On Wed, Jun 21, 2017 at 08:55:28PM +0200, 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 <linux@xxxxxxxxxxxxxxxxxx> >--- >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, >-- >2.11.0 Looks nice. Why I didn't come up with this change. Acked-by: Wei Yang <weiyang@xxxxxxxxx> -- Wei Yang Help you, Help me
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