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

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.dkSigned-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
...@@ -2206,12 +2206,16 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac, ...@@ -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 * list of requested migratetype, possibly along with other pages from the same
* block, depending on fragmentation avoidance heuristics. Returns true if * block, depending on fragmentation avoidance heuristics. Returns true if
* fallback was found so that __rmqueue_smallest() can grab it. * 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 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; struct free_area *area;
unsigned int current_order; int current_order;
struct page *page; struct page *page;
int fallback_mt; int fallback_mt;
bool can_steal; bool can_steal;
...@@ -2221,8 +2225,7 @@ __rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype) ...@@ -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 * approximates finding the pageblock with the most free pages, which
* would be too costly to do exactly. * would be too costly to do exactly.
*/ */
for (current_order = MAX_ORDER-1; for (current_order = MAX_ORDER - 1; current_order >= order;
current_order >= order && current_order <= MAX_ORDER-1;
--current_order) { --current_order) {
area = &(zone->free_area[current_order]); area = &(zone->free_area[current_order]);
fallback_mt = find_suitable_fallback(area, current_order, fallback_mt = find_suitable_fallback(area, current_order,
......
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