On Mon, Sep 26, 2022 at 04:03:06PM +1000, Alistair Popple wrote:

Since 27674ef6c73f ("mm: remove the extra ZONE_DEVICE struct page
refcount") device private pages have no longer had an extra reference
count when the page is in use. However before handing them back to the
owning device driver we add an extra reference count such that free
pages have a reference count of one.

This makes it difficult to tell if a page is free or not because both
free and in use pages will have a non-zero refcount. Instead we should
return pages to the drivers page allocator with a zero reference count.
Kernel code can then safely use kernel functions such as
get_page_unless_zero().

Signed-off-by: Alistair Popple <apopple@nvidia.com>
---
 arch/powerpc/kvm/book3s_hv_uvmem.c       | 1 +
 drivers/gpu/drm/amd/amdkfd/kfd_migrate.c | 1 +
 drivers/gpu/drm/nouveau/nouveau_dmem.c   | 1 +
 lib/test_hmm.c                           | 1 +
 mm/memremap.c                            | 5 -----
 mm/page_alloc.c                          | 6 ++++++
 6 files changed, 10 insertions(+), 5 deletions(-)

I think this is a great idea, but I'm surprised no dax stuff is
touched here?

Jason