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

[zhong jiang <zhongjiang@huawei.com>]

On 2019/10/18 2:01, Ira Weiny wrote:
> 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.
I think the code works.  Because we alway return the ret.  we will return the error value if the ret is negative .
>> +		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?
You're right.  I miss that.  Thanks for pointing out.

Sincerely,
zhong jiang
> Ira
>
>>  }
>>  #else
>>  static long check_and_migrate_cma_pages(struct task_struct *tsk,
>> -- 
>> 1.7.12.4
>>
>>
> .
>