On 06/21/2017 08:55 PM, Rasmus Villemoes wrote:
> Since current_order starts as MAX_ORDER-1 and is then only
> decremented, the second half of the loop condition seems
> superfluous. However, if order is 0, we may decrement current_order
> past 0, making it UINT_MAX. This is obviously too subtle ([1], [2]).
>
> Since we need to add some comment anyway, change the two variables to
> signed, making the counting-down for loop look more familiar, and
> apparently also making gcc generate slightly smaller code.
>
> [1] https://lkml.org/lkml/2016/6/20/493
> [2] https://lkml.org/lkml/2017/6/19/345
>
> Signed-off-by: Rasmus Villemoes <li...@rasmusvillemoes.dk>
Acked-by: Vlastimil Babka <vba...@suse.cz>
> ---
> Michal, something like this, perhaps?
>
> mm/page_alloc.c | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2302f250d6b1..e656f4da9772 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2204,19 +2204,23 @@ static bool unreserve_highatomic_pageblock(const
> struct alloc_context *ac,
> * list of requested migratetype, possibly along with other pages from the
> same
> * block, depending on fragmentation avoidance heuristics. Returns true if
> * fallback was found so that __rmqueue_smallest() can grab it.
> + *
> + * The use of signed ints for order and current_order is a deliberate
> + * deviation from the rest of this file, to make the for loop
> + * condition simpler.
> */
> static inline bool
> -__rmqueue_fallback(struct zone *zone, unsigned int order, int
> start_migratetype)
> +__rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
> {
> struct free_area *area;
> - unsigned int current_order;
> + int current_order;
> struct page *page;
> int fallback_mt;
> 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 >= order;
> --current_order) {
> area = &(zone->free_area[current_order]);
> fallback_mt = find_suitable_fallback(area, current_order,
>
