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

[mpenttil@redhat.com]

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(-)

-- 
2.50.0