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

[zhong jiang <zhongjiang@huawei.com>]

On 2019/10/18 4:42, John Hubbard wrote:
> 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;
>  
I think the ret should be assigned as zero. we will return ret if cma_page_list is empty.
I miss something?

Thanks,
zhong jiang
>  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
>
> .
>