writeback: dirty position control - bdi reserve area

Keep a minimal pool of dirty pages for each bdi, so that the disk IO
queues won't underrun. Also gently increase a small bdi_thresh to avoid
it stuck in 0 for some light dirtied bdi.

It's particularly useful for JBOD and small memory system.

It may result in (pos_ratio > 1) at the setpoint and push the dirty
pages high. This is more or less intended because the bdi is in the
danger of IO queue underflow.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 6a8bb693b429..325f753c80ed 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -599,6 +599,7 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
          */
         if (unlikely(bdi_thresh > thresh))
                 bdi_thresh = thresh;
+        bdi_thresh = max(bdi_thresh, (limit - dirty) / 8);
         /*
          * scale global setpoint to bdi's:
          *      bdi_setpoint = setpoint * bdi_thresh / thresh
@@ -622,6 +623,20 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
         } else
                 pos_ratio /= 4;
 
+        /*
+         * bdi reserve area, safeguard against dirty pool underrun and disk idle
+         * It may push the desired control point of global dirty pages higher
+         * than setpoint.
+         */
+        x_intercept = bdi_thresh / 2;
+        if (bdi_dirty < x_intercept) {
+                if (bdi_dirty > x_intercept / 8) {
+                        pos_ratio *= x_intercept;
+                        do_div(pos_ratio, bdi_dirty);
+                } else
+                        pos_ratio *= 8;
+        }
+
         return pos_ratio;
 }