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Fri, 23 Jan 2026 13:50:51 +0000 Received: from LV8PR12MB9620.namprd12.prod.outlook.com ([fe80::1b59:c8a2:4c00:8a2c]) by LV8PR12MB9620.namprd12.prod.outlook.com ([fe80::1b59:c8a2:4c00:8a2c%3]) with mapi id 15.20.9542.010; Fri, 23 Jan 2026 13:50:51 +0000 Date: Fri, 23 Jan 2026 09:50:50 -0400 From: Jason Gunthorpe To: Alistair Popple Cc: Zi Yan , Jordan Niethe , linux-mm@kvack.org, balbirs@nvidia.com, matthew.brost@intel.com, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, david@redhat.com, lorenzo.stoakes@oracle.com, lyude@redhat.com, dakr@kernel.org, airlied@gmail.com, simona@ffwll.ch, rcampbell@nvidia.com, mpenttil@redhat.com, willy@infradead.org, linuxppc-dev@lists.ozlabs.org, intel-xe@lists.freedesktop.org, Felix.Kuehling@amd.com Subject: Re: [PATCH v2 11/11] mm: Remove device private pages from the physical address space Message-ID: <20260123135050.GV1134360@nvidia.com> References: <20260107091823.68974-12-jniethe@nvidia.com> <36F96303-8FBB-4350-9472-52CC50BAB956@nvidia.com> <6C5F185E-BB12-4B01-8283-F2C956E84AA3@nvidia.com> <16770FCE-A248-4184-ABFC-94C02C0B30F3@nvidia.com> <0C16A79F-5A7B-4358-9806-7F78E7EA8EE6@nvidia.com> Content-Type: text/plain; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?6n5bbpHPt/ZRXAxW/cRAjMitrJ6ClygL5kppuG9OHQ6EygfERfSxKMVcKnaX?= =?us-ascii?Q?EXAJ9Z6PTt+7vvNNqB+a5L5nA1xhnZhSP/DmepcJ+fdoZ1lYbm5jn5pqIuPN?= =?us-ascii?Q?Fh77mzDZRyt4LyNe0eYWAEjd+/NWV/htkko+aJc12VtlNcWmLJEIwT+TcpUy?= =?us-ascii?Q?EqzvrlvkcFpaLGkWFpsMwciaOR/XZ1eTi/lUfvef5gg9EpySurhxP1klqkep?= =?us-ascii?Q?9ytiUBjeSgiCeUF4umD+7AMQc+4++I9MKJvm1HNOroZ94/4S8Flk+qlKOIKM?= =?us-ascii?Q?j4wkLm9u03kjxRY0OizkKYpwue4H84XodEkJzh8hDGdTWP4minypilOaBobS?= =?us-ascii?Q?eOx7ZpPJVneetKQ1UxuR3aTP5l+sjKxxzpiaqIOZNFP1MZurYCH5NIvWaiF3?= =?us-ascii?Q?yYUCRH/j65owbS0Dod/nGgvctaGVT+cq5sCOFwoEOCL1+YpKXav20lM3KCmv?= =?us-ascii?Q?6Ubvru98qtXQR9CY5NyTihnhBq9OGmPbEA+xQ+KwOb6LogdVpZ5fiyPKU6DF?= =?us-ascii?Q?/6cFw/3sJOxRzfYgq0IVIWhcPsJOK20knximwCN2XYQ0SAoCBqhnPq6oBVkq?= =?us-ascii?Q?9yaNx+YO7ATDJ24w/nbUvllcxwjW+AkLHzfmxMBsIzFREuJVoELhuFZhKm/8?= =?us-ascii?Q?bH20imouET3K4uHD1R00XA2HkU4KhnM7Zuq/2J/gTnwkfc7818IF70xOjsXA?= =?us-ascii?Q?xYZBh5IWY97ffjwIac0hOq4G3IlK5SHa6q/kiehpzrE4jq/4RdjtDorErXrl?= =?us-ascii?Q?vRc3t5wcqQBBpcHCBh4TXzwjbMnPMHranGJzB3kKnwWXXlzD+m2oiiJYh8Z+?= =?us-ascii?Q?uvdrodRxJluwG2pQ0j3mYInlAfwD+pAfcls2dy48irnXdopiF4uNDq7xbpdg?= =?us-ascii?Q?ddi0vRbNJ/WnEJ71aUtjBJPVcY0As0ECUj4oU0llYSmLv5aa9rtP+sJ+hY9z?= =?us-ascii?Q?QHiFyOky+ai+Fb0CjBUXiFHOR8m+2VcQGk5/mUdcBNLfRV0tGjM5NAkSYO38?= =?us-ascii?Q?ncAxVK1wrpL+V6ehVQWeSE6GTCquOv4EuogoTw+wOoksOnn3D1gE6jzHFh0U?= =?us-ascii?Q?NxpamDRX9hYgqEqJNbJ4mRd1/LLncCO+RaNJU4Mnk5N/mLvaNIb/q0+61iim?= =?us-ascii?Q?HqzV5FggZSYTUEFBLraCdwoYHQb/UgyBRrqHv6TOVLBle+WPNYXJjU556WQV?= =?us-ascii?Q?HV/ORRRRbgjkmvlS1+hZ3iuY25EVVTp0SZUhxrOPZOiOGDahQ58c4wfIxB6t?