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Message-ID: <aJJ44cZZWqJVFLem@lstrano-desk.jf.intel.com>
Date: Tue, 5 Aug 2025 14:34:25 -0700
From: Matthew Brost <matthew.brost@...el.com>
To: Balbir Singh <balbirs@...dia.com>
CC: <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>, Karol Herbst
<kherbst@...hat.com>, Lyude Paul <lyude@...hat.com>, Danilo Krummrich
<dakr@...nel.org>, David Airlie <airlied@...il.com>, Simona Vetter
<simona@...ll.ch>, Jérôme Glisse <jglisse@...hat.com>,
Shuah Khan <shuah@...nel.org>, David Hildenbrand <david@...hat.com>, "Barry
Song" <baohua@...nel.org>, Baolin Wang <baolin.wang@...ux.alibaba.com>, "Ryan
Roberts" <ryan.roberts@....com>, Matthew Wilcox <willy@...radead.org>, "Peter
Xu" <peterx@...hat.com>, Zi Yan <ziy@...dia.com>, Kefeng Wang
<wangkefeng.wang@...wei.com>, Jane Chu <jane.chu@...cle.com>, Alistair Popple
<apopple@...dia.com>, Donet Tom <donettom@...ux.ibm.com>, Ralph Campbell
<rcampbell@...dia.com>, Mika Penttilä
<mpenttil@...hat.com>, Francois Dugast <francois.dugast@...el.com>
Subject: Re: [v2 00/11] THP support for zone device page migration
On Wed, Jul 30, 2025 at 07:21:28PM +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. Larger order
> pages provide a speedup in throughput and latency.
>
> In my local testing (using lib/test_hmm) and a throughput test, the
> series shows a 350% improvement in data transfer throughput and a
> 500% improvement in latency
>
> 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.
>
> 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.
>
> HMM support for large folios:
>
> Francois Dugast posted patches support for HMM handling [4], the proposed
> changes can build on top of this series to provide support for HMM fault
> handling.
>
> 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/
> [3] https://lore.kernel.org/lkml/20250703233511.2028395-1-balbirs@nvidia.com/
> [4] https://lore.kernel.org/lkml/20250722193445.1588348-1-francois.dugast@intel.com/
>
> These patches are built on top of mm/mm-stable
>
> Cc: Karol Herbst <kherbst@...hat.com>
> Cc: Lyude Paul <lyude@...hat.com>
> Cc: Danilo Krummrich <dakr@...nel.org>
> Cc: David Airlie <airlied@...il.com>
> Cc: Simona Vetter <simona@...ll.ch>
> Cc: "Jérôme Glisse" <jglisse@...hat.com>
> Cc: Shuah Khan <shuah@...nel.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Barry Song <baohua@...nel.org>
> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Peter Xu <peterx@...hat.com>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: Kefeng Wang <wangkefeng.wang@...wei.com>
> Cc: Jane Chu <jane.chu@...cle.com>
> Cc: Alistair Popple <apopple@...dia.com>
> Cc: Donet Tom <donettom@...ux.ibm.com>
> Cc: Ralph Campbell <rcampbell@...dia.com>
> Cc: Mika Penttilä <mpenttil@...hat.com>
> Cc: Matthew Brost <matthew.brost@...el.com>
> Cc: Francois Dugast <francois.dugast@...el.com>
>
> Changelog v2 [3] :
> - Several review comments from David Hildenbrand were addressed, Mika,
> Zi, Matthew also provided helpful review comments
> - In paths where it makes sense a new helper
> is_pmd_device_private_entry() is used
> - anon_exclusive handling of zone device private pages in
> split_huge_pmd_locked() has been fixed
> - Patches that introduced helpers have been folded into where they
> are used
> - Zone device handling in mm/huge_memory.c has benefited from the code
> and testing of Matthew Brost, he helped find bugs related to
> copy_huge_pmd() and partial unmapping of folios.
I see a ton of discussion on this series, particularly patch 2. It looks
like you have landed on a different solution for partial unmaps. I
wanted to pull this series into in for testing but if this is actively
being refactored, likely best to hold off until next post or test off a
WIP branch if you have one.
Matt
> - Zone device THP PMD support via page_vma_mapped_walk() is restricted
> to try_to_migrate_one()
> - There is a new dedicated helper to split large zone device folios
>
> Changelog v1 [2]:
> - 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
>
> Balbir Singh (11):
> mm/zone_device: support large zone device private folios
> 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 | 19 +-
> include/linux/memremap.h | 51 ++-
> include/linux/migrate.h | 2 +
> include/linux/mm.h | 1 +
> include/linux/rmap.h | 2 +
> include/linux/swapops.h | 17 +
> lib/test_hmm.c | 432 ++++++++++++++----
> lib/test_hmm_uapi.h | 3 +
> mm/huge_memory.c | 358 ++++++++++++---
> mm/memory.c | 6 +-
> mm/memremap.c | 48 +-
> mm/migrate_device.c | 517 ++++++++++++++++++---
> mm/page_vma_mapped.c | 13 +-
> mm/pgtable-generic.c | 6 +
> mm/rmap.c | 22 +-
> tools/testing/selftests/mm/hmm-tests.c | 607 ++++++++++++++++++++++++-
> 19 files changed, 2040 insertions(+), 319 deletions(-)
>
> --
> 2.50.1
>
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