writeback: Separate out wb_get_lookup() from wb_get_create()

Separate out wb_get_lookup() which doesn't try to create one if there
isn't already one from wb_get_create().  This will be used by later
patches.
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/backing-dev.h

@@ -230,6 +230,8 @@ static inline int bdi_sched_wait(void *word)
 struct bdi_writeback_congested *
 wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp);
 void wb_congested_put(struct bdi_writeback_congested *congested);
+struct bdi_writeback *wb_get_lookup(struct backing_dev_info *bdi,
+				    struct cgroup_subsys_state *memcg_css);
 struct bdi_writeback *wb_get_create(struct backing_dev_info *bdi,
 				    struct cgroup_subsys_state *memcg_css,
 				    gfp_t gfp);

mm/backing-dev.c

@@ -618,13 +618,12 @@ static int cgwb_create(struct backing_dev_info *bdi,
 }
 
 /**
- * wb_get_create - get wb for a given memcg, create if necessary
+ * wb_get_lookup - get wb for a given memcg
  * @bdi: target bdi
  * @memcg_css: cgroup_subsys_state of the target memcg (must have positive ref)
- * @gfp: allocation mask to use
  *
- * Try to get the wb for @memcg_css on @bdi.  If it doesn't exist, try to
- * create one.  The returned wb has its refcount incremented.
+ * Try to get the wb for @memcg_css on @bdi.  The returned wb has its
+ * refcount incremented.
  *
  * This function uses css_get() on @memcg_css and thus expects its refcnt
  * to be positive on invocation.  IOW, rcu_read_lock() protection on
@@ -641,6 +640,39 @@ static int cgwb_create(struct backing_dev_info *bdi,
  * each lookup.  On mismatch, the existing wb is discarded and a new one is
  * created.
  */
+struct bdi_writeback *wb_get_lookup(struct backing_dev_info *bdi,
+				    struct cgroup_subsys_state *memcg_css)
+{
+	struct bdi_writeback *wb;
+
+	if (!memcg_css->parent)
+		return &bdi->wb;
+
+	rcu_read_lock();
+	wb = radix_tree_lookup(&bdi->cgwb_tree, memcg_css->id);
+	if (wb) {
+		struct cgroup_subsys_state *blkcg_css;
+
+		/* see whether the blkcg association has changed */
+		blkcg_css = cgroup_get_e_css(memcg_css->cgroup, &io_cgrp_subsys);
+		if (unlikely(wb->blkcg_css != blkcg_css || !wb_tryget(wb)))
+			wb = NULL;
+		css_put(blkcg_css);
+	}
+	rcu_read_unlock();
+
+	return wb;
+}
+
+/**
+ * wb_get_create - get wb for a given memcg, create if necessary
+ * @bdi: target bdi
+ * @memcg_css: cgroup_subsys_state of the target memcg (must have positive ref)
+ * @gfp: allocation mask to use
+ *
+ * Try to get the wb for @memcg_css on @bdi.  If it doesn't exist, try to
+ * create one.  See wb_get_lookup() for more details.
+ */
 struct bdi_writeback *wb_get_create(struct backing_dev_info *bdi,
 				    struct cgroup_subsys_state *memcg_css,
 				    gfp_t gfp)
@@ -653,20 +685,7 @@ struct bdi_writeback *wb_get_create(struct backing_dev_info *bdi,
 		return &bdi->wb;
 
 	do {
-		rcu_read_lock();
-		wb = radix_tree_lookup(&bdi->cgwb_tree, memcg_css->id);
-		if (wb) {
-			struct cgroup_subsys_state *blkcg_css;
-
-			/* see whether the blkcg association has changed */
-			blkcg_css = cgroup_get_e_css(memcg_css->cgroup,
-						     &io_cgrp_subsys);
-			if (unlikely(wb->blkcg_css != blkcg_css ||
-				     !wb_tryget(wb)))
-				wb = NULL;
-			css_put(blkcg_css);
-		}
-		rcu_read_unlock();
+		wb = wb_get_lookup(bdi, memcg_css);
 	} while (!wb && !cgwb_create(bdi, memcg_css, gfp));
 
 	return wb;