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Mon, 16 Jun 2025 19:02:16 -0700 Date: Mon, 16 Jun 2025 19:02:08 -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> 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: <20250616162501.GN1174925@nvidia.com> X-NV-OnPremToCloud: AnonymousSubmission X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS2PEPF00003443:EE_|MN2PR12MB4373:EE_ X-MS-Office365-Filtering-Correlation-Id: 7e52e82c-309f-473c-46b8-08ddad4307eb X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|36860700013|82310400026|1800799024|376014; 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X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 17 Jun 2025 02:02:36.2312 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 7e52e82c-309f-473c-46b8-08ddad4307eb 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: DS2PEPF00003443.namprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: MN2PR12MB4373 On Mon, Jun 16, 2025 at 01:25:01PM -0300, Jason Gunthorpe wrote: > On Sun, Jun 15, 2025 at 10:02:03PM -0700, Nicolin Chen wrote: > > FIXTURE_TEARDOWN(iommufd_dirty_tracking) > > { > > - munmap(self->buffer, variant->buffer_size); > > - munmap(self->bitmap, DIV_ROUND_UP(self->bitmap_size, BITS_PER_BYTE)); > > + unsigned long size = variant->buffer_size; > > + > > + if (variant->hugepages) > > + size = __ALIGN_KERNEL(variant->buffer_size, HUGEPAGE_SIZE); > > + munmap(self->buffer, size); > > + free(self->buffer); > > + free(self->bitmap); > > teardown_iommufd(self->fd, _metadata); > > munmap followed by free isn't right.. You are right. I re-checked with Copilot. It says the same thing. I think the whole posix_memalign() + mmap() confuses me.. Yet, should the bitmap pair with free() since it's allocated by a posix_memalign() call? > This code is using the glibc allocator to get a bunch of pages mmap'd > to an aligned location then replacing the pages with MAP_SHARED and > maybe HAP_HUGETLB versions. And I studied some use cases from Copilot. It says that, to use the combination of posix_memalign+mmap, we should do: aligned_ptr = posix_memalign(pagesize, pagesize); unmap(aligned_ptr, pagesize); mapped = mmap(aligned_ptr, pagesize, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0); munmap(mapped, pagesize); // No free() after munmap(). ---breakdown--- Before `posix_memalign()`: [ heap memory unused ] After `posix_memalign()`: [ posix_memalign() memory ] ← managed by malloc/free ↑ aligned_ptr After `munmap(aligned_ptr)`: [ unmapped memory ] ← allocator no longer owns it After `mmap(aligned_ptr, ..., MAP_FIXED)`: [ anonymous mmap region ] ← fully remapped, under your control ↑ mapped ---end--- It points out that the heap bookkeeping will be silently clobbered without the munmap() in-between (like we are doing): ---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--- It also gives a simpler solution for a memory that is not huge page backed but huge page aligned (our !variant->hugepage case): ---code--- void *ptr; size_t alignment = 2 * 1024 * 1024; // or whatever HUGEPAGE_SIZE was size_t size = variant->buffer_size; // Step 1: Use posix_memalign to get an aligned pointer if (posix_memalign(&ptr, alignment, size) != 0) { perror("posix_memalign"); return -1; } // Use the memory directly self->buffer = ptr; // Access/manipulate the memory as needed... // Step 2: Clean up when done free(self->buffer); ---end--- Also, for a huge page case, there is no need of posix_memalign(): "Hugepages are not part of the standard heap, so allocator functions like posix_memalign() or malloc() don't help and can even get in the way." 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--- Should we follow? Thanks Nicolin