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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. Matt > Thanks for the feedback > Balbir Singh