[PATCH] mm/page_alloc.c: eliminate unsigned confusion in __rmqueue_fallback

From: Rasmus Villemoes
Date: Wed Jun 21 2017 - 14:55:58 EST

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,