mm: fix off-by-two in __zone_watermark_ok()

Commit 88f5acf88ae6 ("mm: page allocator: adjust the per-cpu counter
threshold when memory is low") changed the form how free_pages is
calculated but it forgot that we used to do free_pages - ((1 << order) -
1) so we ended up with off-by-two when calculating free_pages.

Reported-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4f95bcf0f2b1..59153da58c69 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1525,7 +1525,7 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark,
         long min = mark;
         int o;
 
-        free_pages -= (1 << order) + 1;
+        free_pages -= (1 << order) - 1;
         if (alloc_flags & ALLOC_HIGH)
                 min -= min / 2;
         if (alloc_flags & ALLOC_HARDER)