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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Shuah Khan , linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH 7.2 v2 05/12] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check in hugepage_pmd_enabled() Date: Mon, 13 Apr 2026 16:42:37 -0400 X-Mailer: MailMate (2.0r6290) Message-ID: <05F00072-7E06-47C9-BC26-FE3736F557FC@nvidia.com> In-Reply-To: References: <20260413192030.3275825-1-ziy@nvidia.com> <20260413192030.3275825-6-ziy@nvidia.com> Content-Type: text/plain X-ClientProxiedBy: MN0P220CA0002.NAMP220.PROD.OUTLOOK.COM (2603:10b6:208:52e::31) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB9473:EE_|CH3PR12MB8727:EE_ X-MS-Office365-Filtering-Correlation-Id: a21175ec-86d2-45eb-ecb4-08de999d3503 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|7416014|376014|22082099003|56012099003|18002099003; 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Previously, with >> READ_ONLY_THP_FOR_FS enabled, hugepage_pmd_enabled() returned true whenever >> /sys/kernel/mm/transparent_hugepage/enabled was set to "always" or >> "madvise". >> >> After this change, hugepage_pmd_enabled() is governed only by the anon and >> shmem PMD THP controls. As a result, khugepaged collapse for file-backed >> folios no longer runs unconditionally under the top-level THP setting, and >> now depends on the anon/shmem PMD configuration. > > This seems like it'll turn off khugepaged too easily. I would have > thought we'd want: > > - if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && > - hugepage_global_enabled()) > + if (hugepage_global_enabled()) > return true; I thought about this, but it means khugepaged is turned on regardless of anon and shmem configs. I tend to think the original code was a bug, since enabling CONFIG_READ_ONLY_THP_FOR_FS would enable khugepaged all the time. > > ... or maybe this whole thing could be simplified? Alternatives could be: 1. to add a file-backed khhugepaged config, but another sysfs? 2. to replace hugepage_pmd_enabled() with hugepage_global_enabled() and let thp_vma_allowable_order() in collapse_scan_mm_slot() skip not qualified VMAs, but that would waste extra CPU cycles for scanning. Maybe not too much waste. Best Regards, Yan, Zi