1 files changed, 14 insertions, 2 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 71252486bc6f..155efca4c123 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -411,8 +411,13 @@ void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
  *
  * Returns @bdi's dirty limit in pages. The term "dirty" in the context of
  * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
- * And the "limit" in the name is not seriously taken as hard limit in
- * balance_dirty_pages().
+ *
+ * Note that balance_dirty_pages() will only seriously take it as a hard limit
+ * when sleeping max_pause per page is not enough to keep the dirty pages under
+ * control. For example, when the device is completely stalled due to some error
+ * conditions, or when there are 1000 dd tasks writing to a slow 10MB/s USB key.
+ * In the other normal situations, it acts more gently by throttling the tasks
+ * more (rather than completely block them) when the bdi dirty pages go high.
  *
  * It allocates high/low dirty limits to fast/slow devices, in order to prevent
  * - starving fast devices
@@ -594,6 +599,13 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
          */
         if (unlikely(bdi_thresh > thresh))
                 bdi_thresh = thresh;
+        /*
+         * It's very possible that bdi_thresh is close to 0 not because the
+         * device is slow, but that it has remained inactive for long time.
+         * Honour such devices a reasonable good (hopefully IO efficient)
+         * threshold, so that the occasional writes won't be blocked and active
+         * writes can rampup the threshold quickly.
+         */
         bdi_thresh = max(bdi_thresh, (limit - dirty) / 8);
         /*
          * scale global setpoint to bdi's: