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Mon, 31 Aug 2026 00:55:55 -0700 Received: from vdi.nvidia.com (10.127.8.14) by mail.nvidia.com (10.129.68.10) with Microsoft SMTP Server id 15.2.2562.46 via Frontend Transport; Mon, 31 Aug 2026 00:55:47 -0700 From: Nimrod Oren To: Andrew Morton , David Hildenbrand , Lorenzo Stoakes , Jonathan Corbet , Vlastimil Babka CC: "Liam R. Howlett" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , "Shuah Khan" , Randy Dunlap , "Zi Yan" , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Nirmoy Das , Dragos Tatulea , , , , Nimrod Oren Subject: [PATCH v2] mm/khugepaged: cap min_free_kbytes recommendation at 1 GiB Date: Mon, 31 Aug 2026 10:56:35 +0300 Message-ID: <20260831075635.2244437-1-noren@nvidia.com> X-Mailer: git-send-email 2.45.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain X-NV-OnPremToCloud: ExternallySecured X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SA2PEPF000015CD:EE_|CY8PR12MB8266:EE_ X-MS-Office365-Filtering-Correlation-Id: d8cc7507-67fe-47e7-a0ab-08df07355490 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|82310400026|36860700016|1800799024|23010399003|376014|7416014|18002099003|56012099006|13003099007|6133799003|11063799006|10067099003; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:216.228.117.161;CTRY:US;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:mail.nvidia.com;PTR:dc6edge2.nvidia.com;CAT:NONE;SFS:(13230040)(82310400026)(36860700016)(1800799024)(23010399003)(376014)(7416014)(18002099003)(56012099006)(13003099007)(6133799003)(11063799006)(10067099003);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: vrnEFyhBEwVs+cDloI3aPzMT1oLZ8gyv7GYdYnTkJbUEHXqVYuR7cliUVHodBybZm1Ses2sHMUEmUGaFGaTyM5CvpuDF0NxCAyRgkBsg1T5/7zuyxiVjYUcipwxbQVt2q6ZkAyul7jjZLGyoPMTB10wPMVTmqDoY1FOc/lmboCPeBT80OLkXlFbmDlcc1WZ0eXqBx6Dm3YLrjo4prVslL7WzTcWW7sidxOkuvdJEDTKvYlX6sxykoFrDADTBT+NHOQaPDu4cvgfHSdvPVHJ1twmZ15kEwyQf9eMlJcSqZCRSy6JRIKVH/3Zz9IE4mEoOXJ75H4nDS4yF56zdsXqmUFTp/+Sd7MWoqKNuu8d/J67MPCqn0RBQMSxg/m5HE5yLnf8wLom5xQYq7zdSflLY8f7PkOrISZ5NAPSah+8tDv8U2bnS9jwn3nPVc1rk+a6p X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 31 Aug 2026 07:56:14.1804 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: d8cc7507-67fe-47e7-a0ab-08df07355490 X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.117.161];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: SA2PEPF000015CD.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY8PR12MB8266 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. Link: https://lore.kernel.org/r/20260716173504.760369-1-noren@nvidia.com/ Suggested-by: Lorenzo Stoakes (ARM) Signed-off-by: Nimrod Oren --- v2: * Use a named constant for the cap. * Drop explicit linux/sizes.h include. * Update min_free_kbytes documentation. v1: * Cap the final recommendation at 1 GiB as suggested by Lorenzo. https://lore.kernel.org/r/20260728202014.2517142-1-noren@nvidia.com/ RFC v1: https://lore.kernel.org/r/20260716173504.760369-1-noren@nvidia.com/ --- Documentation/admin-guide/sysctl/vm.rst | 8 ++++++++ mm/khugepaged.c | 7 ++++++- 2 files changed, 14 insertions(+), 1 deletion(-) diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst index 5b318d17aa4b..c0c925e6f504 100644 --- a/Documentation/admin-guide/sysctl/vm.rst +++ b/Documentation/admin-guide/sysctl/vm.rst @@ -539,6 +539,14 @@ watermark[WMARK_MIN] value for each lowmem zone in the system. Each lowmem zone gets a number of reserved free pages based proportionally on its size. +When Transparent Hugepage (THP) support is enabled through global or +per-size controls, the kernel may raise this value automatically to +help keep pageblocks free and reduce fragmentation for THP allocations. +The automatic recommendation cannot exceed 5% of low memory or 1 GiB, +whichever is lower. This limit applies only to the automatic +recommendation and does not constrain a higher value written by +userspace. + Some minimal amount of memory is needed to satisfy PF_MEMALLOC allocations; if you set this to lower than 1024KB, your system will become subtly broken, and prone to deadlock under high loads. diff --git a/mm/khugepaged.c b/mm/khugepaged.c index f49a6710933b..a8fc061d2488 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -101,6 +101,7 @@ static DEFINE_READ_MOSTLY_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS); static struct kmem_cache *mm_slot_cache __ro_after_init; +#define THP_MIN_FREE_MAX_PAGES (SZ_1G / PAGE_SIZE) #define KHUGEPAGED_MIN_MTHP_ORDER 2 struct collapse_control { @@ -3120,9 +3121,13 @@ void set_recommended_min_free_kbytes(void) recommended_min += pageblock_nr_pages * nr_zones * MIGRATE_PCPTYPES * MIGRATE_PCPTYPES; - /* don't ever allow to reserve more than 5% of the lowmem */ + /* + * Don't allow the THP recommendation to exceed 5% of lowmem or an + * absolute cap, whichever is smaller. + */ recommended_min = min(recommended_min, (unsigned long) nr_free_buffer_pages() / 20); + recommended_min = min(recommended_min, THP_MIN_FREE_MAX_PAGES); recommended_min <<= (PAGE_SHIFT-10); if (recommended_min > min_free_kbytes) { -- 2.45.0