On 9/13/21 9:15 AM, Alex Sierra wrote:

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.

Signed-off-by: Ralph Campbell <redacted>
Signed-off-by: Alex Sierra <redacted>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
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.

v7:
AS: fix condition at try_grab_page added at v5, is invalid. It supposed
to fix xfstests/generic/413 test, however, there's a known issue on
this test where DAX mapped area DIO to non-DAX expect to fail.
https://patchwork.kernel.org/project/fstests/patch/1489463960-3579-1-git-send-email-xzhou@redhat.com
This condition was removed after rebase over patch series
https://lore.kernel.org/r/20210813044133.1536842-4-jhubbard@nvidia.com (local)
---
  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                     | 11 ----
  lib/test_hmm.c                         |  2 +-
  mm/internal.h                          |  8 +++
  mm/memremap.c                          | 69 +++++++-------------------
  mm/migrate.c                           |  5 --
  mm/page_alloc.c                        |  3 ++
  mm/swap.c                              | 45 ++---------------
  12 files changed, 45 insertions(+), 115 deletions(-)

I don't see mm/memcontrol.c listed here so I think you will need this fragment too:

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 389b5766e74f..0cc9c7c71e79 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -5541,11 +5541,7 @@ static struct page *mc_handle_swap_pte(struct vm_area_struct *vma,
  	 */
  	if (is_device_private_entry(ent)) {
  		page = pfn_swap_entry_to_page(ent);
-		/*
-		 * MEMORY_DEVICE_PRIVATE means ZONE_DEVICE page and which have
-		 * a refcount of 1 when free (unlike normal page)
-		 */
-		if (!page_ref_add_unless(page, 1, 1))
+		if (!get_page_unless_zero(page))
  			return NULL;
  		return page;
  	}