On 7/16/26 16:15, David Hildenbrand (Arm) wrote: > On 5/25/26 10:45, [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 v11-v12 > - Fix for issue found by Intel CI in patch 3 > > Changes v10-v11 > - Fix nested mmap_read_lock in test suite > - Addressed review comments from David > > Changes v9-v10 > - Fix for issue Intel CI found, forgotten pte_unmap() before > migration_entry_wait() > > 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]/ > - v9: > https://lore.kernel.org/linux-mm/[email protected]/ > - v10: > https://lore.kernel.org/linux-mm/[email protected]/ > - v11: > 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 | 118 +++- > lib/test_hmm_uapi.h | 19 +- > mm/Kconfig | 2 + > mm/hmm.c | 843 +++++++++++++++++++++++-- > mm/migrate_device.c | 583 +++-------------- > tools/testing/selftests/mm/hmm-tests.c | 54 ++ > 8 files changed, 1086 insertions(+), 578 deletions(-) > What's the status of this? > > Patch #3+#5 are extremely hard to review, I wonder if there could be a better > way to split this up better so we get smaller changes that can be more easily > reviewed. > I will send V13 with few fixes/improvements in couple weeks when back from PTO. I will also address the recent "pmd becomes huge or cleared" affecting mainline migrate_vma_collect() Not sure about the split though, patch 4 is mostly move over from migrate_device.c and patch 5 testcase. But I try to see if there's some sensible way.
Thanks, Mika
