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

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

> Subject: Re: [PATCH] mm: Unsigned 'nr_pages' always larger than zero
> 
> On Wed, 4 Sep 2019 13:24:58 +0200 Vlastimil Babka <vbabka@suse.cz>
> wrote:
> 
> > On 9/4/19 12:26 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.
> > > Hence, Convert the long to compare with zero is feasible.
> >
> > It would be nicer if the parameter nr_pages was long again instead of
> > unsigned long (note there are two variants of the function, so both should
> be changed).
> 
> nr_pages should be unsigned - it's a count of pages!
> 
> The bug is that __get_user_pages_locked() returns a signed long which can
> be a -ve errno.

Ok...  This is my bad...  I think this is the correct fix though.  Not changing the type of nr_pages.

Sorry,
Ira

> 
> I think it's best if __get_user_pages_locked() is to get itself a new local with
> the same type as its return value.  Something like:
> 
> --- a/mm/gup.c~a
> +++ a/mm/gup.c
> @@ -1450,6 +1450,7 @@ static long check_and_migrate_cma_pages(
>  	bool drain_allow = true;
>  	bool migrate_allow = true;
>  	LIST_HEAD(cma_page_list);
> +	long ret;
> 
>  check_again:
>  	for (i = 0; i < nr_pages;) {
> @@ -1511,17 +1512,18 @@ check_again:
>  		 * 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;
> +		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,
> 
>