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

[John Hubbard <jhubbard@nvidia.com>]

On 10/16/19 8:19 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.
> 
> The patch use a new local variable to store the return value of
> __get_user_pages_locked(). Then use it to compare with zero.

Hi Zhong,

The above are actually more like notes to yourself, and while those are
good to have, but it's not yet a perfect commit description: it talks 
about how you got here, which is only part of the story. A higher level
summary is better, and let the code itself cover the details. 

Like this:

First line (subject): 

   mm/gup: allow CMA migration to propagate errors back to caller

Commit description:

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: 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;

Ira pointed out that this needs initialization, see below.

>  
>  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;

Although technically correct, it makes me feel odd to see the assignment
done from signed to unsigned, *before* checking for >0. And Ira is hinting
at the same thing, when he asks if we can return early here. See below...

> +		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,
> 

So, overall, I'd recommend this, instead:

diff --git a/mm/gup.c b/mm/gup.c
index 23a9f9c9d377..72bc027037fa 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,


thanks,

John Hubbard
NVIDIA