Re: [PATCH 0/3] Migrate on fault for device pages

["Mika Penttilä" <mpenttil@redhat.com>]

Hi Balbir!

On 1/16/26 01:16, Balbir Singh wrote:

> On 1/14/26 20:19, mpenttil@redhat.com wrote:
>> From: Mika Penttilä <mpenttil@redhat.com>
>>
>> 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 flags, MIGRATE_VMA_FAULT,
>> is added to tell to add fault handling to migrate.
>>
>> Tested in X86-64 VM with HMM test device, passing the selftests.
>> Tested also rebased on the
>> "Remove device private pages from physical address space" series:
>> https://lore.kernel.org/linux-mm/20260107091823.68974-1-jniethe@nvidia.com/
>> plus a small patch to adjust with no problems.
>>
>> Changes from RFC:
>>   - 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@redhat.com/
>>   
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Jason Gunthorpe <jgg@nvidia.com>
>> Cc: Leon Romanovsky <leonro@nvidia.com>
>> Cc: Alistair Popple <apopple@nvidia.com>
>> Cc: Balbir Singh <balbirs@nvidia.com>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Cc: Matthew Brost <matthew.brost@intel.com>
>> Suggested-by: Alistair Popple <apopple@nvidia.com>
>> Signed-off-by: Mika Penttilä <mpenttil@redhat.com>
>>
>> Mika Penttilä (3):
>>   mm: unified hmm fault and migrate device pagewalk paths
>>   mm: add new testcase for the migrate on fault case
>>   mm:/migrate_device.c: remove migrate_vma_collect_*() functions
>>
>>  include/linux/hmm.h                    |  17 +-
>>  include/linux/migrate.h                |   6 +-
>>  lib/test_hmm.c                         | 100 +++-
>>  lib/test_hmm_uapi.h                    |  19 +-
>>  mm/hmm.c                               | 657 +++++++++++++++++++++++--
>>  mm/migrate_device.c                    | 589 +++-------------------
>>  tools/testing/selftests/mm/hmm-tests.c |  54 ++
>>  7 files changed, 869 insertions(+), 573 deletions(-)
>>
>
> I see some kernel test robot failures, I assume there will be a new version
> for review?
>
> Balbir
>
Yes I will fix those and send a new version. The test robot failures are basically 
missing guards for !MIGRATE configs.

Thanks,
Mika