Re: [HMM v13 04/18] mm/ZONE_DEVICE/free-page: callback when page is freed

[Anshuman Khandual]

On 11/18/2016 11:48 PM, JÃrÃme Glisse wrote:
> When a ZONE_DEVICE page refcount reach 1 it means it is free and nobody
> is holding a reference on it (only device to which the memory belong do).
> Add a callback and call it when that happen so device driver can implement
> their own free page management.
> 
> Signed-off-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> ---
>  include/linux/memremap.h | 4 ++++
>  kernel/memremap.c        | 8 ++++++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index fe61dca..469c88d 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -37,17 +37,21 @@ static inline struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
>  
>  /**
>   * struct dev_pagemap - metadata for ZONE_DEVICE mappings
> + * @free_devpage: free page callback when page refcount reach 1
>   * @altmap: pre-allocated/reserved memory for vmemmap allocations
>   * @res: physical address range covered by @ref
>   * @ref: reference count that pins the devm_memremap_pages() mapping
>   * @dev: host device of the mapping for debug
> + * @data: privata data pointer for free_devpage
>   * @flags: memory flags (look for MEMORY_FLAGS_NONE in memory_hotplug.h)
>   */
>  struct dev_pagemap {
> +	void (*free_devpage)(struct page *page, void *data);
>  	struct vmem_altmap *altmap;
>  	const struct resource *res;
>  	struct percpu_ref *ref;
>  	struct device *dev;
> +	void *data;
>  	int flags;
>  };
>  
> diff --git a/kernel/memremap.c b/kernel/memremap.c
> index 438a73aa2..3d28048 100644
> --- a/kernel/memremap.c
> +++ b/kernel/memremap.c
> @@ -190,6 +190,12 @@ EXPORT_SYMBOL(get_zone_device_page);
>  
>  void put_zone_device_page(struct page *page)
>  {
> +	/*
> +	 * If refcount is 1 then page is freed and refcount is stable as nobody
> +	 * holds a reference on the page.
> +	 */
> +	if (page->pgmap->free_devpage && page_count(page) == 1)
> +		page->pgmap->free_devpage(page, page->pgmap->data);
>  	put_dev_pagemap(page->pgmap);
>  }
>  EXPORT_SYMBOL(put_zone_device_page);
> @@ -326,6 +332,8 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
>  	pgmap->ref = ref;
>  	pgmap->res = &page_map->res;
>  	pgmap->flags = flags | MEMORY_DEVICE;
> +	pgmap->free_devpage = NULL;
> +	pgmap->data = NULL;

When is the driver expected to load up pgmap->free_devpage ? I thought
this function is one of the right places. Though as all the pages in
the same hotplug operation point to the same dev_pagemap structure this
loading can be done at later point of time as well.