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Howlett" , Mike Rapoport , linux-mm@kvack.org, stable@vger.kernel.org Subject: Re: [RFC PATCH 1/5] mm/page_alloc: count guard pages as free again when their buddy merges Date: Wed, 14 Oct 2026 16:57:45 -0400 X-Mailer: MailMate (3.0r7032) Message-ID: In-Reply-To: <20261007021250.1665929-2-riel@surriel.com> References: <20261007021250.1665929-1-riel@surriel.com> <20261007021250.1665929-2-riel@surriel.com> Content-Type: text/plain X-MS-Reactions: disallow X-ClientProxiedBy: BY3PR04CA0006.namprd04.prod.outlook.com (2603:10b6:a03:217::11) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) 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: IA0PR12MB8374:EE_|DS0PR12MB7607:EE_ X-MS-Office365-Filtering-Correlation-Id: fb8de088-0032-4609-abd4-08df2a35d064 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|376014|7416014|1800799024|366016|18002099003|22082099003|261009223027099003|10067099003|4143699003|11063799006|56012099006; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?NyosQfoIcR8EfmULEnEuNGI5euzHCcO9W6qEAECwNd83P66SuFqVof6xXpWR?= =?us-ascii?Q?Tt8eWnAcK14tXBDUMUmfbzxqokkTPLUIauvoISg3pG0+UxbNouSdBpqq0hW4?= =?us-ascii?Q?AFTM8VBIgBHbPFVTEEgHoiMzeHpnsXaRpn111DjWfm+f+xWpKadzePRrg1B5?= =?us-ascii?Q?KvlCtdTRBPwLFJU7+sbnxUpfDhMcomsjABYUhePjvwHhPrNU3h0dTpYiRD9M?= =?us-ascii?Q?xpTdeT/TkHiKLWEIyOwqB+DTS1Aw8iRKmPrERj5kuWkERTMVzxFPNx9S1lNX?= =?us-ascii?Q?1PZdX50QrLBXIGJBRPKRVkVzm1me8iMAw3uk3U7mVXVqhhSDb4ewAgQqB6oQ?= =?us-ascii?Q?x0OnLtSIataUvbu0o3ZiUvA9WmFXeNh6tcBGChnX9BAkApm5bX4ZRQLf8O7j?= =?us-ascii?Q?saOOlvLL0HICXOT7xCI6dyPdmhwhA9fWgDyEh8gBIucuayFClkdxeMjYXuJa?= =?us-ascii?Q?z6cYGziAmfEsZf6Ueo7lDQDGbdbL+RRoomA4xXv3E7Zh8toubizJplmYU86l?= =?us-ascii?Q?ctb+f9u4peOk6LM0B3EdXhKG5Fg75MpkWbghYiqAi21O6yXFYK3r/dgW3vbQ?= =?us-ascii?Q?fNNBsx3k3APfjxrYthAqzsCqdbI+59SIsecsa3RmJxPGtaCy/q0JsHj7Np8O?= =?us-ascii?Q?nq4LW9MbXuJ5pmpssz1+2rXJas9DopFZC0PRBXKD0Jf8kkqfUm1eEMY5+D0C?= =?us-ascii?Q?5u2Eh4EvfpnS3MgEipD9G4RRT1iPCBBwCR2iHwNmzNVlRc+bjwSWXfQsL4sK?= =?us-ascii?Q?4SGGM8/2QPxqNrvc8W1UsCzCvcfau6mNw/pNnIlwmrlYHlsPi3JahtX5rFAM?= =?us-ascii?Q?O/q8tk3ZAo+Bbh0/IL70jiPpUibFh3zdRnz/OT+A64NkyF6PYfNJR+gebOlt?= =?us-ascii?Q?nwu60obAAYGROntWj/KkPJ9B/2/0oejAJoEGlkrIsOc9/mHUHJd7EIpSqtW3?= =?us-ascii?Q?rFF6hjz18ggMkiCAZ4gdGn/vfOBb+t2NYpPONAo+z7mQNGUtlJ5N1uuI+15/?= =?us-ascii?Q?AjFkgS3MoP3N4o/0HSiUJb6Vya2Yxhsgjekk+G0aT/1IekoDRKjUFYs2VIOs?= =?us-ascii?Q?/7UKQNpewrB9niUIO/WVo9b1bBG5MIbBaiUkl/7ntX+EOdZ0lR1zYICQPizk?= =?us-ascii?Q?/EXs09lIBaOMVLZlXGTExSdUmozZfPToBV6ZBdUbIKrrUzA0RhGfdxYQLJ35?= =?us-ascii?Q?T4flZc4IEuvM1y7b6aWnJCLr7Lx7loO7eOmbzYdEYxjVkyNdcnUFk9rCDTOJ?= =?us-ascii?Q?sydBQm2E/QDFfzgGLeodJ/wXKR5gpfPE3Tle1gsoptEaZQg//ZoNGbvzZ99C?= =?us-ascii?Q?JqSovFMQVZLESlrobaRwDJBs5vxm6li8kX3n/2r7L4NWWoFKEDNma58As4ap?