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Wed, 11 Jun 2025 03:45:34 +0000 Message-ID: Date: Wed, 11 Jun 2025 09:15:28 +0530 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] mm: Allow lockless kernel pagetable walking To: Lorenzo Stoakes , David Hildenbrand Cc: akpm@linux-foundation.org, catalin.marinas@arm.com, will@kernel.org, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, suzuki.poulose@arm.com, steven.price@arm.com, gshan@redhat.com, linux-arm-kernel@lists.infradead.org, yang@os.amperecomputing.com, ryan.roberts@arm.com, anshuman.khandual@arm.com References: <20250610114401.7097-1-dev.jain@arm.com> <20250610114401.7097-2-dev.jain@arm.com> <970e1b84-7f47-4a51-ad99-4df9026b8f7d@redhat.com> <359668fb-df61-4135-89ee-2675a06a5c1b@lucifer.local> Content-Language: en-US From: Dev Jain In-Reply-To: <359668fb-df61-4135-89ee-2675a06a5c1b@lucifer.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MA0PR01CA0019.INDPRD01.PROD.OUTLOOK.COM (2603:1096:a01:b8::6) To AM9PR08MB7120.eurprd08.prod.outlook.com (2603:10a6:20b:3dc::22) MIME-Version: 1.0 X-MS-TrafficTypeDiagnostic: AM9PR08MB7120:EE_|PAVPR08MB9282:EE_|AM4PEPF00025F99:EE_|DB9PR08MB10377:EE_ X-MS-Office365-Filtering-Correlation-Id: 3494be12-3c31-402b-0353-08dda89a7f8a x-checkrecipientrouted: true NoDisclaimer: true X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam-Untrusted: BCL:0;ARA:13230040|1800799024|366016|376014|7416014; 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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 11 Jun 2025 03:46:07.3755 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 3494be12-3c31-402b-0353-08dda89a7f8a X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: AM4PEPF00025F99.EURPRD83.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB9PR08MB10377 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250611_044616_183346_B11B1A73 X-CRM114-Status: GOOD ( 12.05 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 10/06/25 6:57 pm, Lorenzo Stoakes wrote: > On Tue, Jun 10, 2025 at 03:24:16PM +0200, David Hildenbrand wrote: >> On 10.06.25 14:07, Lorenzo Stoakes wrote: >>> OK so I think the best solution here is to just update check_ops_valid(), which >>> was kind of sucky anyway (we check everywhere but walk_page_range_mm() to >>> enforce the install pte thing). >>> >>> Let's do something like: >>> >>> #define OPS_MAY_INSTALL_PTE (1<<0) >>> #define OPS_MAY_AVOID_LOCK (1<<1) >>> >>> and update check_ops_valid() to take a flags or maybe 'capabilities' field. >>> >>> Then check based on this e.g.: >>> >>> if (ops->install_pte && !(capabilities & OPS_MAY_INSTALL_PTE)) >>> return false; >>> >>> if (ops->walk_lock == PGWALK_NOLOCK && !(capabilities & OPS_MAY_AVOID_LOCK)) >>> return false; >>> >> Hm. I mean, we really only want to allow this lockless check for >> walk_kernel_page_table_range(), right? >> >> Having a walk_kernel_page_table_range_lockeless() might (or might not) be >> better, to really only special-case this specific path. > Agree completely, Dev - let's definitely do this. Makes sense. > >> So, I am wondering if we should further start splitting the >> kernel-page-table walker up from the mm walker, at least on the "entry" >> function for now. > How do you mean? > >> -- >> Cheers, >> >> David / dhildenb >>