Re: [PATCH] mm/page_alloc: Add __free_pages documentation

[Mike Rapoport <rppt@kernel.org>]

On Tue, Oct 27, 2020 at 02:55:23AM +0000, Matthew Wilcox (Oracle) wrote:
> Provide some guidance towards when this might not be the right interface
> to use.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>

> ---
>  mm/page_alloc.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 23f5066bd4a5..e613177b8041 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5007,6 +5007,26 @@ static inline void free_the_page(struct page *page, unsigned int order)
>  		__free_pages_ok(page, order, FPI_NONE);
>  }
>  
> +/**
> + * __free_pages - Free pages allocated with alloc_pages().
> + * @page: The page pointer returned from alloc_pages().
> + * @order: The order of the allocation.
> + *
> + * This function can free multi-page allocations that are not compound
> + * pages.  It does not check that the @order passed in matches that of
> + * the allocation, so it is easy to leak memory.  Freeing more memory
> + * than was allocated will probably emit a warning.
> + *
> + * If the last reference to this page is speculative, it will be released
> + * by put_page() which only frees the first page of a non-compound
> + * allocation.  To prevent the remaining pages from being leaked, we free
> + * the subsequent pages here.  If you want to use the page's reference
> + * count to decide when to free the allocation, you should allocate a
> + * compound page, and use put_page() instead of __free_pages().
> + *
> + * Context: May be called in interrupt context or while holding a normal
> + * spinlock, but not in NMI context or while holding a raw spinlock.
> + */
>  void __free_pages(struct page *page, unsigned int order)
>  {
>  	if (put_page_testzero(page))
> -- 
> 2.28.0
> 
> 

-- 
Sincerely yours,
Mike.