On 5/5/26 10:09, Alistair Popple wrote:
> Thanks for doing this work Mika. I've been meaning to take a look at this > series > for a while. I'm currently at LSFMM but will try and take a look this week or > next as it sounds quite useful. > > - Alistair Thanks Alistair and no problem, appreciate your insights whenever you have time. --Mika > > On 2026-05-05 at 15:16 +1000, [email protected] wrote... >> From: Mika Penttilä <[email protected]> >> >> Currently, the way device page faulting and migration works >> is not optimal, if you want to do both fault handling and >> migration at once. >> >> Being able to migrate not present pages (or pages mapped with incorrect >> permissions, eg. COW) to the GPU requires doing either of the >> following sequences: >> >> 1. hmm_range_fault() - fault in non-present pages with correct permissions, >> etc. >> 2. migrate_vma_*() - migrate the pages >> >> Or: >> >> 1. migrate_vma_*() - migrate present pages >> 2. If non-present pages detected by migrate_vma_*(): >> a) call hmm_range_fault() to fault pages in >> b) call migrate_vma_*() again to migrate now present pages >> >> The problem with the first sequence is that you always have to do two >> page walks even when most of the time the pages are present or zero page >> mappings so the common case takes a performance hit. >> >> The second sequence is better for the common case, but far worse if >> pages aren't present because now you have to walk the page tables three >> times (once to find the page is not present, once so hmm_range_fault() >> can find a non-present page to fault in and once again to setup the >> migration). It is also tricky to code correctly. One page table walk >> could costs over 1000 cpu cycles on X86-64, which is a significant hit. >> >> We should be able to walk the page table once, faulting >> pages in as required and replacing them with migration entries if >> requested. >> >> Add a new flag to HMM APIs, HMM_PFN_REQ_MIGRATE, >> which tells to prepare for migration also during fault handling. >> Also, for the migrate_vma_setup() call paths, a flag, MIGRATE_VMA_FAULT, >> is added to tell to add fault handling to migrate. >> >> One extra benefit of migrating with hmm_range_fault() path >> is the migrate_vma.vma gets populated, so no need to >> retrieve that separataly. >> >> Tested in X86-64 VM with HMM test device, passing the selftests. >> For performance, the migrate throughput tests from the selftests >> show similar numbers (within error margin) as unmodified kernel. >> Tested also rebased on the >> "Remove device private pages from physical address space" series: >> https://lore.kernel.org/linux-mm/[email protected]/ >> plus a small patch to adjust with no problems. >> >> Changes v8-v9 >> - rebase on drm-tip >> - fixed uaf around migrate_vma_split_folio() usage >> - added missing pmd unlock >> >> Changes v7-v8 >> - rebase on 7.0 >> - fixed subject in two patches >> - enhanced commit messages >> - squashed patch 6 into patch 4 to fix kernel test robot warning >> - readded dropped Cc block from cover letter >> - fixed white space >> >> Changes v6-v7 >> - rebase on 7.0.0-rc6 >> - added documentation and comments >> - denote to be migrated zero page as HMM_PFN_MIGRATE alone >> - got rid of HMM_PFN_INOUT_FLAGS movement in patch 2 >> - picked up Acked-By from David for patch 1 >> >> Changes v5-v6 >> - rebase on 7.0.0-rc4 >> - use range based TLB flushing while unmapping ptes >> - gate migration behind HMM_PFN_REQ_MIGRATE for fault and >> migrate paths >> - always infer migration flags from migrate->flags only >> >> Changes v4-v5 >> - rebase on 6.19 >> - fixed David's email address >> - fixed link issue without CONFIG_TRANSPARENT_HUGEPAGE >> - refactored into smaller commits >> - added more comments to code >> >> Changes v3-v4: >> - rebase on 6.19-rc8 >> - fixed issues found by kernel test robot with random configs >> - fixed typos >> >> Changes v2-v3: >> - rebase on 6.19-rc7 >> - fixed issues found by kernel test robot >> - fixed smatch issues reported by Dan Carpenter <[email protected]> >> - fixes to lock handling (pmd/pte) on errors >> - added assertions for pmd/pte lock states >> - other issues discovered by Matthew, thanks! >> >> Changes v1-v2: >> - rebase on 6.19-rc6 >> - fixed issues found by kernel test robot >> - fixed locking (pmd/ptl) to cover handle_ and prepare_ regions >> parts if migrating >> - other issues discovered by Matthew, thanks! >> >> Changes RFC-v1: >> - rebase on 6.19-rc5 >> - adjust for the device THP >> - changes from feedback >> >> Revisions: >> - RFC >> https://lore.kernel.org/linux-mm/[email protected]/ >> - v1: >> https://lore.kernel.org/all/[email protected]/ >> - v2: >> https://lore.kernel.org/all/[email protected]/ >> - v3: >> https://lore.kernel.org/all/[email protected]/ >> - v4: >> https://lore.kernel.org/all/[email protected]/ >> - v5: >> https://lore.kernel.org/linux-mm/[email protected]/ >> - v6: >> https://lore.kernel.org/linux-mm/[email protected]/ >> - v7: >> https://lore.kernel.org/linux-mm/[email protected]/ >> - v8: >> https://lore.kernel.org/linux-mm/[email protected]/ >> >> Cc: David Hildenbrand <[email protected]> >> Cc: Jason Gunthorpe <[email protected]> >> Cc: Leon Romanovsky <[email protected]> >> Cc: Alistair Popple <[email protected]> >> Cc: Balbir Singh <[email protected]> >> Cc: Zi Yan <[email protected]> >> Cc: Matthew Brost <[email protected]> >> Cc: Andrew Morton <[email protected]> >> Cc: Lorenzo Stoakes <[email protected]> >> Cc: "Liam R. Howlett" <[email protected]> >> Cc: Vlastimil Babka <[email protected]> >> Cc: Mike Rapoport <[email protected]> >> Cc: Suren Baghdasaryan <[email protected]> >> Cc: Michal Hocko <[email protected]> >> >> Mika Penttilä (5): >> mm/Kconfig: changes for migrate on fault for device pages >> mm: Add helper to convert HMM pfn to migrate pfn >> mm/hmm: do the plumbing for HMM to participate in migration >> mm: setup device page migration in HMM pagewalk >> lib/test_hmm:: add a new testcase for the migrate on fault >> >> include/linux/hmm.h | 19 +- >> include/linux/migrate.h | 26 +- >> lib/test_hmm.c | 101 ++- >> lib/test_hmm_uapi.h | 19 +- >> mm/Kconfig | 2 + >> mm/hmm.c | 835 +++++++++++++++++++++++-- >> mm/migrate_device.c | 583 +++-------------- >> tools/testing/selftests/mm/hmm-tests.c | 54 ++ >> 8 files changed, 1066 insertions(+), 573 deletions(-) >> >> drm-tip >> base-commit: 94d56a898a2db27f841b17f6966a81ba502fe63c >> -- >> 2.50.0 >>
