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Commit b002529d authored by Rasmus Villemoes's avatar Rasmus Villemoes Committed by Linus Torvalds
Browse files

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: default avatarRasmus Villemoes <linux@rasmusvillemoes.dk>
Reported-by: default avatarHao Lee <haolee.swjtu@gmail.com>
Acked-by: default avatarWei Yang <weiyang@gmail.com>
Acked-by: default avatarMichal Hocko <mhocko@suse.com>
Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 8c03cc85

mm/page_alloc.c

+7 −4
Original line number Diff line number Diff line
@@ -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,