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Tue, 26 Aug 2025 09:00:59 +0000 Message-ID: Date: Tue, 26 Aug 2025 11:00:53 +0200 User-Agent: Mozilla Thunderbird Subject: Re: To: David Hildenbrand , intel-xe@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, amd-gfx@lists.freedesktop.org, x86@kernel.org Cc: airlied@gmail.com, thomas.hellstrom@linux.intel.com, matthew.brost@intel.com, dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org, Lorenzo Stoakes References: <20250820143739.3422-1-christian.koenig@amd.com> <4e5f4ef0-53f1-417e-8f3b-76fd7c64cd23@amd.com> <6591105b-969d-44d6-80e1-233c1b84b121@redhat.com> Content-Language: en-US From: =?UTF-8?Q?Christian_K=C3=B6nig?= In-Reply-To: <6591105b-969d-44d6-80e1-233c1b84b121@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: FR2P281CA0028.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:14::15) To PH7PR12MB5685.namprd12.prod.outlook.com (2603:10b6:510:13c::22) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH7PR12MB5685:EE_|PH7PR12MB6835:EE_ X-MS-Office365-Filtering-Correlation-Id: 66c69398-f169-4795-e7a8-08dde47f1344 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|7416014|376014; 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But what drivers do here is that they have single pages (usually for get_free_page or similar) and want to apply a certain caching attribute to it. >> >> So what would happen is that we completely clutter the R/B tree used by the PAT with thousands if not millions of entries. >> > > Hm, above you're saying that there is no direct map, but now you are saying that the pages were obtained through get_free_page()? The problem only happens with highmem pages on 32bit kernels. Those pages are not in the linear mapping. > I agree that what you describe here sounds suboptimal. But if the pages where obtained from the buddy, there surely is a direct map -- unless we explicitly remove it :( > > If we're talking about individual pages without a directmap, I would wonder if they are actually part of a bigger memory region that can just be reserved in one go (similar to how remap_pfn_range()) would handle it. > > Can you briefly describe how your use case obtains these PFNs, and how scattered tehy + their caching attributes might be? What drivers do is to call get_free_page() or alloc_pages_node() with the GFP_HIGHUSER flag set. For non highmem pages drivers then calls set_pages_wc/uc() which changes the caching of the linear mapping, but for highmem pages there is no linear mapping so set_pages_wc() or set_pages_uc() doesn't work and drivers avoid calling it. Those are basically just random system memory pages. So they are potentially scattered over the whole memory address space. Regards, Christian.