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

[Ira Weiny <ira.weiny@intel.com>]

On Thu, Oct 17, 2019 at 11:19:46AM +0800, 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.
> 
> The patch use a new local variable to store the return value of
> __get_user_pages_locked(). Then use it to compare with zero.
> 
> Suggested-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: zhong jiang <zhongjiang@huawei.com>
> ---
>  mm/gup.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 8f236a3..1fe0ceb 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1443,6 +1443,7 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk,
>  	bool drain_allow = true;
>  	bool migrate_allow = true;
>  	LIST_HEAD(cma_page_list);
> +	long ret;
>  
>  check_again:
>  	for (i = 0; i < nr_pages;) {
> @@ -1504,17 +1505,18 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk,
>  		 * 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;

I think if ret is negative we want to return the error here.

> +		if ((ret > 0) && migrate_allow) {
>  			drain_allow = true;
>  			goto check_again;
>  		}
>  	}
>  
> -	return nr_pages;
> +	return ret;

If the cma_page_list is empty doesn't this return uninitialized data?

Ira

>  }
>  #else
>  static long check_and_migrate_cma_pages(struct task_struct *tsk,
> -- 
> 1.7.12.4
> 
>