3 files changed, 29 insertions, 7 deletions

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index f0ca569..a978414 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -425,20 +425,30 @@ static ssize_t queue_wb_lat_store(struct request_queue *q, const char *page,
 	ssize_t ret;
 	s64 val;
 
-	rwb = q->rq_wb;
-	if (!rwb)
-		return -EINVAL;
-
 	ret = queue_var_store64(&val, page);
 	if (ret < 0)
 		return ret;
+	if (val < -1)
+		return -EINVAL;
+
+	rwb = q->rq_wb;
+	if (!rwb) {
+		ret = wbt_init(q);
+		if (ret)
+			return ret;
+
+		rwb = q->rq_wb;
+		if (!rwb)
+			return -EINVAL;
+	}
 
 	if (val == -1)
 		rwb->min_lat_nsec = wbt_default_latency_nsec(q);
 	else if (val >= 0)
 		rwb->min_lat_nsec = val * 1000ULL;
-	else
-		return -EINVAL;
+
+	if (rwb->enable_state == WBT_STATE_ON_DEFAULT)
+		rwb->enable_state = WBT_STATE_ON_MANUAL;
 
 	wbt_update_limits(rwb);
 	return count;
diff --git a/block/blk-wbt.c b/block/blk-wbt.c
index 7c0e618..b864734 100644
--- a/block/blk-wbt.c
+++ b/block/blk-wbt.c
@@ -673,7 +673,7 @@ void wbt_disable_default(struct request_queue *q)
 {
 	struct rq_wb *rwb = q->rq_wb;
 
-	if (rwb) {
+	if (rwb && rwb->enable_state == WBT_STATE_ON_DEFAULT) {
 		del_timer_sync(&rwb->window_timer);
 		rwb->win_nsec = rwb->min_lat_nsec = 0;
 		wbt_update_limits(rwb);
@@ -721,6 +721,7 @@ int wbt_init(struct request_queue *q)
 	rwb->last_comp = rwb->last_issue = jiffies;
 	rwb->queue = q;
 	rwb->win_nsec = RWB_WINDOW_NSEC;
+	rwb->enable_state = WBT_STATE_ON_DEFAULT;
 	wbt_update_limits(rwb);
 
 	/*
diff --git a/block/blk-wbt.h b/block/blk-wbt.h
index 8f485f8..65f1de5 100644
--- a/block/blk-wbt.h
+++ b/block/blk-wbt.h
@@ -21,6 +21,15 @@ enum {
 	WBT_NUM_RWQ		= 2,
 };
 
+/*
+ * Enable states. Either off, or on by default (done at init time),
+ * or on through manual setup in sysfs.
+ */
+enum {
+	WBT_STATE_ON_DEFAULT	= 1,
+	WBT_STATE_ON_MANUAL	= 2,
+};
+
 static inline void wbt_clear_state(struct blk_issue_stat *stat)
 {
 	stat->time &= BLK_STAT_TIME_MASK;
@@ -61,6 +70,8 @@ struct rq_wb {
 	int scale_step;
 	bool scaled_max;
 
+	short enable_state;			/* WBT_STATE_* */
+
 	/*
 	 * Number of consecutive periods where we don't have enough
 	 * information to make a firm scale up/down decision.