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



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