Re: [PATCH] mm/gup: allow CMA migration to propagate errors back to caller

[Vlastimil Babka <vbabka@suse.cz>]

On 10/21/19 5:17 PM, zhong jiang wrote:
> check_and_migrate_cma_pages() was recording the result of
> __get_user_pages_locked() in an unsigned "nr_pages" variable. Because
> __get_user_pages_locked() returns a signed value that can include
> negative errno values, this had the effect of hiding errors.
> 
> Change check_and_migrate_cma_pages() implementation so that it
> uses a signed variable instead, and propagates the results back
> to the caller just as other gup internal functions do.
> 
> This was discovered with the help of unsigned_lesser_than_zero.cocci.
> 
> Suggested-by: John Hubbard <jhubbard@nvidia.com>
> Signed-off-by: zhong jiang <zhongjiang@huawei.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/gup.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 8f236a3..c2b3e11 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 = nr_pages;
>  
>  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) {
> +		if ((ret > 0) && migrate_allow) {
> +			nr_pages = ret;
>  			drain_allow = true;
>  			goto check_again;
>  		}
>  	}
>  
> -	return nr_pages;
> +	return ret;
>  }
>  #else
>  static long check_and_migrate_cma_pages(struct task_struct *tsk,
>