From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from CY3PR05CU001.outbound.protection.outlook.com (mail-westcentralusazon11013042.outbound.protection.outlook.com [40.93.201.42]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8D1AB303C97 for ; Fri, 25 Sep 2026 02:44:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=fail smtp.client-ip=40.93.201.42 ARC-Seal:i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790304293; cv=fail; b=mCGc0KHiD+HocDhJwjzCFGkM25ha9LpfENRi4t6830/zpGgrKWv3M5zFmF5xjR4yjJKY7l8uAGHFeymXg+FcRcusXyPTdWU9/mf5oc1+CxZcx1XsLmgOWxQrWxWBCbF4bnOHp4HhUz0X3Gdt8LRClSH6h0vb+j0+LZaf58sWMOI= ARC-Message-Signature:i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790304293; c=relaxed/simple; bh=gsGvYcZOY37UE0Z8R4VQoxRJFw0dAo4RDe+NqoUH7X4=; h=Content-Type:Date:Message-Id:Cc:To:From:Subject:References: In-Reply-To:MIME-Version; b=XqxSdO+Yk7Thb3uM0bDPV1glGUXev/p4dmLuuEmyrTzTfET7ERYIJ4kDGQaFPAoQndsf/FpMQJqEXqIItbGte1dUcaKgvLo9UxZHS/1PLkczic4wCdTAyx+z8u2nqsvIwrGXGqhWw+Q2VkzQC9Vk9LtzP2tBZFLc+OtlT0tkQ84= ARC-Authentication-Results:i=2; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=nvidia.com; spf=fail smtp.mailfrom=nvidia.com; dkim=pass (2048-bit key) header.d=Nvidia.com header.i=@Nvidia.com header.b=op358lV3; arc=fail smtp.client-ip=40.93.201.42 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=nvidia.com Authentication-Results: smtp.subspace.kernel.org; spf=fail smtp.mailfrom=nvidia.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=Nvidia.com header.i=@Nvidia.com header.b="op358lV3" ARC-Seal: i=1; a=rsa-sha256; s=arcselector10001; d=microsoft.com; cv=none; b=Zm93Iy6ZYVvOojVlG3wrqDeeJ1z/9Kor3oXJSe+x9vFaxwSrKr8jTE3BlDgxY8jwbO0STLvruCHD15FS+sM1SVHS42F6yPHHH63lJDZ/k6HaGBCGaLxdv82DEunTrPKu5YzY7gyTnK41Khg7ZZCpvnPpuFC1c23OeiXHPd+W6NZiziyOLNM0hxGGWVmgukWXd2IExmLj+AqzYBeKA9Sqw/5KNSBjfR1McF78fcGUE56DdFXV5PdK9uRgvvenh7j6P/WIEEJD83P6Tbd7NiBQ6NpGPbh2vCpX3m42J17V8x5jrdl60Mrg+L2Yp3qtII4vjhX5SLU3xSwFDe3xEWp1Lw== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com; s=arcselector10001; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-AntiSpam-MessageData-ChunkCount:X-MS-Exchange-AntiSpam-MessageData-0:X-MS-Exchange-AntiSpam-MessageData-1; bh=oD/ac/D6J0+BKIbZE7nTIglrsiPdFdiGbDxN5UoC+G4=; b=mwy2SkYfjk1k3Vb35PPj4/+PO87sRv9mD5ADrDweQhD/6vj95O+5oYoEe+j5JRqnETH7l1FDW8PbxwnF0kuCfE7hL1I7gp0NwvZuT+KkV3FTUU5+nxx0kpVZKJO/1wqcc7T5noUrakaHFMlmDZCtpUfR9N1lY3zAgGpMjIiHc+n6xDIQP0h8s99JNG4KaakNZzS/1ZqO8w14Db5ZLMHCDFySVuuMU84yBgATLixOdQMlr/tfBVbVuUpwBpfYkr5DbiEvsqDUAvDI3cukp10EHu36MQtQxIMXVhUpt4YiPZ5DDEs/I+9+nC4kjV9X/dh0SjWRNKe7Y1z4f4jt0eSaxA== ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=pass smtp.mailfrom=nvidia.com; dmarc=pass