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Tue, 18 May 2021 19:45:10 +0000 Received: from DM6PR12MB3834.namprd12.prod.outlook.com ([fe80::ddb4:2cbb:4589:f039]) by DM6PR12MB3834.namprd12.prod.outlook.com ([fe80::ddb4:2cbb:4589:f039%4]) with mapi id 15.20.4129.031; Tue, 18 May 2021 19:45:10 +0000 Date: Tue, 18 May 2021 16:45:09 -0300 From: Jason Gunthorpe To: Peter Xu Cc: Alistair Popple , linux-mm@kvack.org, nouveau@lists.freedesktop.org, bskeggs@redhat.com, akpm@linux-foundation.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, jhubbard@nvidia.com, rcampbell@nvidia.com, jglisse@redhat.com, hch@infradead.org, daniel@ffwll.ch, willy@infradead.org, bsingharora@gmail.com, Christoph Hellwig Subject: Re: [PATCH v8 5/8] mm: Device exclusive memory access Message-ID: <20210518194509.GF1002214@nvidia.com> References: <20210407084238.20443-1-apopple@nvidia.com> <20210407084238.20443-6-apopple@nvidia.com> <47694715.suB6H4Uo8R@nvdebian> <20210518173334.GE1002214@nvidia.com> Content-Type: text/plain; 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X-MS-Exchange-AntiSpam-MessageData: 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 X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: 405f46c4-5a36-4831-4ba2-08d91a35727c X-MS-Exchange-CrossTenant-AuthSource: DM6PR12MB3834.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 18 May 2021 19:45:10.8541 (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: PlKj/0ol5HPuVdPinr9uw+4RrA+jlPJ/GU4TYo/OSuWrUqTK53QZM9mP5Ceahvqz X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM6PR12MB3212 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 18, 2021 at 02:01:36PM -0400, Peter Xu wrote: > > > Indeed it'll be odd for a COW page since for COW page then it means after > > > parent/child writting to the page it'll clone into two, then it's a mistery on > > > which one will be the one that "exclusived owned" by the device.. > > > > For COW pages it is like every other fork case.. We can't reliably > > write-protect the device_exclusive page during fork so we must copy it > > at fork time. > > > > Thus three reasonable choices: > > - Copy to a new CPU page > > - Migrate back to a CPU page and write protect it > > - Copy to a new device exclusive page > > IMHO the ownership question would really help us to answer this one.. I'm confused about what device ownership you are talking about It is just a page and it is tied to some specific pgmap? If the thing providing the migration stuff goes away then all device_exclusive pages should revert back to CPU pages and destroy the pgmap? Jason