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Howlett" , Mike Rapoport , Suren Baghdasaryan , Shuah Khan , Randy Dunlap , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Nirmoy Das , Dragos Tatulea , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm/khugepaged: cap min_free_kbytes recommendation at 1 GiB Date: Mon, 31 Aug 2026 11:00:13 -0400 X-Mailer: MailMate (3.0r7024) Message-ID: <784BE9B6-F891-4DFE-A4C5-D1986CE0F242@nvidia.com> In-Reply-To: References: <20260831075635.2244437-1-noren@nvidia.com> Content-Type: text/plain X-MS-Reactions: disallow X-ClientProxiedBy: BN0PR04CA0111.namprd04.prod.outlook.com (2603:10b6:408:ec::26) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR12MB8374:EE_|DS8PR12MB333896:EE_ X-MS-Office365-Filtering-Correlation-Id: bab25141-e792-4231-7973-08df077090c1 X-MS-Exchange-SenderADCheck: 1 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 /I82Wlry iXXxAMZ71rnAP8IMCSJtjJb7cONlHQceBZuANE+AiyDDq8mDhqmDIYm9exNw1BJli2OA0xlm4Nj0irIPB/eA212XSuJ44vSH3ySKknz+TCPkcJ8GMXxgMRp7FsWYBhB1wzlO0L+iasXr0wVUC61t+s2kyLnbgSiyKvpUZCzqC/6PZLI0C70Ylnpq0gwd6CPGV8LA3KDHg5WrqdWfcq4X/d3+qwcc23yPcBYjorRgDcijnc7PKl6JiJavVxrpF91Pj13g26MoW2aPJx00EYDuXnxipzD77hz5GXZ8aPkRfvIGIA7X/rD+PyS4gZGL6fkFRbdrDe9f33Ua87wnMB4/Gp8qYHzfVzQLY3IbJ+7Dk7O0MpMNPuu6L4uf6sqG3pju4oYFOBUTCGJC/+ElCjGTTotj+v0oyFnYU5GRC Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 31 Aug 2026, at 5:34, Lorenzo Stoakes (ARM) wrote: > On Mon, Aug 31, 2026 at 11:20:30AM +0200, Michal Hocko wrote: >> On Mon 31-08-26 10:56:35, Nimrod Oren wrote: >>> When THP is enabled, set_recommended_min_free_kbytes() may raise >>> min_free_kbytes using a heuristic that scales with pageblock_nr_pages. >>> With MIGRATE_PCPTYPES equal to 3, the formula accounts for 11 >>> pageblocks for every eligible populated zone before capping the result >>> at 5% of low memory. >>> >>> This is reasonable when a pageblock is 2 MiB, as on common 4 KiB page >>> configurations, but scales poorly with larger base page sizes. With >>> the default arm64 pageblock sizes, the contribution per eligible zone >>> before the 5% cap is: >>> >>> 4 KiB pages: 2 MiB pageblock, 22 MiB per zone >>> 16 KiB pages: 32 MiB pageblock, 352 MiB per zone >>> 64 KiB pages: 512 MiB pageblock, 5.5 GiB per zone >>> >>> Consequently, min_free_kbytes can reach excessive and unwanted levels. >>> >>> Add an absolute 1 GiB cap to the recommendation, in addition to the >>> existing percentage cap. This bounds the automatic recommendation to a >>> sane value on systems with large pageblocks while preserving existing >>> behavior for typical systems with 2 MiB pageblocks. >> >> I would argue that the whole model of increasing min_free_kbytes for THP >> is wrong. This will trigger memory reclaim sooner and make the THP >> availability more likely but this was at times where we didn't have >> pro-active compaction and many changes in the compaction. So is this >> actually needed in general resp. only large pageblocks systems? > > Ohh even better :) > > I did find it strange that we increased accordingly. > > I'm more than happy to see this just die altogether if people agree that's > a sensible way forward... It sounds reasonable to me. My first reaction was that THP generation might be hurt due to fewer free pages. But probably the extra min_free_kbytes just moves reclaim earlier, like Michal said. After removing this automatic min_free_kbytes bump, user can retain the old behavior by setting a higher min_free_kbytes at boot time. Best Regards, Yan, Zi