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

[Mika Penttilä]

Hi,

On 3/17/26 12:06, Balbir Singh wrote:

> 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
>
No problem, and thanks! btw there's also v6 it has only two minor changes:
https://lore.kernel.org/linux-mm/20260316062407.3354636-1-mpenttil@xxxxxxxxxx/

--Mika