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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jonathan Corbet , Shuah Khan , Randy Dunlap , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Brendan Jackman , Johannes Weiner , David Laight , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org References: <20261014140859.1006967-1-noren@nvidia.com> <20261014144823.8c89da26c547fa95936a2fe5@linux-foundation.org> Content-Language: en-US From: Nimrod Oren In-Reply-To: <20261014144823.8c89da26c547fa95936a2fe5@linux-foundation.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MR1P264CA0035.FRAP264.PROD.OUTLOOK.COM (2603:10a6:501:2f::22) To MW4PR12MB6802.namprd12.prod.outlook.com (2603:10b6:303:20f::19) Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: MW4PR12MB6802:EE_|DS4PR12MB9681:EE_ X-MS-Office365-Filtering-Correlation-Id: 382cadd2-8808-4c69-b472-08df2ad7b6c6 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|366016|376014|1800799024|23010399003|56012099006|4143699003|11063799006|261009223027099003|10067099003|18002099003|22082099003|6133799003; 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The new value comes from calculate_min_free_kbytes() or, while >> THP is enabled, from set_recommended_min_free_kbytes(). A value written >> to the vm.min_free_kbytes sysctl is replaced whenever the new value is >> higher, so a lower value does not stick. This matters most when THP is >> enabled on configurations with large pageblocks, where the new value >> can be excessive [1]. >> >> Stop recalculating min_free_kbytes once a value has been written to the >> sysctl. Systems where the sysctl is never written keep the current >> behavior. > > This problem lies somewhere in the range of "minor annoyance" to > "catastrophically horrible", but it isn't clear where in that range it > falls. > > So can we please can we have a paragraph which explains the real-world > userspace-visible runtime effects of this problem? Sure. Can we add the following after the first paragraph? For example, with default arm64 pageblock sizes, the THP recommendation can reach 5.5 GiB per eligible zone with 64 KiB pages vs 22 MiB with 4 KiB pages. This keeps a significant amount of memory free, reducing what applications can use and triggering reclaim much earlier. Users can lower vm.min_free_kbytes to avoid this, but the lower value only sticks until the next recalculation. Thanks