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Thu, 10 Jul 2025 12:08:26 -0700 Date: Thu, 10 Jul 2025 12:08:25 -0700 From: Nicolin Chen To: Xu Yilun CC: , , , , , , , , , , , , , Subject: Re: [PATCH v4 2/7] iommufd: Add iommufd_object_tombstone_user() helper Message-ID: References: <20250709040234.1773573-1-yilun.xu@linux.intel.com> <20250709040234.1773573-3-yilun.xu@linux.intel.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20250709040234.1773573-3-yilun.xu@linux.intel.com> X-NV-OnPremToCloud: AnonymousSubmission X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: MWH0EPF000A6731:EE_|CH1PR12MB9693:EE_ X-MS-Office365-Filtering-Correlation-Id: cbb8a8dd-7df0-4618-2157-08ddbfe52f3c X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|1800799024|36860700013|82310400026|7053199007; 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X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 10 Jul 2025 19:08:41.6369 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: cbb8a8dd-7df0-4618-2157-08ddbfe52f3c X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.118.233];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: MWH0EPF000A6731.namprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH1PR12MB9693 On Wed, Jul 09, 2025 at 12:02:29PM +0800, Xu Yilun wrote: > Add the iommufd_object_tombstone_user() helper, which allows the caller > to destroy an iommufd object created by userspace. > > This is useful on some destroy paths when the kernel caller finds the > object should have been removed by userspace but is still alive. With > this helper, the caller destroys the object but leave the object ID > reserved (so called tombstone). The tombstone prevents repurposing the > object ID without awareness of the original user. > > Since this happens for abnormal userspace behavior, for simplicity, the > tombstoned object ID would be permanently leaked until > iommufd_fops_release(). I.e. the original user gets an error when > calling ioctl(IOMMU_DESTROY) on that ID. > > The first use case would be to ensure the iommufd_vdevice can't outlive > the associated iommufd_device. > > Suggested-by: Jason Gunthorpe > Reviewed-by: Lu Baolu > Co-developed-by: Aneesh Kumar K.V (Arm) > Signed-off-by: Aneesh Kumar K.V (Arm) > Signed-off-by: Xu Yilun Reviewed-by: Nicolin Chen With one nit: > - while (!xa_empty(&ictx->objects)) { > + for (;;) { > unsigned int destroyed = 0; > unsigned long index; > + bool empty = true; > > + /* > + * xa_for_each() will not return tomestones (zeroed entries), > + * which prevent the xarray being empty. So use an empty flags Since the first "empty" and the second "empty" are different things, > + * instead of xa_empty() to indicate all entries are either > + * NULLed or tomestoned. > + */ let's write something like this (correcting typos too): /* * We can't use xa_empty(), as a tombstone (NULLed entry) would * prevent it returning true, unlike xa_for_each() ignoring the * NULLed entries. So use an empty flag instead of xa_empty() to * indicate all entries are either NULLed or tombstoned. */