Z3a2xoWUhj?= =?utf-8?B?QVUvbEtCR1hsTXBoODZSeFRKVjd0MndHbmhKR3Ywdzg0dlJBRjlQOUl6YWtE?= =?utf-8?B?eUI0ZzJqajd6NDhHMWhFUkRmc3RTcGgxcTZMWFRZRURtODhqUjFuSnhTMWFT?= =?utf-8?B?UzQ1STluWFpXOGZBWFdWZGIwdzJ1QzY0Sms5NlIrMmxnWUtRRCt3SDlqRUhR?= =?utf-8?B?bis1MExWOFBKV0tNRWtvcjROcXhKT1JjWk5OWjQwR2NublRCRWFqWWhCbkhG?= =?utf-8?B?c1RHN1Q0ZXNTNUc4N1Yxd2hkN09rS1I0dkoyc0drTWdobnp6b1FJODZhUTND?= =?utf-8?B?NUtvZVJleUxWa2F4WUJJcVhVZmxVR2VBdDJVWE1nZFJpa3NrUWpUZ3g0Tmx2?= =?utf-8?B?RlRBNlpidnM5K2dROHgxS3hJalFSa3ZLWGtCUzhpbEoyMEFITk5XTG5YUWQr?= =?utf-8?B?bC9DVmIvbXFyZW9XNDJmWmQxZDdJZ1gxMnlobXFXTnR2eEZWK0ZRMFNlRkJI?= =?utf-8?B?MHFTa09BU0p2bHpkWkMvQkcxTWtwSkdlU1NaLzZteGlIT2JXdTZrM1dDaFNk?= =?utf-8?B?Z0dPL2VFUWREa2tMK2JtblMzWEVOZlNQeUpyYjZIRktNcjN3N0d5VGtwRzFB?= =?utf-8?B?eEp1R2tGbWVNSE1JY1phRzd0VkJkclVWSHZBNFNLOEhPRSt1UythWDZMVXl3?= =?utf-8?B?TVk0VU92a2ZzWWQxdUZwVm5sc0paTFZqK0p2OWY1Tk45dDZLZUdUTHI3cTVW?= =?utf-8?B?eEViRjllajdib0x1RzI4bHFCaU1BVXpwb3Uvd2QvOGUyYUZzalQzaDdGMzg4?= =?utf-8?B?ZC9EWEJIRzRsN21ZSmhGeTliSXFyYlJ6TVFWeEVzTnM0L2h2L1A1ZEhmbUJ1?= =?utf-8?B?ODNtbFFRWExIblMyUmhzQWE0TE12d2x6eVJsMjdCU2NKUjQvMlA4SkM3OURP?= =?utf-8?B?ejh1MWhmRmZZZGVMemM5SmlRZW05cjRBTitoZUo0YmoweTFLc1R5eVBBaTI0?= =?utf-8?B?bFZvb09uTXJsUnh0azl2NG81OUpQbUtnQ3Nrbi8yRkpIL3pRRW5nNmhWZEFW?= =?utf-8?B?cGpUZmthR2p5UVIycFgwbHpmZmU5clp5Q1FMNjgvZlJ4UmQ3VmplVW9Pd1pm?= =?utf-8?B?VHJiQSt5WVIxQkJsMW5hNmRGT1kybFFzRnAxR1RROUxwcWpDVnQvMFkwTnhQ?= =?utf-8?B?NU50WFlEUWhnelB1N01xUVdrVGRkeUNtMm1XL1dnRXNEdHBQdWJ4MzQ0SWFL?= =?utf-8?B?Tk5yY0JCRFdKNkk2UXl1TStIOUR6M1lYMjFpUVE5UllJblIybkdLTmhYTm45?= =?utf-8?B?SEpybG1md2VqVTZCYnk5YWJNRkU4bHhzWHg5ZXJxaHR6bkFYT05mQWtKS3g2?= =?utf-8?B?ZjFrV0w3cWxyY0RNRDd1eEx5VHdTakQ2TmZkc0ZOc0pYWENtVHgwd1YwT3Zl?= =?utf-8?B?cUJlMVRuV08xRHEvNVBicXBIVEV5bFEzWTRzMThKeXhSNlV3Ti82c1lJOS9T?= =?utf-8?Q?q6Z8=3D?= X-Forefront-Antispam-Report: CIP:255.255.255.255; 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This step is necessary when the folio’s order changes > > > dynamically between zone_device_page_init calls to avoid building a > > > corrupt folio. As part of the metadata reinitialization, the dev_pagemap > > > must be passed in from the caller because the pgmap stored in the folio > > > page may have been overwritten with a compound head. > > > > Thanks. What are the worst-case userspace-visible effects of the bug? > > If you reallocate a subset of pages from what was originally a large > device folio, the pgmap mapping becomes invalid because it was > overwritten by the compound head, and this can crash the kernel. > > Alternatively, consider the case where the original folio had an order > of 9 and _nr_pages was set. If you then reallocate the folio plus one as s/_nr_pages/the order was encoded the page flags. Not clearing _nr_pages is probably bad too, not sure what the side affect of that is, but it can't be good. > an individual page, the flags would still have PG_locked set, causing a > hang the next time you try to lock the page. > > This is pretty bad if drivers implement a buddy allocator for device > pages (Xe does; Nouveau doesn’t, which is why they haven’t hit this > issue). Only Nouveau enables large device pages in 6.19 but probably > best to have kernel flying around with known issues. s/best to have kernel/best to not have kernels Matt > > Matt