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authorRasmus Villemoes <linux@rasmusvillemoes.dk>2017-07-10 15:49:26 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2017-07-10 16:32:32 -0700
commitb002529d256307602c669d1886c0b953b52b8700 (patch)
tree9219954404a5b6124356dd8a1785d6f5c0653126
parent8c03cc85a035ae7a208c28c4382ecfeb6adf79a6 (diff)
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 <linux@rasmusvillemoes.dk> Reported-by: Hao Lee <haolee.swjtu@gmail.com> Acked-by: Wei Yang <weiyang@gmail.com> Acked-by: Michal Hocko <mhocko@suse.com> Acked-by: Vlastimil Babka <vbabka@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--mm/page_alloc.c11
1 files changed, 7 insertions, 4 deletions
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 869035717048..d90c31951b90 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,