Subject: Re: [PATCH 2/7] mm: Free device private pages have zero refcount

From: Jason Gunthorpe <[email-masked]>
Date: Mon, 26 Sep 2022 11:36:11 -0300

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 <[email-masked]>
> ---
>  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