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

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

[Andrew Morton]

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

[Akinobu Mita]

2009/7/18 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.

Maybe. But we had such a bug in c51b1a160b63304720d49479986915e4c475a2cf
(xip: fix get_zeroed_page with __GFP_HIGHME). Even the VM code
had it and did not fixed for a long time.

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