mm/page_alloc.c: eliminate unsigned confusion in __rmqueue_fallback (b002529d) · Commits · 戴 / test

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>

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,

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