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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: [PATCH v5 02/14] mm/khugepaged: add folio dirty check after try_to_unmap() Date: Wed, 29 Apr 2026 11:29:12 -0400 Message-ID: <20260429152924.727124-3-ziy@nvidia.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260429152924.727124-1-ziy@nvidia.com> References: <20260429152924.727124-1-ziy@nvidia.com> Content-Transfer-Encoding: 8bit Content-Type: text/plain X-ClientProxiedBy: BN0PR04CA0108.namprd04.prod.outlook.com (2603:10b6:408:ec::23) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB9473:EE_|IA0PPF12042BF6F:EE_ X-MS-Office365-Filtering-Correlation-Id: 08203d36-d65e-4943-47ce-08dea604356a X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|7416014|1800799024|22082099003|56012099003|18002099003; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?LloPMyzjXTEeHhCZWn+PA3zHHWvREQWLUvGNbxK6cETYBTzCGuAVMlQUKFSU?= =?us-ascii?Q?9LCZ6RzGYTs+1H+0PETY/klTImibm1fxg01o4okOZM6aDUGO3mDwMYwkM/lJ?= =?us-ascii?Q?VjWXEjgHdsO6M33CtyS7TciUDYfp65l1Bh9j+gxgs6R45pFsLC0IKs+FW4ca?= =?us-ascii?Q?t8dUI/ammkUmwyuevovxWxtLCo0ApPJvphfPUxdJ5Jv+UoXvWbkNv+T2g8oS?= =?us-ascii?Q?1rmfqTzOeRqJW/WPBBoKq5TSs1sa6TJCQtXLuIwHebswdZhS9QTkdiEmenIt?= =?us-ascii?Q?fUhxqbPZwtOzNd178Xj7yNrfwkmEqJI5XRXgMV1h7uNm+tgDZu6B5mKGheFs?= =?us-ascii?Q?FQ8wCFUZeMhQFrJ0RfeQDY9PQgdUfDYVsSl7IKkdCB9y+DnlChLXv3xJNU/H?= =?us-ascii?Q?4reZs2Lwc+NK2LQ1iXKx6qijIGdPOQXhNLvFQ+ytr9xXRoQxLGBLD3nl65bZ?= =?us-ascii?Q?/4eEyCmADrRtHDJMBRN8Cndv8/9EKtm7KJpH6mOd0bnRaLWMm2FaxBp/KVI4?= =?us-ascii?Q?27mvrkuVs64ENn1HBzeD5Es4VTCUqEVt3BTUaxQNUWYcWdGGzutOBzhrXmnv?= =?us-ascii?Q?kULNzY8uuGcuTfBfq8ZEHe2L3nO/2SsAnSAXP2TfwK9LLkGcoB6FknZkeKmc?= =?us-ascii?Q?AjHWefaf7iHMMZOvJsIHuGF9r0XtyFlRg7LHP/Ia4/5UqhVQfzUR9mtfsi5Q?= =?us-ascii?Q?NYJBJJmikoeC0oP5Li/EtCWKvcj48i7RhC0TvHBICEEXYEAybg33XAV3YagI?= =?us-ascii?Q?QDU3oaDYsak2X4Gm0MtsT0xLifsDrKRGa/JjvP4f8n4+RqqHo+1jTQEwvfxm?= =?us-ascii?Q?x+L10ai9TLTIanBpthbZXRQlZjurjBJ2MAdlxXqVyfg3pRtDvnLVqDehlZNa?= =?us-ascii?Q?E+ihf1z6HtL+XHoyx9Vgv9oOArz+XhIM6U7z8ZoOCjfel9hc1lPKCI6T7y5W?= =?us-ascii?Q?XyiNfnO12b8rkfql1CxPE3fpxRC6khd9kej3tjmDyB8z+VqEarJREZtstamB?= =?us-ascii?Q?NBXW5OUZnTEH6/gfS4MM+cF8Jm9VvYGjXM4XzKG/QHYKHwdYndT8wWp6WHbj?= =?us-ascii?Q?bR6BcQgPh6plFRYM6dHygRf5KWNu8j18NUle+isQ/CKwO4MgbiHpJngiJrjO?= =?us-ascii?Q?aODsIufuULhu6QrA3h+JXtPHGiaw2WKY/rvE7yuY4WcXjcIPzMQ/xbfWknUd?= =?us-ascii?Q?W3FnPMXIs+Ljz8iPq6hViXR/VXiyFrLzr/NL3kXh7gDNL+5ESGiLk8xXATTL?= =?us-ascii?Q?nMFKPIHZBXOGvFrYMpRx+/RN2Rb6Qj0M716p9zzCD/Wu29ARrLeboFwbV3qH?= =?us-ascii?Q?BBsteDTZl5wG7y58gETNmLK+00t6XZpEKr0k8mHsh6rdYTSV0j4VoEVFF5Bw?= =?us-ascii?Q?/Ie6zDtTwu0X5T5qkEZiIy8v5tMThXSnvB/83m4kLkf0xhfNT8sEOILy84Ca?= =?us-ascii?Q?al6tHgt7U8sPeboqqhuNyXfMJUJEexzWagU4r5aamvV8QUDFARjejLPuJIr1?= =?us-ascii?Q?1LbZOY51YUrCB9nTLSOFbXQNAkU5iB9MDhVCjriShb0ILgX5EBaEBzN9yQX2?= =?us-ascii?Q?XK67TaPW4PIp+vhIrPyBMkn3ZtDBfrAN+O5J7F0CMT3M6ycy2xyH5ygWzppB?= =?us-ascii?Q?OgASr8WPHkK8LYCqGYMR4qMx0MLUCCDF0lmRA27bVMYTLwilygsLzatgoKZs?= =?us-ascii?Q?kfgh/r9cL2+pemNGdykGbkYY5Pl+ZXvPJb+jLr/p0bCk6Gkz?