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Tue, 22 Jul 2025 00:06:00 +0000 Date: Mon, 21 Jul 2025 17:07:46 -0700 From: Matthew Brost To: Balbir Singh CC: , , , Karol Herbst , Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , =?iso-8859-1?B?Suly9G1l?= Glisse , Shuah Khan , David Hildenbrand , Barry Song , Baolin Wang , "Ryan Roberts" , Matthew Wilcox , "Peter Xu" , Zi Yan , Kefeng Wang , Jane Chu , Alistair Popple , Donet Tom Subject: Re: [v1 resend 00/12] THP support for zone device page migration Message-ID: References: <20250703233511.2028395-1-balbirs@nvidia.com> <09c5ce76-f7c1-41c7-97e1-74448e6143f7@nvidia.com> <0340e202-8ec8-4226-b136-e924eca00309@nvidia.com> Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <0340e202-8ec8-4226-b136-e924eca00309@nvidia.com> X-ClientProxiedBy: SJ0PR05CA0199.namprd05.prod.outlook.com (2603:10b6:a03:330::24) To DM4PR11MB6527.namprd11.prod.outlook.com (2603:10b6:8:8e::19) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR11MB6527:EE_|DM3PR11MB8733:EE_ X-MS-Office365-Filtering-Correlation-Id: b7c70b75-6c88-441b-3aa2-08ddc8b38a8c X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|7416014|1800799024; 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I wanted to give > >>> some quick feedback: > >>> > >> > >> Are you seeing improvements with the patchset? > >> > > > > We're nowhere near stable yet, but basic testing shows that CPU time > > from the start of migrate_vma_* to the end drops from ~300µs to ~6µs on > > a 2MB GPU fault. A lot of this improvement is dma-mapping at 2M > > grainularity for the CPU<->GPU copy rather than mapping 512 4k pages > > too. > > > >>> - You appear to have missed updating hmm_range_fault, specifically > >>> hmm_vma_handle_pmd, to check for device-private entries and populate the > >>> HMM PFNs accordingly. My colleague François has a fix for this if you're > >>> interested. > >>> > >> > >> Sure, please feel free to post them. > >> > >>> - I believe copy_huge_pmd also needs to be updated to avoid installing a > >>> migration entry if the swap entry is device-private. I don't have an > >>> exact fix yet due to my limited experience with core MM. The test case > >>> that triggers this is fairly simple: fault in a 2MB device page on the > >>> GPU, then fork a process that reads the page — the kernel crashes in > >>> this scenario. > >>> > >> > >> I'd be happy to look at any traces you have or post any fixes you have > >> > > > > I've got it so the kernel doesn't explode but still get warnings like: > > > > [ 3564.850036] mm/pgtable-generic.c:54: bad pmd ffff8881290408e0(efffff80042bfe00) > > [ 3565.298186] BUG: Bad rss-counter state mm:ffff88810a100c40 type:MM_ANONPAGES val:114688 > > [ 3565.306108] BUG: non-zero pgtables_bytes on freeing mm: 917504 > > > > I'm basically just skip is_swap_pmd clause if the entry is device > > private, and let the rest of the function execute. This avoids > > installing a migration entry—which isn't required and cause the > > crash—and allows the rmap code to run, which flips the pages to not > > anonymous exclusive (effectively making them copy-on-write (?), though > > that doesn't fully apply to device pages). It's not 100% correct yet, > > but it's a step in the right direction. > > > > > Thanks, could you post the stack trace as well. This is usually a symptom of > not freeing up the page table cleanly. > Did you see my reply here [1]? I've got this working cleanly. I actually have all my tests passing with a few additional core MM changes. I'll reply shortly to a few other patches with those details and will also send over a complete set of the core MM changes we've made to get things stable. Matt [1] https://lore.kernel.org/linux-mm/aHrsdvjjliBBdVQm@lstrano-desk.jf.intel.com/ > Do you have my latest patches that have > > if (is_swap_pmd(pmdval)) { > swp_entry_t entry = pmd_to_swp_entry(pmdval); > > if (is_device_private_entry(entry)) > goto nomap; > } > > in __pte_offset_map()? > > Balbir Singh