[PATCH 1/3] mm/page-writeback.c: avoid potential division by zero in wb_min_max_ratio()

[Wen Yang <wenyang@linux.alibaba.com>]

The variables 'min' and 'max' are unsigned long and
do_div truncates them to 32 bits, which means it can test
non-zero and be truncated to zero for division.
Fix this issue by using div64_ul() instead.

Fixes: 693108a8a667 ("writeback: make bdi->min/max_ratio handling cgroup writeback aware")
Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Qian Cai <cai@lca.pw>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
---
 mm/page-writeback.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 50055d2..2d658b2 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -201,11 +201,11 @@ static void wb_min_max_ratio(struct bdi_writeback *wb,
 	if (this_bw < tot_bw) {
 		if (min) {
 			min *= this_bw;
-			do_div(min, tot_bw);
+			min = div64_ul(min, tot_bw);
 		}
 		if (max < 100) {
 			max *= this_bw;
-			do_div(max, tot_bw);
+			max = div64_ul(max, tot_bw);
 		}
 	}
 
-- 
1.8.3.1