Re: [PATCH] mm: Unsigned 'nr_pages' always larger than zero

[John Hubbard <jhubbard@nvidia.com>]

On 9/4/19 11:48 AM, Andrew Morton wrote:
> On Wed, 4 Sep 2019 13:24:58 +0200 Vlastimil Babka <vbabka@suse.cz> wrote:
> 
>> On 9/4/19 12:26 PM, zhong jiang wrote:
>>> With the help of unsigned_lesser_than_zero.cocci. Unsigned 'nr_pages"'
>>> compare with zero. And __get_user_pages_locked will return an long value.
>>> Hence, Convert the long to compare with zero is feasible.
>>
>> It would be nicer if the parameter nr_pages was long again instead of unsigned
>> long (note there are two variants of the function, so both should be changed).
> 
> nr_pages should be unsigned - it's a count of pages!
> 

Yes!

> The bug is that __get_user_pages_locked() returns a signed long which
> can be a -ve errno.
> 
> I think it's best if __get_user_pages_locked() is to get itself a new
> local with the same type as its return value.  Something like:
> 
> --- a/mm/gup.c~a
> +++ a/mm/gup.c
> @@ -1450,6 +1450,7 @@ static long check_and_migrate_cma_pages(
>   	bool drain_allow = true;
>   	bool migrate_allow = true;
>   	LIST_HEAD(cma_page_list);
> +	long ret;
>   
>   check_again:
>   	for (i = 0; i < nr_pages;) {
> @@ -1511,17 +1512,18 @@ check_again:
>   		 * again migrating any new CMA pages which we failed to isolate
>   		 * earlier.
>   		 */
> -		nr_pages = __get_user_pages_locked(tsk, mm, start, nr_pages,
> +		ret = __get_user_pages_locked(tsk, mm, start, nr_pages,
>   						   pages, vmas, NULL,
>   						   gup_flags);
>   
> -		if ((nr_pages > 0) && migrate_allow) {
> +		nr_pages = ret;
> +		if (ret > 0 && migrate_allow) {
>   			drain_allow = true;
>   			goto check_again;
>   		}
>   	}
>   
> -	return nr_pages;
> +	return ret;
>   }
>   #else
>   static long check_and_migrate_cma_pages(struct task_struct *tsk,
> 

+1 for this approach, please.


thanks,
-- 
John Hubbard
NVIDIA