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Tue, 6 May 2025 13:54:13 -0700 Date: Tue, 6 May 2025 13:54:10 -0700 From: Nicolin Chen To: Jason Gunthorpe CC: Baolu Lu , , , , , , , , , , , , , , , , , , , , , , , , , , , Subject: Re: [PATCH v2 13/22] iommufd: Add mmap interface Message-ID: References: <7be26560c604b0cbc2fd218997b97a47e4ed11ff.1745646960.git.nicolinc@nvidia.com> <20250505165019.GM2260709@nvidia.com> <20250505172813.GS2260709@nvidia.com> <20250506125425.GX2260709@nvidia.com> Precedence: bulk X-Mailing-List: linux-doc@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: <20250506125425.GX2260709@nvidia.com> X-NV-OnPremToCloud: AnonymousSubmission X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SN1PEPF000252A4:EE_|DS0PR12MB9423:EE_ X-MS-Office365-Filtering-Correlation-Id: bb339c00-ab95-4991-0735-08dd8ce036c5 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|7416014|82310400026|376014|36860700013; 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X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 06 May 2025 20:54:37.4551 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: bb339c00-ab95-4991-0735-08dd8ce036c5 X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.117.160];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: SN1PEPF000252A4.namprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DS0PR12MB9423 On Tue, May 06, 2025 at 09:54:25AM -0300, Jason Gunthorpe wrote: > On Mon, May 05, 2025 at 01:07:34PM -0700, Nicolin Chen wrote: > > On Mon, May 05, 2025 at 02:28:13PM -0300, Jason Gunthorpe wrote: > > > On Mon, May 05, 2025 at 10:21:03AM -0700, Nicolin Chen wrote: > > > > > > > +void iommufd_ctx_free_mmap(struct iommufd_ctx *ictx, unsigned long immap_id) > > > > > > > +{ > > > > > > > + kfree(mtree_erase(&ictx->mt_mmap, immap_id >> PAGE_SHIFT)); > > > > > > > > > > > > MMIO lifecycle question: what happens if a region is removed from the > > > > > > maple tree (and is therefore no longer mappable), but is still mapped > > > > > > and in use by userspace? > > > > > > > > > > I think we should probably zap it and make any existing VMAs > > > > > SIGBUS... Otherwise it is hard to reason about from the kernel side > > > > > > > > I added in v3 a pair of open/close op that would refcount the > > > > vIOMMU object (owner of the mmap region). This would EBUSY the > > > > vIOMMU destroy ioctl that would call this function. > > > > > > That's no good, we can't have VMAs prevent cleaning up iommufd > > > > Hmm, would you please elaborate why? > > It would create weird dependencies in the kernel that become hard to > manage. If we really need to unplug the PFNs behind the VMA for some > reason then we should always allow that to progress. Requiring the VMA > to be unmapped first is not great. OK. Now I start to think about the FD situation: either a fault queue or an eventq returns an FD and holds a refcount on the event object. So the event object can't be destroyed unless the FD is released first. Are we doing it incorrectly here? > > > objects, the right thing is to zap it with invalidate_mapping_range() > > > > Also, I can't find much info about invalidate_mapping_range(). Is > > it a user space call? > > unmap_mapping_range(), see how VFIO uses it. I see! It just needs to call that function when we remove the mmap for a vIOMMU destroy(). > > I do see a few drivers defining a fault op in vm_operations_struct, > > where VM_FAULT_SIGBUS is returned when there is page backing up the > > VMAs. Is it what we need here? > > Probably as well Once the vma is unmapped, any further access would trigger a fault, right? That's how it works. I see do_fault() report a VM_FAULT_SIGBUS, when !vma->vm_ops->fault. So, perhaps we don't need an iommufd fault op at this moment. Thanks Nicolin