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

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

> 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).

Why?  What does it mean for nr_pages to be negative?  The check below seems valid.  Unsigned can be 0 so the check can fail.  IOW Checking unsigned > 0 seems ok.

What am I missing?

Ira

> 
> > Signed-off-by: zhong jiang <zhongjiang@huawei.com>
> 
> Fixes: 932f4a630a69 ("mm/gup: replace get_user_pages_longterm() with
> FOLL_LONGTERM")
> 
> (which changed long to unsigned long)
> 
> AFAICS... stable shouldn't be needed as the only "risk" is that we goto
> check_again even when we fail, which should be harmless.
> 
> Vlastimil
> 
> > ---
> >  mm/gup.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 23a9f9c..956d5a1 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -1508,7 +1508,7 @@ static long check_and_migrate_cma_pages(struct
> task_struct *tsk,
> >  						   pages, vmas, NULL,
> >  						   gup_flags);
> >
> > -		if ((nr_pages > 0) && migrate_allow) {
> > +		if (((long)nr_pages > 0) && migrate_allow) {
> >  			drain_allow = true;
> >  			goto check_again;
> >  		}
> >