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Howlett" , Mike Rapoport , Anshuman Khandual , Catalin Marinas , Will Deacon , Samuel Holland , linux-s390@vger.kernel.org Subject: Re: mm: opaque hardware page-table entry handles To: Pedro Falcato References: <74182e50-b54f-4d2d-a27f-3a59a538d6bc@arm.com> <31d36023-d728-4eee-90f8-158c7066f565@kernel.org> <4bfeb697-9c1f-4316-96bb-9bfd66f959df@kernel.org> <6110202c-057b-4701-8c04-1a76ee7bb9ab@arm.com> <20260721124057.2820903Adf-agordeev@linux.ibm.com> <3c21586c-0ffe-4ebc-97f1-803e8acfc5c1@arm.com> <716f73c1-41cc-48a5-b0e1-a47403a30da1@kernel.org> <47b4e62e-b001-465c-9c3b-ea0636925387@arm.com> From: Muhammad Usama Anjum Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: LO4P265CA0056.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:2af::23) To VI1PR08MB3421.eurprd08.prod.outlook.com (2603:10a6:803:80::16) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-TrafficTypeDiagnostic: VI1PR08MB3421:EE_|GVXPR08MB11448:EE_|DB1PEPF000509FB:EE_|DB9PR08MB11461:EE_ X-MS-Office365-Filtering-Correlation-Id: af072100-b024-4022-cab4-08dee96f5915 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|23010399003|7416014|376014|6133799003|56012099006|4143699003|11063799006|10067099003|18002099003|22082099003; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: b7vTL8BHwNOTxKYLplXbrPWfjX34rWPneVdy2O10zqsoxxSBa3HtGWTSzF4N30Z1w1PO1GU8hY/WNzwk7LcLgX+6drtxOxHqUEf5mq61e6JbOGva/mXsEHeApx8N5eBcPABem1FZ0PPbAkMYv4fhr3jRV8HlZuXJ2ggt5UBQopEdVU/ndsTzz+5FAPW2rM3l0Y5BSk5zD1t2xYguDkDy7ZM2PXFoiMParkYn43YJAQN7R4QfHBJzQcg/rA9To6IVX9wm5xpEvIQ5XolaglCcr7fFmZFbPolJT3xELwx5inBLokKiRWNF40dwXwYd0C8K4RH7ks0K8vRb3a0hmoIpcGbqSxMb+bM3ekdEisMh1za3SmNqkhqdJUKjqlQb+9Hlq/Bsf3HuPbik76+uAnvTTS9ljG4Dvhc8W4DHamXDL7RuYdi6QBNy9qdzddi2sIni X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 24 Jul 2026 10:35:57.7408 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: af072100-b024-4022-cab4-08dee96f5915 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: DB1PEPF000509FB.eurprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB9PR08MB11461 On 24/07/2026 10:30 am, Pedro Falcato wrote: > On Thu, Jul 23, 2026 at 06:01:49PM +0100, Muhammad Usama Anjum wrote: >> On 23/07/2026 5:38 pm, David Hildenbrand (Arm) wrote: >>> >>>> So after the conversion, it doesn't makes sense to keep STRICT_MM_TYPECHECKS >>>> around. Please correct me if I'm wrong. >>>> >>>> This brings another thing in my mind that in ideal world we would just >>>> turn STRICT_MM_TYPECHECKS on for all arches all the time. But the problem >>>> with it is that STRICT_MM_TYPECHECKS defines pte_t which will stay double >>>> meaning still. So we want sort of STRICT_MM_TYPECHECKS with different name >>>> and separate stack and hardware types. >>> >>> I think STRICT_MM_TYPECHECKS is also to avoid catching when passing a pte_t into >>> a function that consumes a pmd_t, and vice versa. >>> >>> STRICT_MM_TYPECHECKS makes the compiler generate worse code today, which is why >>> we don't enable it always. >>> >>>> >>>>> >>>>> We would like to keep it, and I guess some other architectures too. >>>> David had summarized it well. We'll convert generic code to use hw_p*_t >>>> types which would be typedef to p*_t for those architectures which don't >>>> care about. But if an architecture wants to interpret these types >>>> differently or want to make sure the type is enforced and not directly >>>> dereferenced, they need to convert the arch code a well. But it can be >>>> done slowly. >>> >>> I think what Alexander meant is: they want to keep STRICT_MM_TYPECHECKS (or >>> something that achieves something similar :) ). So as long as that keeps on >>> working, all good :) >> Thank you for clarifying. >> >> It has been a month since I started this discussion. We don't have any >> disagreement on it. Should I proceed to post first part patch series considering >> most people must already have viewed these emails and are on-board with it? > > Besides the vague disagreement I have (about whether this is worth it or > not), I have an actual objection: I've been doing some constifying of pte_t* > for various reasons lately, and something that just just came up is how > hw_ptep will _not_ work with const, unless you do something like > const_hw_ptep, in which case welcome to Win32 programming or something :| > > (I'm assuming the idea still is typedef struct { pte_t *pte; } hw_ptep;) We have moved on from this as it was very disruptive. Now we envision the following: typedef struct {pte_t __pte;} hw_pte_t; With this, const wouldn't be a problem. > > Have you tried sparse instead? We want to ensure safety during compile time. Without going into more detail, more features would be build on top of this type conversion. -- Thanks, Usama