action=none header.from=nvidia.com; dkim=pass header.d=nvidia.com; arc=none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=Nvidia.com; s=selector2; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=oD/ac/D6J0+BKIbZE7nTIglrsiPdFdiGbDxN5UoC+G4=; b=op358lV3Pvjy8NKL2jt+NyUJgrn7oK7xJKkpfSGdTxhi6z/s3YGZRc++iqTG8DcVnxWv5TS9QuOSEqNiyN/2faZbgunhhfIZMChDw/d9yAKV4rU5ouTWpVE6GcD/w9gkxle2XKsRPMV+GAVtE327J/Bt8DQ4j+0OlhaqPyPBN/6nu4Sm2nEAPttc03EmKmlGmNFoVEjSGgfia21FIgdqtZ/ZjoZ5EWF4Ax2VxJTNvF+8sJTBTNl1PnZ+XuGBJTU/eSFDGzss1igG5h6uq6ZO2dC+SmirSTjEkIEHpDJhpY5esVHPzD9bH+CgJqI+Q/Ais7mkZ6TKY35fdXbRZsM/OQ== Authentication-Results: mx.microsoft.com 1; dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=nvidia.com; Received: from IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) by PH8PR12MB6914.namprd12.prod.outlook.com (2603:10b6:510:1cb::21) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.451.18; Fri, 25 Sep 2026 02:44:47 +0000 Received: from IA0PR12MB8374.namprd12.prod.outlook.com ([fe80::d85f:4c87:ae84:3f16]) by IA0PR12MB8374.namprd12.prod.outlook.com ([fe80::d85f:4c87:ae84:3f16%6]) with mapi id 15.21.0451.014; Fri, 25 Sep 2026 02:44:47 +0000 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Thu, 24 Sep 2026 22:44:44 -0400 Message-Id: Cc: "Ilya Gladyshev" , , , , , , , , , , , , , , , , , , , , , , , To: "Linus Torvalds" , "David Hildenbrand (Arm)" From: "Zi Yan" Subject: Re: [PATCH v6 3/3] mm: implement page refcount locking via dedicated bit X-Mailer: aerc 0.22.0 References: <56e036665dbeb0a8f51d697220d5ed84f210994c@linux.dev> <570cf649-a8bf-4073-be71-207fbb2bef40@kernel.org> In-Reply-To: X-ClientProxiedBy: YQZPR01CA0157.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:8c::6) 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_|PH8PR12MB6914:EE_ X-MS-Office365-Filtering-Correlation-Id: bd19f3fe-3509-4fec-1398-08df1aaef645 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|376014|7416014|366016|1800799024|6133799003|4143699003|10067099003|11063799006|56012099006|18002099003|22082099003; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:IA0PR12MB8374.namprd12.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230040)(23010399003)(376014)(7416014)(366016)(1800799024)(6133799003)(4143699003)(10067099003)(11063799006)(56012099006)(18002099003)(22082099003);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?MWN2SjJkb0VVaGlMVldaamNFWUdPa0hFTFpsR0NQSVpHc3BmN3NLZkhRS0wr?= =?utf-8?B?eFJnSDlMRmVpM1lGY1g3TTY1SklxV3JCdE01L0NtcCszbzQrUmJtWUduc1Ew?= =?utf-8?B?SnlZRWErNStaamhrNHRqZXhDeENlTU81S0ZQeUdqSkZubmhnMVdGcmdnMDZN?= =?utf-8?B?eU1IcEZLa05DK2FWOWcwcUJVekEveXNYSXA4aDNwcFRsMkJXaCtyRWJMNFNl?= =?utf-8?B?STB2ZExqcFhhb1ZyNjZVTTFLbjRtVEZoa3dtV0E2VjFQRkdGWDlKc0MvUWlS?