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charset=UTF-8 Date: Mon, 24 Aug 2026 21:04:03 +0900 Message-Id: Cc: "Alice Ryhl" , "Burak Emir" , "Yury Norov" , "Miguel Ojeda" , "Boqun Feng" , "Gary Guo" , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , "Benno Lossin" , "Andreas Hindborg" , "Trevor Gross" , "Danilo Krummrich" , "Daniel Almeida" , "Tamir Duberstein" , =?utf-8?q?Onur_=C3=96zkan?= , "David Airlie" , "Simona Vetter" , "Greg Kroah-Hartman" , "John Hubbard" , "Alistair Popple" , "Timur Tabi" , "Zhi Wang" , , , , , "Yury Norov" , "dri-devel" Subject: Re: [PATCH v7 02/10] rust: bitmap: restrict bitmap length to at most i32::MAX From: "Eliot Courtney" To: "Alexandre Courbot" , "Eliot Courtney" X-Mailer: aerc 0.21.0-0-g5549850facc2 References: <20260817-chid-v7-0-a5872e64d8f4@nvidia.com> <20260817-chid-v7-2-a5872e64d8f4@nvidia.com> In-Reply-To: X-ClientProxiedBy: TY4PR01CA0026.jpnprd01.prod.outlook.com (2603:1096:405:2bf::12) To DS0PR12MB6413.namprd12.prod.outlook.com (2603:10b6:8:ce::10) Precedence: bulk X-Mailing-List: nova-gpu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR12MB6413:EE_|PH7PR12MB8122:EE_ X-MS-Office365-Filtering-Correlation-Id: 6f00238f-77c3-4768-8dc2-08df01d7cbfa X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|1800799024|23010399003|366016|10070799003|6133799003|3023799007|10067099003|11063799006|5023799004|56012099006|4143699003|22082099003|18002099003; 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Restricting all bitmaps >> (even non-`BitmapVec` backed ones) to a maximum size of `i32::MAX` >> simplifies a few things and matches `BitmapVec::MAX_LEN`. > > Can we be more explicit (if possible in the doccomments as well) about > what "a few things" is? IIUC we are doing this because some of the C > APIs take an `int`, so we can just state that much. Yeah, it somewhat depends on which version of the underlying C function is being called. The exact details vary between drm-rust-next's version and bitmap-for-next's version. Here is a (non-exhaustive?) list: - `copy_and_extend` truncates `len` to u32, which is wrong for > u32::MAX size - `__bitmap_set` and `__bitmap_clear` need i32 for size and u32 for start - so you couldn't run these on a `Bitmap` with a > i32::MAX size. - `next_zero_area_off` on bitmap-for-next can overflow without this restriction (we still have some overflow cases even with this restriction on 32-bit, or through `align_offset` in general) I'll add a note to the commit message and doccomment about the reasoning which IMO should more be about keeping the set of invariants between BitmapVec and Bitmap the same. > > Note also (as a point to consider, not a review ask) that we could > enforce this requirement in the type system if we made `nbits` a > `Bounded`. Currently `from_raw` and `from_raw_mut` are > unsafe, so this becomes one more invariant for the caller to uphold, but > would we have made them unsafe just for this if they weren't already? I heard from Yury [1] that there is some work to eventually unify on unsigned longs. So I expect that the i32::MAX restriction may not be around forever. That's why I didn't do a change like Bounded here cos it's quite intrusive. [1]: https://lore.kernel.org/all/anELwosZSI5MZR4d@yury/