Re: [PATCH v3 04/12] mm: introduce page_ref_sub_return()

[Kirill A. Shutemov]

On Fri, Jan 31, 2020 at 07:40:21PM -0800, John Hubbard wrote:
> An upcoming patch requires subtracting a large chunk of refcounts from
> a page, and checking what the resulting refcount is. This is a little
> different than the usual "check for zero refcount" that many of the
> page ref functions already do. However, it is similar to a few other
> routines that (like this one) are generally useful for things such as
> 1-based refcounting.
> 
> Add page_ref_sub_return(), that subtracts a chunk of refcounts
> atomically, and returns an atomic snapshot of the result.
> 
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> ---
>  include/linux/page_ref.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
> index 14d14beb1f7f..b9cbe553d1e7 100644
> --- a/include/linux/page_ref.h
> +++ b/include/linux/page_ref.h
> @@ -102,6 +102,16 @@ static inline void page_ref_sub(struct page *page, int nr)
>  		__page_ref_mod(page, -nr);
>  }
>  
> +static inline int page_ref_sub_return(struct page *page, int nr)
> +{
> +	int ret = atomic_sub_return(nr, &page->_refcount);
> +
> +	if (page_ref_tracepoint_active(__tracepoint_page_ref_mod))
> +		__page_ref_mod(page, -nr);

Shouldn't it be __page_ref_mod_and_return() and relevant tracepoint?

> +
> +	return ret;
> +}
> +
>  static inline void page_ref_inc(struct page *page)
>  {
>  	atomic_inc(&page->_refcount);
> -- 
> 2.25.0
> 

-- 
 Kirill A. Shutemov