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

Dan Williams
2019-12-19T05:28:04Z

On Mon, Dec 16, 2019 at 2:26 PM John Hubbard <jhubbard@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@lst.de>
> Suggested-by: Jérôme Glisse <jglisse@redhat.com>
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> Reviewed-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: John Hubbard <jhubbard@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.