= =?us-ascii?Q?NLb/VhcHhvel2pJvXmUdTJ8UsK5j29tRDVmsvpNRWQ9osGygmpIX91PoYtRA?= =?us-ascii?Q?82XwvKaNY2X55/mi0qTSrtPbNOkLzwQKkI52+T/hu6dJycbJns7+mn2/hZIo?= =?us-ascii?Q?ube6jZXivx+jTxh7hvXdGJZS0SZYKxxjPbMUIBurCtxMzG3RSYJt08NMo5U3?= =?us-ascii?Q?/S8rN1SS9izv3WFL7qGIU7FT/JGfPls9ONMA47n+a0Td78Zou6fNr53j0+bt?= =?us-ascii?Q?LA4Whh3putd/MRgGG0rzuOsffbYQb2NV4i2tIB+4kAXsKnp3GA+7vkL2lzZd?= =?us-ascii?Q?keBqK3FNb8H9RHCnnpsDnzauTqE57f+RFMtUZJaHNq+KugzD?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: fb8de088-0032-4609-abd4-08df2a35d064 X-MS-Exchange-CrossTenant-AuthSource: IA0PR12MB8374.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 14 Oct 2026 20:57:53.0550 (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: UdRfAHv88kMoDpDiAnBpovJRHucKyWAFyrHF2NE8css3sp917R1g+T3VxIQxJtMb X-MS-Exchange-Transport-CrossTenantHeadersStamped: DS0PR12MB7607 On 6 Oct 2026, at 22:12, Rik van Riel wrote: > With debug_guardpage_minorder set, expand() turns the unused halves of a > split into guard pages, which page_del_and_expand() leaves out of the free > counts. Freeing the allocated half credits only that half, and > __free_one_page() then merges the guard buddy with clear_page_guard(), > which no longer touches the counters. > > The merged block goes on a free list with the guard pages never counted, > so NR_FREE_PAGES falls behind the free lists by every guard that merges. > Before commit e0932b6c1f94 ("mm: page_alloc: consolidate free page > accounting"), __set_page_guard() and __clear_page_guard() adjusted the > counts themselves; that commit dropped both adjustments but kept the > subtraction implicit in expand(). > > Count the guard pages under the merged block's migratetype when the > buddy is cleared. account_freepages() skips an isolated block as it does > for every other free page, and moving the block off the isolated list > counts it then. > > Booting a 16GB VM with debug_pagealloc=on debug_guardpage_minorder=1, > check the difference in free pages reported between /proc/vmstat > and /proc/buddyinfo. > > At three points (idle, after a read and file-creation load, after > a second load), check the difference between nr_free_pages and the > buddyinfo numbers in the normal zone, as a number of pages: > > idle after load after second load > unpatched -130 79205 99121 > patched -16 45 0 > > The small remaining difference seems to be due to the numbers > not being read at exactly the same time, and is also seen > without guard pages. > > Fixes: e0932b6c1f94 ("mm: page_alloc: consolidate free page accounting") > Cc: stable@vger.kernel.org > Assisted-by: LLM > Signed-off-by: Rik van Riel > --- > mm/page_alloc.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > Good catch. Reviewed-by: Zi Yan Best Regards, Yan, Zi