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spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/18/25 14:57, Matthew Brost wrote: > On Fri, Jul 18, 2025 at 01:57:13PM +1000, Balbir Singh wrote: >> On 7/18/25 09:40, Matthew Brost wrote: >>> On Fri, Jul 04, 2025 at 09:34:59AM +1000, Balbir Singh wrote: >> ... >>>> >>>> The nouveau dmem code has been enhanced to use the new THP migration >>>> capability. >>>> >>>> Feedback from the RFC [2]: >>>> >>> >>> Thanks for the patches, results look very promising. 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. 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