Re: [HMM 08/15] mm/ZONE_DEVICE: special case put_page() for device private pages

[Dan Williams]

On Mon, May 22, 2017 at 9:51 AM, JÃrÃme Glisse <jglisse@xxxxxxxxxx> wrote:
> A ZONE_DEVICE page that reach a refcount of 1 is free ie no longer
> have any user. For device private pages this is important to catch
> and thus we need to special case put_page() for this.
>
> Signed-off-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> ---
>  include/linux/mm.h | 30 ++++++++++++++++++++++++++++++
>  kernel/memremap.c  |  1 -
>  2 files changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a825dab..11f7bac 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -23,6 +23,7 @@
>  #include <linux/page_ext.h>
>  #include <linux/err.h>
>  #include <linux/page_ref.h>
> +#include <linux/memremap.h>
>
>  struct mempolicy;
>  struct anon_vma;
> @@ -795,6 +796,20 @@ static inline bool is_device_private_page(const struct page *page)
>         return ((page_zonenum(page) == ZONE_DEVICE) &&
>                 (page->pgmap->type == MEMORY_DEVICE_PRIVATE));
>  }
> +
> +static inline void put_zone_device_private_page(struct page *page)
> +{
> +       int count = page_ref_dec_return(page);
> +
> +       /*
> +        * If refcount is 1 then page is freed and refcount is stable as nobody
> +        * holds a reference on the page.
> +        */
> +       if (count == 1)
> +               page->pgmap->page_free(page, page->pgmap->data);
> +       else if (!count)
> +               __put_page(page);
> +}
>  #else
>  static inline bool is_zone_device_page(const struct page *page)
>  {
> @@ -805,6 +820,10 @@ static inline bool is_device_private_page(const struct page *page)
>  {
>         return false;
>  }
> +
> +static inline void put_zone_device_private_page(struct page *page)
> +{
> +}
>  #endif
>
>  static inline void get_page(struct page *page)
> @@ -822,6 +841,17 @@ static inline void put_page(struct page *page)
>  {
>         page = compound_head(page);
>
> +       /*
> +        * For private device pages we need to catch refcount transition from
> +        * 2 to 1, when refcount reach one it means the private device page is
> +        * free and we need to inform the device driver through callback. See
> +        * include/linux/memremap.h and HMM for details.
> +        */
> +       if (unlikely(is_device_private_page(page))) {

Since I presume HMM is a niche use case can we make this a
"static_branch_unlikely(&hmm_key) && is_device_private_page(page))"?
That way non-hmm platforms see minimal overhead.