1 files changed, 5 insertions, 2 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 45e187b2d971..644bcb665773 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -857,8 +857,11 @@ static void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
          *                   bw * elapsed + write_bandwidth * (period - elapsed)
          * write_bandwidth = ---------------------------------------------------
          *                                          period
+         *
+         * @written may have decreased due to account_page_redirty().
+         * Avoid underflowing @bw calculation.
          */
-        bw = written - bdi->written_stamp;
+        bw = written - min(written, bdi->written_stamp);
         bw *= HZ;
         if (unlikely(elapsed > period)) {
                 do_div(bw, elapsed);
@@ -922,7 +925,7 @@ static void global_update_bandwidth(unsigned long thresh,
                                     unsigned long now)
 {
         static DEFINE_SPINLOCK(dirty_lock);
-        static unsigned long update_time;
+        static unsigned long update_time = INITIAL_JIFFIES;
 
         /*
          * check locklessly first to optimize away locking for the most time