mm/page-writeback.c: use div64_ul() for u64-by-unsigned-long divide

The two variables 'numerator' and 'denominator', though they are
declared as long, they should actually be unsigned long (according to
the implementation of the fprop_fraction_percpu() function)

And do_div() does a 64-by-32 division, while the divisor 'denominator'
is unsigned long, thus 64-bit on 64-bit platforms.  Hence the proper
function to call is div64_ul().

Link: http://lkml.kernel.org/r/20200102081442.8273-3-wenyang@linux.alibaba.comSigned-off-by: Wen Yang <wenyang@linux.alibaba.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Qian Cai <cai@lca.pw>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page-writeback.c

@@ -766,7 +766,7 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
 	struct wb_domain *dom = dtc_dom(dtc);
 	unsigned long thresh = dtc->thresh;
 	u64 wb_thresh;
-	long numerator, denominator;
+	unsigned long numerator, denominator;
 	unsigned long wb_min_ratio, wb_max_ratio;
 
 	/*
@@ -777,7 +777,7 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
 
 	wb_thresh = (thresh * (100 - bdi_min_ratio)) / 100;
 	wb_thresh *= numerator;
-	do_div(wb_thresh, denominator);
+	wb_thresh = div64_ul(wb_thresh, denominator);
 
 	wb_min_max_ratio(dtc->wb, &wb_min_ratio, &wb_max_ratio);