Am 2021-08-17 um 8:01 p.m. schrieb Ralph Campbell:

On 8/12/21 11:31 PM, Alex Sierra wrote:

quoted

From: Ralph Campbell <redacted>

ZONE_DEVICE struct pages have an extra reference count that
complicates the
code for put_page() and several places in the kernel that need to
check the
reference count to see that a page is not being used (gup, compaction,
migration, etc.). Clean up the code so the reference count doesn't
need to
be treated specially for ZONE_DEVICE.

v2:
AS: merged this patch in linux 5.11 version

v5:
AS: add condition at try_grab_page to check for the zone device type,
while
page ref counter is checked less/equal to zero. In case of device
zone, pages
ref counter are initialized to zero.

Signed-off-by: Ralph Campbell <redacted>
Signed-off-by: Alex Sierra <redacted>
---
  arch/powerpc/kvm/book3s_hv_uvmem.c     |  2 +-
  drivers/gpu/drm/nouveau/nouveau_dmem.c |  2 +-
  fs/dax.c                               |  4 +-
  include/linux/dax.h                    |  2 +-
  include/linux/memremap.h               |  7 +--
  include/linux/mm.h                     | 13 +----
  lib/test_hmm.c                         |  2 +-
  mm/internal.h                          |  8 +++
  mm/memremap.c                          | 68 +++++++-------------------
  mm/migrate.c                           |  5 --
  mm/page_alloc.c                        |  3 ++
  mm/swap.c                              | 45 ++---------------
  12 files changed, 46 insertions(+), 115 deletions(-)

I haven't seen a response to the issues I raised back at v3 of this
series.
https://lore.kernel.org/linux-mm/4f6dd918-d79b-1aa7-3a4c-caa67ddc29bc@nvidia.com/ (local)


Did I miss something?

I think part of the response was that we did more testing. Alex added
support for DEVICE_GENERIC pages to test_hmm and he ran DAX tests
recommended by Theodore Tso. In that testing he ran into a WARN_ON_ONCE
about a zero page refcount in try_get_page. The fix is in the latest
version of patch 2. But it's already obsolete because John Hubbard is
about to remove that function altogether.

I think the issues you raised were more uncertainty than known bugs. It
seems the fact that you can have DAX pages with 0 refcount is a feature
more than a bug.

Regards,
  Felix