Re: [PATCH] mm: remove a redundant condition in the for loop

[Michal Hocko]

On Mon 19-06-17 21:05:29, Rasmus Villemoes wrote:
> On Mon, Jun 19 2017, Vlastimil Babka <vbabka@xxxxxxx> wrote:
> 
> > On 06/19/2017 03:54 PM, Hao Lee wrote:
> >> The variable current_order decreases from MAX_ORDER-1 to order, so the
> >> condition current_order <= MAX_ORDER-1 is always true.
> >> 
> >> Signed-off-by: Hao Lee <haolee.swjtu@xxxxxxxxx>
> >
> > Sounds right.
> >
> > Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> 
> current_order and order are both unsigned, and if order==0,
> current_order >= order is always true, and we may decrement
> current_order past 0 making it UINT_MAX... A comment would be in order,
> though.

Yes, not the first time this has been brought up
https://lkml.org/lkml/2016/6/20/493. I guess a comment is long overdue.
Or just get rid of the unsigned trap which would be probably more clean.
 
> >> ---
> >>  mm/page_alloc.c | 5 ++---
> >>  1 file changed, 2 insertions(+), 3 deletions(-)
> >> 
> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >> index 2302f25..9120c2b 100644
> >> --- a/mm/page_alloc.c
> >> +++ b/mm/page_alloc.c
> >> @@ -2215,9 +2215,8 @@ __rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
> >>  	bool can_steal;
> >>  
> >>  	/* Find the largest possible block of pages in the other list */
> >> -	for (current_order = MAX_ORDER-1;
> >> -				current_order >= order && current_order <= MAX_ORDER-1;
> >> -				--current_order) {
> >> +	for (current_order = MAX_ORDER-1; current_order >= order;
> >> +							--current_order) {
> >>  		area = &(zone->free_area[current_order]);
> >>  		fallback_mt = find_suitable_fallback(area, current_order,
> >>  				start_migratetype, false, &can_steal);
> >> 

-- 
Michal Hocko
SUSE Labs