blk-wbt: cleanup disable-by-default for CFQ

Make it clear that we are disabling wbt for the specified queued,
if it was enabled by default. This is in preparation for allowing
users to re-enable wbt, and not have it disabled automatically
again.
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-wbt.c

@@ -665,15 +665,21 @@ void wbt_set_write_cache(struct rq_wb *rwb, bool write_cache_on)
 		rwb->wc = write_cache_on;
 }
 
-void wbt_disable(struct rq_wb *rwb)
+ /*
+ * Disable wbt, if enabled by default. Only called from CFQ, if we have
+ * cgroups enabled
+ */
+void wbt_disable_default(struct request_queue *q)
 {
+	struct rq_wb *rwb = q->rq_wb;
+
 	if (rwb) {
 		del_timer_sync(&rwb->window_timer);
 		rwb->win_nsec = rwb->min_lat_nsec = 0;
 		wbt_update_limits(rwb);
 	}
 }
-EXPORT_SYMBOL_GPL(wbt_disable);
+EXPORT_SYMBOL_GPL(wbt_disable_default);
 
 u64 wbt_default_latency_nsec(struct request_queue *q)
 {

block/blk-wbt.h

@@ -105,7 +105,7 @@ void wbt_exit(struct request_queue *);
 void wbt_update_limits(struct rq_wb *);
 void wbt_requeue(struct rq_wb *, struct blk_issue_stat *);
 void wbt_issue(struct rq_wb *, struct blk_issue_stat *);
-void wbt_disable(struct rq_wb *);
+void wbt_disable_default(struct request_queue *);
 
 void wbt_set_queue_depth(struct rq_wb *, unsigned int);
 void wbt_set_write_cache(struct rq_wb *, bool);
@@ -141,7 +141,7 @@ static inline void wbt_requeue(struct rq_wb *rwb, struct blk_issue_stat *stat)
 static inline void wbt_issue(struct rq_wb *rwb, struct blk_issue_stat *stat)
 {
 }
-static inline void wbt_disable(struct rq_wb *rwb)
+static inline void wbt_disable_default(struct request_queue *q)
 {
 }
 static inline void wbt_set_queue_depth(struct rq_wb *rwb, unsigned int depth)

block/cfq-iosched.c

@@ -3780,13 +3780,10 @@ static void check_blkcg_changed(struct cfq_io_cq *cic, struct bio *bio)
 	/*
 	 * If we have a non-root cgroup, we can depend on that to
 	 * do proper throttling of writes. Turn off wbt for that
-	 * case.
+	 * case, if it was enabled by default.
 	 */
-	if (nonroot_cg) {
-		struct request_queue *q = cfqd->queue;
-
-		wbt_disable(q->rq_wb);
-	}
+	if (nonroot_cg)
+		wbt_disable_default(cfqd->queue);
 
 	/*
 	 * Drop reference to queues.  New queues will be assigned in new