Re: [PATCH] mm: add gfp mask checking for __get_free_pages()

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 4 Jul 2009 11:09:50 +0900
Akinobu Mita <akinobu.mita@gmail.com> wrote:

> __get_free_pages() with __GFP_HIGHMEM is not safe because the return
> address cannot represent a highmem page. get_zeroed_page() already has
> such a debug checking.
> 
> Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
> ---
>  mm/page_alloc.c |   24 +++++++++---------------
>  1 files changed, 9 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e0f2cdf..4a1a374 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1903,31 +1903,25 @@ EXPORT_SYMBOL(__alloc_pages_nodemask);
>   */
>  unsigned long __get_free_pages(gfp_t gfp_mask, unsigned int order)
>  {
> -	struct page * page;
> +	struct page *page;
> +
> +	/*
> +	 * __get_free_pages() returns a 32-bit address, which cannot represent
> +	 * a highmem page
> +	 */
> +	VM_BUG_ON((gfp_mask & __GFP_HIGHMEM) != 0);
> +
>  	page = alloc_pages(gfp_mask, order);
>  	if (!page)
>  		return 0;
>  	return (unsigned long) page_address(page);
>  }
> -
>  EXPORT_SYMBOL(__get_free_pages);
>  
>  unsigned long get_zeroed_page(gfp_t gfp_mask)
>  {
> -	struct page * page;
> -
> -	/*
> -	 * get_zeroed_page() returns a 32-bit address, which cannot represent
> -	 * a highmem page
> -	 */
> -	VM_BUG_ON((gfp_mask & __GFP_HIGHMEM) != 0);
> -
> -	page = alloc_pages(gfp_mask | __GFP_ZERO, 0);
> -	if (page)
> -		return (unsigned long) page_address(page);
> -	return 0;
> +	return __get_free_pages(gfp_mask | __GFP_ZERO, 0);
>  }
> -
>  EXPORT_SYMBOL(get_zeroed_page);
>  
>  void __pagevec_free(struct pagevec *pvec)

Fair enough.

I suspect we could just delete that VM_BUG_ON() - we can't go and do
runtime checking for every darn programmer error, and this would be a
pretty dumb one.


Your patch turns get_zeroed_page() into a simple one-liner wrapper
around __get_free_pages().  We could perhaps save some .text, some
kernel stack and some CPU cycles by inlining get_zeroed_page().