= =?utf-8?B?QS91UzBTZHZzeWwxYy83ZHk3Qk5GRlN2cUNZNkh3OG1JR25EMGo4QU41WEJm?= =?utf-8?B?Mko3Z2ZlWDF4YjRNbmFPSWoyeEl6TFBlVFJVRG1sRWhhVXZrUE4wbXY2TUJR?= =?utf-8?B?Sk5SYXBVdnNkWWtCd1dNWTRNR1E4eWIwc0dDR0lBTDgwMHpzMm9aNHRzU3Nx?= =?utf-8?B?NzA4UEpEOUp2NnhaamVTRThWVURkMHNwZmdHMlpKQ2tPNm5hdHpCVTI2bTln?= =?utf-8?B?L3pXS05xZ0w5QUxyUElzZ01HTE8valJjY2pTQkRMZnlvd0lnay9VNDRaR080?= =?utf-8?B?TGluNHFBZXBLVjZBUHRuK2I3SjZFRktLaVZxZ0NEZ2toaE5oYXRTTTl3TzJY?= =?utf-8?B?cWl6eVJOTE9xSHVUVk56MHlzNlExRWlCbW9KT1hvOVZ6dlA1R2plUUlYRFVp?= =?utf-8?B?V283M2w2elBHZlcweTQ5Zms5em43WGp1eTVpS3FPZ0p0ckxSaFk4alZnN2ZV?= =?utf-8?B?S0N6OUFYWnlBRGZGY296eCsyd0lhVUhFTVFrWE5vVktiR2NJb242YktKVzRN?= =?utf-8?B?L0Z4VVo5dktrRmZQb04zekVYVHVMNjBGOE1nVmRhWlAxM1k2VjZVaXpGT0p2?= =?utf-8?B?bUljZzBIS3hmUjJOdnBEYmhLczcrQ3JSUkl5bkJDSmNJV2xEdDRCbWpOWmpE?= =?utf-8?B?TG5Bd04zK0dxdVllRGJoaHl3c2g0UEt4T3prdy9WMGZoaytOaW8zQmsxTHdQ?= =?utf-8?B?dVFKQ1pscnBia1ZiTWRvdlFjaXZmL0UzdUFSSENUbStpdnBWQWZwRzdXd2k3?= =?utf-8?B?L3VzUHdldGtndVJHbkZtQzdQVGpBN0xuU3l3UzhrZGE0VUx4L0dzUGU1dUhF?= =?utf-8?B?aFJEWXhwSFZKemF3b01NVzZjSmY5V2syNVVhT1NiUXhpc0VaV3lEWU5tYXlh?= =?utf-8?B?L24wUWVOYnNqVjM4L01HZ0llMkliUlF3NXBrQUFpT0NwZ0oxM1U1L0VJVmJ0?= =?utf-8?B?SjkrUUNQOEpVY0R0SCsxYzdEcGxCQS9seThQeGk0L0swenMwYnhWM1JYcDNa?= =?utf-8?B?SC9IV3poNkJzMExWcDFqLzh6Z1JJUVZua29FcjVkWE9pUVp6UmpRMWF4dVJl?= =?utf-8?B?dTc0bHFyZjR2bWxUQW9DN3ZwNkFtamxUWHBWZk0zNVRQZmprM21BZ2hBdnpq?= =?utf-8?B?d1BQeDlVVktIN0c4aU8zRkFsY3V1TlR6bTlNeXlZcjhJbkgvTFJ5V2hxM0cr?= =?utf-8?B?Wk1jOTVaMmRXV2FkWklyYjdLL05Pd2pLMXFZVU94Ui91MlRGSzMvckVjS0R2?= =?utf-8?B?alZpY1czbklWeS9QR09pS092Mk9HWGRKUmtiQnlZZjlmMVVlKzA2SnpkanVZ?= =?utf-8?B?SitLa2x0S3F5TDJHZmd6ZWtkbVpYK25zNjVDcnJ0ZjJiVUVlUGt4YTcrQXNx?= =?utf-8?B?N1VtRVBuNHpaaFFXR0hlZ2ZSUExYdjMxUlhZbEhBbFRLUzNNVTJRUUc0ZnUy?= =?utf-8?B?cGpSYU5sckcveGpWRHZ6MmhZckdibnpFZVk4YnprZEl2MnB2cCt5bDJzeUtI?= =?utf-8?B?R045aUFjVy80M3dRdHNsY2FuTllhVmp2UVc2azh2cjBmQVZaanNtUTVlN1hL?= =?utf-8?B?T05hOGJmcUhqSHlJLzZMQzcxTGY4VnhKTkM3Q0tUU1V1N1RoY2c3cjdEaWhu?= =?utf-8?B?Y05SSElpQzEvVmVwMVpGS1BPVHY0a24vTGwyOWFXTWlZNit2NFJRcjZIcE1E?= =?utf-8?B?NXRpbnNXdFZVZVVmZHpmM21HcDV5RHkvdEtGbUMrYXZ2N0ZqTVhlaGI5VitI?= =?utf-8?B?a245SU9WOE83TW5qazE1djR6cDZWTHowTFVIQmVWeG5ZODZyZjZDeFEzWXds?= =?utf-8?B?SFdRZUhDNnQzbUNIUUVEcXd0MDAxWndBdWZNSDRPdndqdjhUeVJGaEQzWFFP?= =?utf-8?B?aWxLZ3pvMFFvK0tMTlF4N2RlSGlFa0lDZURXQmxhVHp5QXRKcldRTWllWTdE?= =?utf-8?B?YXNHciszVnN1OE5KQ2lFOGJieHFnMFJwUlpOOHZhcGdKcU94NTV4bHFWSTZF?= =?utf-8?Q?hxAKFIr7QwDG0muVPg?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: bd19f3fe-3509-4fec-1398-08df1aaef645 X-MS-Exchange-CrossTenant-AuthSource: IA0PR12MB8374.