[PATCH v11 04/25] mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages

[Dan Williams]

On Mon, Dec 16, 2019 at 2:26 PM John Hubbard <jhubbard at nvidia.com> wrote:
>
> An upcoming patch changes and complicates the refcounting and
> especially the "put page" aspects of it. In order to keep
> everything clean, refactor the devmap page release routines:
>
> * Rename put_devmap_managed_page() to page_is_devmap_managed(),
>   and limit the functionality to "read only": return a bool,
>   with no side effects.
>
> * Add a new routine, put_devmap_managed_page(), to handle checking
>   what kind of page it is, and what kind of refcount handling it
>   requires.
>
> * Rename __put_devmap_managed_page() to free_devmap_managed_page(),
>   and limit the functionality to unconditionally freeing a devmap
>   page.
>
> This is originally based on a separate patch by Ira Weiny, which
> applied to an early version of the put_user_page() experiments.
> Since then, Jérôme Glisse suggested the refactoring described above.
>
> Cc: Christoph Hellwig <hch at lst.de>
> Suggested-by: Jérôme Glisse <jglisse at redhat.com>
> Reviewed-by: Dan Williams <dan.j.williams at intel.com>
> Reviewed-by: Jan Kara <jack at suse.cz>
> Signed-off-by: Ira Weiny <ira.weiny at intel.com>
> Signed-off-by: John Hubbard <jhubbard at nvidia.com>
> ---
>  include/linux/mm.h | 17 +++++++++++++----
>  mm/memremap.c      | 16 ++--------------
>  mm/swap.c          | 24 ++++++++++++++++++++++++
>  3 files changed, 39 insertions(+), 18 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index c97ea3b694e6..77a4df06c8a7 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -952,9 +952,10 @@ static inline bool is_zone_device_page(const struct page *page)
>  #endif
>
>  #ifdef CONFIG_DEV_PAGEMAP_OPS
> -void __put_devmap_managed_page(struct page *page);
> +void free_devmap_managed_page(struct page *page);
>  DECLARE_STATIC_KEY_FALSE(devmap_managed_key);
> -static inline bool put_devmap_managed_page(struct page *page)
> +
> +static inline bool page_is_devmap_managed(struct page *page)
>  {
>         if (!static_branch_unlikely(&devmap_managed_key))
>                 return false;
> @@ -963,7 +964,6 @@ static inline bool put_devmap_managed_page(struct page *page)
>         switch (page->pgmap->type) {
>         case MEMORY_DEVICE_PRIVATE:
>         case MEMORY_DEVICE_FS_DAX:
> -               __put_devmap_managed_page(page);
>                 return true;
>         default:
>                 break;
> @@ -971,7 +971,14 @@ static inline bool put_devmap_managed_page(struct page *page)
>         return false;
>  }
>
> +bool put_devmap_managed_page(struct page *page);
> +
>  #else /* CONFIG_DEV_PAGEMAP_OPS */
> +static inline bool page_is_devmap_managed(struct page *page)
> +{
> +       return false;
> +}
> +
>  static inline bool put_devmap_managed_page(struct page *page)
>  {
>         return false;
> @@ -1028,8 +1035,10 @@ static inline void put_page(struct page *page)
>          * need to inform the device driver through callback. See
>          * include/linux/memremap.h and HMM for details.
>          */
> -       if (put_devmap_managed_page(page))
> +       if (page_is_devmap_managed(page)) {
> +               put_devmap_managed_page(page);
>                 return;
> +       }
>
>         if (put_page_testzero(page))
>                 __put_page(page);
> diff --git a/mm/memremap.c b/mm/memremap.c
> index e899fa876a62..2ba773859031 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -411,20 +411,8 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
>  EXPORT_SYMBOL_GPL(get_dev_pagemap);
>
>  #ifdef CONFIG_DEV_PAGEMAP_OPS
> -void __put_devmap_managed_page(struct page *page)
> +void free_devmap_managed_page(struct page *page)
>  {
> -       int count = page_ref_dec_return(page);
> -
> -       /* still busy */
> -       if (count > 1)
> -               return;
> -
> -       /* only triggered by the dev_pagemap shutdown path */
> -       if (count == 0) {
> -               __put_page(page);
> -               return;
> -       }
> -
>         /* notify page idle for dax */
>         if (!is_device_private_page(page)) {
>                 wake_up_var(&page->_refcount);
> @@ -461,5 +449,5 @@ void __put_devmap_managed_page(struct page *page)
>         page->mapping = NULL;
>         page->pgmap->ops->page_free(page);
>  }
> -EXPORT_SYMBOL(__put_devmap_managed_page);
> +EXPORT_SYMBOL(free_devmap_managed_page);

This patch does not have a module consumer for
free_devmap_managed_page(), so the export should move to the patch
that needs the new export.

Also the only reason that put_devmap_managed_page() is EXPORT_SYMBOL
instead of EXPORT_SYMBOL_GPL is that there was no practical way to
hide the devmap details from evey module in the kernel that did
put_page(). I would expect free_devmap_managed_page() to
EXPORT_SYMBOL_GPL if it is not inlined into an existing exported
static inline api.