Re: [RFC 00/11] THP support for zone device pages

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

On Thu, Mar 06, 2025 at 03:42:28PM +1100, Balbir Singh wrote:

This is an exciting series to see. As of today, we have just merged this
series into the DRM subsystem / Xe [2], which adds very basic SVM
support. One of the performance bottlenecks we quickly identified was
the lack of THP for device pages—I believe our profiling showed that 96%
of the time spent on 2M page GPU faults was within the migrate_vma_*
functions. Presumably, this will help significantly.

We will likely attempt to pull this code into GPU SVM / Xe fairly soon.
I believe we will encounter a conflict since [2] includes these patches
[3] [4], but we should be able to resolve that. These patches might make
it into the 6.15 PR — TBD but I can get back to you on that.

I have one question—does this series contain all the required core MM
changes for us to give it a try? That is, do I need to include any other
code from the list to test this out?

Matt

[2] https://patchwork.freedesktop.org/series/137870/
[3] https://patchwork.freedesktop.org/patch/641207/?series=137870&rev=8
[4] https://patchwork.freedesktop.org/patch/641214/?series=137870&rev=8

> 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.
> 
> The nouveau dmem code has been enhanced to use the new THP migration
> capability.
> 
> 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.
> 
> References:
> [1] https://lore.kernel.org/linux-mm/20201106005147.20113-1-rcampbell@nvidia.com/
> 
> These patches are built on top of mm-everything-2025-03-04-05-51
> 
> 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>
> 
> Balbir Singh (11):
>   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
> 
>  drivers/gpu/drm/nouveau/nouveau_dmem.c | 244 +++++++++----
>  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                         | 387 ++++++++++++++++----
>  lib/test_hmm_uapi.h                    |   3 +
>  mm/huge_memory.c                       | 242 +++++++++---
>  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/rmap.c                              |  19 +-
>  tools/testing/selftests/mm/hmm-tests.c | 407 +++++++++++++++++++++
>  18 files changed, 1630 insertions(+), 288 deletions(-)
> 
> -- 
> 2.48.1
>