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>

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;
 }