[RFC,0/6] Allow file-backed or shared device private pages

Message

Alistair Popple March 16, 2025, 4:29 a.m. UTC

To simplify the initial implementation device private pages were restricted to
only being used for private anonymous. This avoided having to deal with issues
related to shared and/or file-backed pagesi early on.

This series lifts that restriction by allowing ZONE_DEVICE private pages to
exist in the pagecache. As the CPU cannot directly access these pages special
care needs to be taken when looking them up in the page-cache. This series
solves the problem by always migrating such pages back from device memory when
looking them up in the pagecache. This is similar to how device private pages
work for anonymous memory, where a CPU fault on the device memory will always
trigger a migration back to CPU system memory.

Initially this series only allows for read-only migration - this is because the
call to migrate pages back will always reload the data from backing storage.
It then introduces a callback that drivers may implement to actually copy any
modified data back as required.

Drivers are expected to call set_page_dirty() when copying data back to ensure
it hits the backing store.

This series is an early draft implementation - in particular error handling
is not dealt with and I'm not sure that the management of PTE write bits is
entirely correct. Much more testing of all the various filesystem corner cases
is also required. The aim of this series is to get early feedback on the overall
concept of putting device private pages in the pagecache before fleshing out the
implementation further.

Signed-off-by: Alistair Popple <[e-mail]>

Alistair Popple (6):
  mm/migrate_device.c: Don't read dirty bit of non-present PTEs
  mm/migrate: Support file-backed pages with migrate_vma
  mm: Allow device private pages to exist in page cache
  mm: Implement writeback for share device private pages
  selftests/hmm: Add file-backed migration tests
  nouveau: Add SVM support for migrating file-backed pages to the GPU

 drivers/gpu/drm/nouveau/nouveau_dmem.c |  24 ++-
 include/linux/memremap.h               |   2 +-
 include/linux/migrate.h                |   6 +-
 lib/test_hmm.c                         |  27 ++-
 mm/filemap.c                           |  41 ++++-
 mm/memory.c                            |   9 +-
 mm/memremap.c                          |   1 +-
 mm/migrate.c                           |  42 ++--
 mm/migrate_device.c                    | 114 +++++++++++-
 mm/rmap.c                              |   2 +-
 tools/testing/selftests/mm/hmm-tests.c | 252 +++++++++++++++++++++++++-
 11 files changed, 489 insertions(+), 31 deletions(-)

base-commit: 0ad2507d5d93f39619fc42372c347d6006b64319

Comments

Christoph Hellwig March 17, 2025, 6:04 a.m. UTC | #1

On Sun, Mar 16, 2025 at 03:29:23PM +1100, Alistair Popple wrote:
> This series lifts that restriction by allowing ZONE_DEVICE private pages to
> exist in the pagecache.

You'd better provide a really good argument for why we'd even want
to do that.  So far this cover letter fails to do that.