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: 08203d36-d65e-4943-47ce-08dea604356a X-MS-Exchange-CrossTenant-AuthSource: DS7PR12MB9473.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 29 Apr 2026 15:30:14.0369 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: U78LTz8WpnH3LaTCBS5/JKxz+CWHUibcwQGl6YQ/vC0a9xPO/SKo/h1AmHajHY+P X-MS-Exchange-Transport-CrossTenantHeadersStamped: IA0PPF12042BF6F This check ensures the correctness of read-only PMD folio collapse after it is enabled for all FSes supporting PMD pagecache folios and replaces READ_ONLY_THP_FOR_FS. READ_ONLY_THP_FOR_FS only supports read-only fd and uses mapping->nr_thps and inode->i_writecount to prevent any write to read-only to-be-collapsed folios. In upcoming commits, READ_ONLY_THP_FOR_FS will be removed and the aforementioned mechanism will go away too. To ensure khugepaged functions as expected after the changes, skip if any folio is dirty after try_to_unmap(), since a dirty folio at that point means this read-only folio can get writes between try_to_unmap() and try_to_unmap_flush() via cached TLB entries and khugepaged does not support writable pagecache folio collapse yet. Signed-off-by: Zi Yan Reviewed-by: Baolin Wang Acked-by: David Hildenbrand (Arm) --- mm/khugepaged.c | 28 ++++++++++++++++++++++++---- 1 file changed, 24 insertions(+), 4 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 6808f2b48d864..71209a72195ab 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -2327,8 +2327,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, } } else if (folio_test_dirty(folio)) { /* - * khugepaged only works on read-only fd, - * so this page is dirty because it hasn't + * This page is dirty because it hasn't * been flushed since first write. There * won't be new dirty pages. * @@ -2386,8 +2385,8 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, if (!is_shmem && (folio_test_dirty(folio) || folio_test_writeback(folio))) { /* - * khugepaged only works on read-only fd, so this - * folio is dirty because it hasn't been flushed + * khugepaged only works on clean file-backed folios, + * so this folio is dirty because it hasn't been flushed * since first write. */ result = SCAN_PAGE_DIRTY_OR_WRITEBACK; @@ -2431,6 +2430,27 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, goto out_unlock; } + /* + * At this point, the folio is locked and unmapped. If the PTE + * was dirty, try_to_unmap() has transferred the dirty bit to + * the folio and we must not collapse it into a clean + * file-backed folio. + * + * If the folio is clean here, no one can write it until we + * drop the folio lock. A write through a stale TLB entry came + * from a clean PTE and must fault because the PTE has been + * cleared; the fault path has to take the folio lock before + * installing a writable mapping. Buffered write paths also + * have to take the folio lock before modifying file contents + * without a mapping, typically via write_begin_get_folio(). + */ + if (!is_shmem && folio_test_dirty(folio)) { + result = SCAN_PAGE_DIRTY_OR_WRITEBACK; + xas_unlock_irq(&xas); + folio_putback_lru(folio); + goto out_unlock; + } + /* * Accumulate the folios that are being collapsed. */ -- 2.53.0