Re: [v1 resend 00/12] THP support for zone device page migration

[Matthew Brost <matthew.brost@intel.com>]

On Fri, Jul 04, 2025 at 09:34:59AM +1000, Balbir Singh wrote:
> This patch series adds support for THP migration of zone device pages.
> To do so, the patches implement support for folio zone device pages
> by adding support for setting up larger order pages.
> 
> These patches build on the earlier posts by Ralph Campbell [1]
> 
> Two new flags are added in vma_migration to select and mark compound pages.
> migrate_vma_setup(), migrate_vma_pages() and migrate_vma_finalize()
> support migration of these pages when MIGRATE_VMA_SELECT_COMPOUND
> is passed in as arguments.
> 
> The series also adds zone device awareness to (m)THP pages along
> with fault handling of large zone device private pages. page vma walk
> and the rmap code is also zone device aware. Support has also been
> added for folios that might need to be split in the middle
> of migration (when the src and dst do not agree on
> MIGRATE_PFN_COMPOUND), that occurs when src side of the migration can
> migrate large pages, but the destination has not been able to allocate
> large pages. The code supported and used folio_split() when migrating
> THP pages, this is used when MIGRATE_VMA_SELECT_COMPOUND is not passed
> as an argument to migrate_vma_setup().
> 
> The test infrastructure lib/test_hmm.c has been enhanced to support THP
> migration. A new ioctl to emulate failure of large page allocations has
> been added to test the folio split code path. hmm-tests.c has new test
> cases for huge page migration and to test the folio split path. A new
> throughput test has been added as well.
> 
> The nouveau dmem code has been enhanced to use the new THP migration
> capability.
> 
> Feedback from the RFC [2]:
> 

Thanks for the patches, results look very promising. I wanted to give
some quick feedback:

- You appear to have missed updating hmm_range_fault, specifically
hmm_vma_handle_pmd, to check for device-private entries and populate the
HMM PFNs accordingly. My colleague François has a fix for this if you're
interested.

- I believe copy_huge_pmd also needs to be updated to avoid installing a
migration entry if the swap entry is device-private. I don't have an
exact fix yet due to my limited experience with core MM. The test case
that triggers this is fairly simple: fault in a 2MB device page on the
GPU, then fork a process that reads the page — the kernel crashes in
this scenario.

Matt

> It was advised that prep_compound_page() not be exposed just for the purposes
> of testing (test driver lib/test_hmm.c). Work arounds of copy and split the
> folios did not work due to lock order dependency in the callback for
> split folio.
> 
> mTHP support:
> 
> The patches hard code, HPAGE_PMD_NR in a few places, but the code has
> been kept generic to support various order sizes. With additional
> refactoring of the code support of different order sizes should be
> possible.
> 
> The future plan is to post enhancements to support mTHP with a rough
> design as follows:
> 
> 1. Add the notion of allowable thp orders to the HMM based test driver
> 2. For non PMD based THP paths in migrate_device.c, check to see if
>    a suitable order is found and supported by the driver
> 3. Iterate across orders to check the highest supported order for migration
> 4. Migrate and finalize
> 
> The mTHP patches can be built on top of this series, the key design elements
> that need to be worked out are infrastructure and driver support for multiple
> ordered pages and their migration.
> 
> References:
> [1] https://lore.kernel.org/linux-mm/20201106005147.20113-1-rcampbell@nvidia.com/
> [2] https://lore.kernel.org/linux-mm/20250306044239.3874247-3-balbirs@nvidia.com/T/
> 
> These patches are built on top of mm-unstable
> 
> Cc: Karol Herbst <kherbst@redhat.com>
> Cc: Lyude Paul <lyude@redhat.com>
> Cc: Danilo Krummrich <dakr@kernel.org>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Simona Vetter <simona@ffwll.ch>
> Cc: "Jérôme Glisse" <jglisse@redhat.com>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Barry Song <baohua@kernel.org>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
> Cc: Jane Chu <jane.chu@oracle.com>
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: Donet Tom <donettom@linux.ibm.com>
> 
> Changelog v1:
> - Changes from RFC [2], include support for handling fault_folio and using
>   trylock in the fault path
> - A new test case has been added to measure the throughput improvement
> - General refactoring of code to keep up with the changes in mm
> - New split folio callback when the entire split is complete/done. The
>   callback is used to know when the head order needs to be reset.
> 
> Testing:
> - Testing was done with ZONE_DEVICE private pages on an x86 VM
> - Throughput showed upto 5x improvement with THP migration, system to device
>   migration is slower due to the mirroring of data (see buffer->mirror)
> 
> 
> Balbir Singh (12):
>   mm/zone_device: support large zone device private folios
>   mm/migrate_device: flags for selecting device private THP pages
>   mm/thp: zone_device awareness in THP handling code
>   mm/migrate_device: THP migration of zone device pages
>   mm/memory/fault: add support for zone device THP fault handling
>   lib/test_hmm: test cases and support for zone device private THP
>   mm/memremap: add folio_split support
>   mm/thp: add split during migration support
>   lib/test_hmm: add test case for split pages
>   selftests/mm/hmm-tests: new tests for zone device THP migration
>   gpu/drm/nouveau: add THP migration support
>   selftests/mm/hmm-tests: new throughput tests including THP
> 
>  drivers/gpu/drm/nouveau/nouveau_dmem.c | 246 +++++++---
>  drivers/gpu/drm/nouveau/nouveau_svm.c  |   6 +-
>  drivers/gpu/drm/nouveau/nouveau_svm.h  |   3 +-
>  include/linux/huge_mm.h                |  18 +-
>  include/linux/memremap.h               |  29 +-
>  include/linux/migrate.h                |   2 +
>  include/linux/mm.h                     |   1 +
>  lib/test_hmm.c                         | 428 +++++++++++++----
>  lib/test_hmm_uapi.h                    |   3 +
>  mm/huge_memory.c                       | 261 ++++++++---
>  mm/memory.c                            |   6 +-
>  mm/memremap.c                          |  50 +-
>  mm/migrate.c                           |   2 +
>  mm/migrate_device.c                    | 488 +++++++++++++++++---
>  mm/page_alloc.c                        |   1 +
>  mm/page_vma_mapped.c                   |  10 +
>  mm/pgtable-generic.c                   |   6 +
>  mm/rmap.c                              |  19 +-
>  tools/testing/selftests/mm/hmm-tests.c | 607 ++++++++++++++++++++++++-
>  19 files changed, 1874 insertions(+), 312 deletions(-)
> 
> -- 
> 2.49.0
>