From: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> Subject: 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@xxxxxxxxxxxxxxxxxxxx: fix up reject fixupping] Link: http://lkml.kernel.org/r/20170621185529.2265-1-linux@xxxxxxxxxxxxxxxxxx Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> Reported-by: Hao Lee <haolee.swjtu@xxxxxxxxx> Acked-by: Wei Yang <weiyang@xxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Acked-by: Vlastimil Babka <vbabka@xxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/page_alloc.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff -puN mm/page_alloc.c~mm-page_allocc-eliminate-unsigned-confusion-in-__rmqueue_fallback mm/page_alloc.c --- a/mm/page_alloc.c~mm-page_allocc-eliminate-unsigned-confusion-in-__rmqueue_fallback +++ a/mm/page_alloc.c @@ -2206,12 +2206,16 @@ static bool unreserve_highatomic_pageblo * 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, un * 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, _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
