Re: [PATCH v5 0/6] Migrate on fault for device pages

[Balbir Singh]

On 2/11/26 19:12, mpenttil@xxxxxxxxxx wrote:
> From: Mika Penttilä <mpenttil@xxxxxxxxxx>
> 
> 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/20260130111050.53670-1-jniethe@xxxxxxxxxx/
> plus a small patch to adjust with no problems.
> 
> 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 <dan.carpenter@xxxxxxxxxx>
>   - 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/20250814072045.3637192-1-mpenttil@xxxxxxxxxx/
>   - v1: https://lore.kernel.org/all/20260114091923.3950465-1-mpenttil@xxxxxxxxxx/
>   - v2: https://lore.kernel.org/all/20260119112502.645059-1-mpenttil@xxxxxxxxxx/
>   - v3: https://lore.kernel.org/all/20260126111939.1332983-2-mpenttil@xxxxxxxxxx/
>   - v4: https://lore.kernel.org/all/20260202112622.2104213-1-mpenttil@xxxxxxxxxx/
> 
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Cc: Leon Romanovsky <leonro@xxxxxxxxxx>
> Cc: Alistair Popple <apopple@xxxxxxxxxx>
> Cc: Balbir Singh <balbirs@xxxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
> 
> Mika Penttilä (6):
>   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: implement device page migration for HMM pagewalk
>   mm: add new testcase for the migrate on fault case
>   mm:/migrate_device.c: remove migrate_vma_collect_*()
> 
>  include/linux/hmm.h                    |  18 +-
>  include/linux/migrate.h                |  27 +-
>  lib/test_hmm.c                         | 100 ++-
>  lib/test_hmm_uapi.h                    |  19 +-
>  mm/Kconfig                             |   2 +
>  mm/hmm.c                               | 814 +++++++++++++++++++++++--
>  mm/migrate_device.c                    | 602 +++---------------
>  tools/testing/selftests/mm/hmm-tests.c |  54 ++
>  8 files changed, 1053 insertions(+), 583 deletions(-)
> 
> 
> base-commit: 05f7e89ab9731565d8a62e3b5d1ec206485eeb0b


Thanks for your patience, I am just trying out the patches and reviewing them in parallel

Balbir