= =?us-ascii?Q?PufVzs0BfRUPQxjAntD1gxsk/rQ0+JXmI7QyBkzWO74P/i1dFV/0wTnp43OG?= =?us-ascii?Q?nJtQvJ8E7N/1LALj+lPePNyGisA3czpxZR2kivDJpZatLyjy98CN/oH41xoq?= =?us-ascii?Q?v4R3uVRTYWI1rB+fCc1JR9FMpJDX6xIjiVwDlChA1O8jApsQd6CjnaoW4rqu?= =?us-ascii?Q?+K1zF9mWcsQq//DdDH+lOeaBvZgWkjVPCJyAIPA5lPrwwWJBDSCf05VXlR/u?= =?us-ascii?Q?wP4lCle2VLx45LOMRQYqXTH8C7faBTS2KQms4nQBBjc17WeF7cf70P57BHCx?= =?us-ascii?Q?HP3zntQ80Ikt9pfd/+f7cgFv2BBYM/yQBDNPrfHzPIrjZDIxViX4eCmDs9Tq?= =?us-ascii?Q?JyADniIUhyZrF/ghdob41Ecbex8QP9lwwKxRctsEdOUjPkmHosvUqUAi2uT2?= =?us-ascii?Q?XRuuzlioMzCwody3epeVaLh0x3hdCVvUn+6WKR1pBaNTEXxIOg9aJ3T/FKLu?= =?us-ascii?Q?p0cw5dzvQg=3D=3D?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: fcc29591-dc62-40fd-74fd-08de5a866b85 X-MS-Exchange-CrossTenant-AuthSource: LV8PR12MB9620.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 23 Jan 2026 13:50:51.1056 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: o0Dcu+lYo9RzqG09W5FZTgTlAgHF0BbKGUhWGGo1ryzJTCTzDCMQ+18J/9Hb54CE X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY3PR12MB9655 On Fri, Jan 23, 2026 at 04:38:49PM +1100, Alistair Popple wrote: > > >> We need to be a bit careful by what we mean when we say "HMM" in the kernel. > > >> > > >> Specifically MIGRATE_PFN_SHIFT is used with migrate_vma/migrate_device, which > > >> is the migration half of "HMM" which does depend on CONFIG_DEVICE_MIGRATION or > > >> really just CONFIG_ZONE_DEVICE making it somewhat arch specific. > > >> > > >> However hmm_range_fault() does something similar - see the definition of > > >> hmm_pfn_flags - it actually steals the top 11 bits of a pfn for flags, and it is > > >> not architecture specific. It only depends on CONFIG_MMU. > > > > > > Oh, that is hacky. But are HMM PFNs with any flag exposed to code outside HMM? > > > Currently, device private needs to reserve PFNs for struct page, so I assume > > > only the reserved PFNs are seen by outsiders. Otherwise, when outsiders see > > > a HMM PFN with a flag, pfn_to_page() on such a PFN will read non exist > > > struct page, right? > > Any user of hmm_range_fault() would be exposed to an issue - most users of > hmm_range_fault() use it to grab a PFN (ie. physical address) to map into some > remote page table. So potentially if some important bit in the PFN is dropped > that could potentially result in users mapping the wrong physical address or > page. Trim the quotes guys.. hmm is arguably returning phys_addr_t >> PAGE_SHIFT. This is a lossless translation because everything is aligned, it isn't hacky. The value it returns is not a "pfn", it is a hmm structure that has to be decoded to something else using a hmm helper function. I think we take a number of liberties going between pte, phys_addr_t, pfn. If there are arches that use a special encoding for the mm PFN then range_fault would need to call converter functions to get to/from phys_addr_t. Jason