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 25 Sep 2026 02:44:47.0521 (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: KAI96qAGOIjYW/aJWvxpG9cKiRMwbt+5QCEnfsmy0UsS04K69TcOhOB/nlRvitj1 X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH8PR12MB6914 On Thu Sep 24, 2026 at 4:06 PM EDT, Linus Torvalds wrote: > On Thu, 24 Sept 2026 at 11:52, David Hildenbrand (Arm) = wrote: >> >> There needs to be a common page freeing function that dipatches stuff. > > I think this all needs to be entirely generic to all pages. Agreed. > Everybody needs to do it right. It assumes all callers of page_ref_dec_and_test() are free page functions, but there are two exceptions: 1. compaction_free() uses folio_put_testzero(), which calls page_ref_dec_and_test() to put not used migration dst page back to isolated freelist instead of freeing them. 2. virtio_mem_fake_offline_going_offline() drops the last reference of fake-offline pages, so that they can be offline. They will need to be converted to not use page_ref_dec_and_test() in addition to a common page freeing function. And any such future use needs to be banned. > > But I think that in the scenario that Ilya mentioned: > > Refcount operations as follows (FR =3D FROZEN): > > A: DEC 1 -> 0 [ page_ref_dec_and_test() ] > B: INC 0 -> 1 [ folio_try_get() ] =3D> success > B: DEC 1 -> 0 [ page_ref_dec_and_test() ] > B: CAS 0 -> FR [ page_ref_dec_and_test() ] > B: * deallocates via __folio_put() * > > C: * re-allocates page * > C: DEC 1 -> 0 [ page_ref_dec_and_test() ] > A: CAS 0 -> FR [ page_ref_dec_and_test() ] =3D> success > A: * deallocates directly via free_frozen_pages() * =3D> BUG > > the big here is that the de-allocation was done not from the state of > the *page*, but from the state of who finished it off. > > So that > > A: CAS 0 -> FR [ page_ref_dec_and_test() ] =3D> success > > is not a problem per se. It's a fine case of "we had one single unique > final free". > > The failure is that A then goes on to do that > > "deallocates directly via free_frozen_pages" > > because A mis-judged the page as having the old stale state that it > had when *A* was using it. > > IOW, I think the soluition is either: > > (a) every user of put_page_testzero(page) agrees tro do exactly the > same thing and there is no difference between how that final > "0->FROZEN" is dealt with, so it doesn't matter that A is freeing a > page that C allocated for something else > > or > > (b) the CAS 0 -> FR state sequence always looks at the *page* to > decide what to do, never at the caller state (ie A will look at > whatever C wrote when it allocated the page to know how to do that > final free). > > Hmm? > Just wonder if we could make 1 -> FROZEN atomic to get rid of the gap. --=20 Best Regards, Yan, Zi