From: Rasmus Villemoes Date: Mon, 10 Jul 2017 22:49:26 +0000 (-0700) Subject: mm/page_alloc.c: eliminate unsigned confusion in __rmqueue_fallback X-Git-Tag: v4.13-rc1~62^2~70 X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=b002529d256307602c669d1886c0b953b52b8700;p=platform%2Fkernel%2Flinux-exynos.git mm/page_alloc.c: eliminate unsigned confusion in __rmqueue_fallback 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 [akpm@linux-foundation.org: fix up reject fixupping] Link: http://lkml.kernel.org/r/20170621185529.2265-1-linux@rasmusvillemoes.dk Signed-off-by: Rasmus Villemoes Reported-by: Hao Lee Acked-by: Wei Yang Acked-by: Michal Hocko Acked-by: Vlastimil Babka Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 8690357..d90c319 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -2206,12 +2206,16 @@ 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; @@ -2221,8 +2225,7 @@ __rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype) * approximates finding the pageblock with the most free pages, which * would be too costly to do exactly. */ - for (current_order = MAX_ORDER-1; - current_order >= order && current_order <= MAX_ORDER-1; + 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,