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Mon, 30 Aug 2021 17:04:46 +0000 Subject: Re: [PATCH v1 03/14] mm: add iomem vma selection for memory migration To: Christoph Hellwig Cc: "Sierra Guiza, Alejandro (Alex)" , akpm@linux-foundation.org, linux-mm@kvack.org, rcampbell@nvidia.com, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, jgg@nvidia.com, jglisse@redhat.com References: <20210825034828.12927-1-alex.sierra@amd.com> <20210825034828.12927-4-alex.sierra@amd.com> <20210825074602.GA29620@lst.de> <20210830082800.GA6836@lst.de> From: Felix Kuehling Message-ID: Date: Mon, 30 Aug 2021 13:04:43 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 In-Reply-To: <20210830082800.GA6836@lst.de> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US X-ClientProxiedBy: YQBPR0101CA0152.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:e::25) To BN9PR12MB5129.namprd12.prod.outlook.com (2603:10b6:408:136::12) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.2.100] (142.186.47.3) by YQBPR0101CA0152.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:e::25) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4457.21 via Frontend Transport; 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As far as I can tell, all the work >> we did first with DEVICE_GENERIC and now DEVICE_PUBLIC always used >> normal pages. Are we missing something in our driver code that would >> make these PTEs special? I don't understand how that can be, because >> driver code is not really involved in updating the CPU mappings. Maybe >> it's something we need to do in the migration helpers. > It looks like I'm totally misunderstanding what you are adding here > then. Why do we need any special treatment at all for memory that > has normal struct pages and is part of the direct kernel map? The pages are like normal memory for purposes of mapping them in CPU page tables and for coherent access from the CPU. From an application perspective, we want file-backed and anonymous mappings to be able to use DEVICE_PUBLIC pages with coherent CPU access. The goal is to optimize performance for GPU heavy workloads while minimizing the need to migrate data back-and-forth between system memory and device memory. The pages are special in two ways: 1. The memory is managed not by the Linux buddy allocator, but by the GPU driver's TTM memory manager 2. We want to migrate data in response to GPU page faults and application hints using the migrate_vma helpers It's the second part that we're really trying to address with this patch series. Regards,   Felix