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Tue, 17 Jun 2025 14:23:43 -0700 Date: Tue, 17 Jun 2025 14:23:41 -0700 From: Nicolin Chen To: Jason Gunthorpe CC: , , , , , , , Subject: Re: [PATCH rc 1/4] iommufd/selftest: Fix iommufd_dirty_tracking with large hugepage sizes Message-ID: References: <9515eb5cb58bc8dfec083df51550bd9ae6d60da9.1750049883.git.nicolinc@nvidia.com> <20250616162501.GN1174925@nvidia.com> <20250617115948.GV1174925@nvidia.com> Precedence: bulk X-Mailing-List: iommu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20250617115948.GV1174925@nvidia.com> X-NV-OnPremToCloud: AnonymousSubmission X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BN2PEPF000055DA:EE_|CY8PR12MB7514:EE_ X-MS-Office365-Filtering-Correlation-Id: 0476c257-efd1-4ae9-e044-08ddade54b2f X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|82310400026|376014|1800799024|36860700013; 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X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 17 Jun 2025 21:24:07.5052 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 0476c257-efd1-4ae9-e044-08ddade54b2f X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.117.161];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: BN2PEPF000055DA.namprd21.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY8PR12MB7514 On Tue, Jun 17, 2025 at 08:59:48AM -0300, Jason Gunthorpe wrote: > On Mon, Jun 16, 2025 at 07:02:08PM -0700, Nicolin Chen wrote: > > ---breakdown--- > > After `posix_memalign()`: > > [ posix_memalign() memory ] ← malloc thinks it owns this > > > > Then `mmap(aligned_ptr, ..., MAP_FIXED)`: > > [ anonymous mmap region ] ← malloc still thinks it owns this (!) > > ↑ mapped > > ---end--- > > Yes, this is correct and what we are doing here. The allocator always > owns it and we are just replacing the memory with a different mmap. Hmm, if allocator always owns it. Does that mean the munmap() [3] will release what [1] and [2] do (allocating and replacing)? [1] posix_memalign() [2] mmap() [3] munmap() > > // Step 1: Use posix_memalign to get an aligned pointer > > if (posix_memalign(&ptr, alignment, size) != 0) { > > perror("posix_memalign"); > > return -1; > > } > > Also no, the main point of this is to inject MAP_SHARED which > posix_memalign cannot not do. I see. > > Instead, it suggests a cleaner version without posix_memalign(): > > ---code--- > > void *addr = mmap(NULL, variant->buffer_size, PROT_READ | PROT_WRITE, > > MAP_SHARED | MAP_ANONYMOUS | MAP_HUGETLB | MAP_POPULATE, > > -1, 0); > > if (addr == MAP_FAILED) { perror("mmap"); return -1; } > > ---end--- > > Yes, we could do this only for MAP_HUGETLB, but it doesn't help the > normal case with MAP_SHARED. > > So I would leave it alone, use the version I showed. OK. Will respin a v2 with that. Thanks Nicolin