> Subject: Re: [PATCH] mm: Unsigned 'nr_pages' always larger than zero
>
> On Wed, 4 Sep 2019 13:24:58 +0200 Vlastimil Babka <vbabka@xxxxxxx>
> 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,
>
>
