diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 8a8bb87..c360f6a 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -1,5 +1,7 @@
+// SPDX-License-Identifier: GPL-2.0-only
 
 #include <linux/wait.h>
+#include <linux/rbtree.h>
 #include <linux/backing-dev.h>
 #include <linux/kthread.h>
 #include <linux/freezer.h>
@@ -21,10 +23,12 @@
 static struct class *bdi_class;
 
 /*
- * bdi_lock protects updates to bdi_list. bdi_list has RCU reader side
- * locking.
+ * bdi_lock protects bdi_tree and updates to bdi_list. bdi_list has RCU
+ * reader side locking.
  */
 DEFINE_SPINLOCK(bdi_lock);
+static u64 bdi_id_cursor;
+static struct rb_root bdi_tree = RB_ROOT;
 LIST_HEAD(bdi_list);
 
 /* bdi_wq serves all asynchronous writeback tasks */
@@ -102,39 +106,25 @@
 }
 DEFINE_SHOW_ATTRIBUTE(bdi_debug_stats);
 
-static int bdi_debug_register(struct backing_dev_info *bdi, const char *name)
+static void bdi_debug_register(struct backing_dev_info *bdi, const char *name)
 {
-	if (!bdi_debug_root)
-		return -ENOMEM;
-
 	bdi->debug_dir = debugfs_create_dir(name, bdi_debug_root);
-	if (!bdi->debug_dir)
-		return -ENOMEM;
 
-	bdi->debug_stats = debugfs_create_file("stats", 0444, bdi->debug_dir,
-					       bdi, &bdi_debug_stats_fops);
-	if (!bdi->debug_stats) {
-		debugfs_remove(bdi->debug_dir);
-		bdi->debug_dir = NULL;
-		return -ENOMEM;
-	}
-
-	return 0;
+	debugfs_create_file("stats", 0444, bdi->debug_dir, bdi,
+			    &bdi_debug_stats_fops);
 }
 
 static void bdi_debug_unregister(struct backing_dev_info *bdi)
 {
-	debugfs_remove(bdi->debug_stats);
-	debugfs_remove(bdi->debug_dir);
+	debugfs_remove_recursive(bdi->debug_dir);
 }
 #else
 static inline void bdi_debug_init(void)
 {
 }
-static inline int bdi_debug_register(struct backing_dev_info *bdi,
+static inline void bdi_debug_register(struct backing_dev_info *bdi,
 				      const char *name)
 {
-	return 0;
 }
 static inline void bdi_debug_unregister(struct backing_dev_info *bdi)
 {
@@ -249,8 +239,8 @@
 {
 	int err;
 
-	bdi_wq = alloc_workqueue("writeback", WQ_MEM_RECLAIM | WQ_FREEZABLE |
-					      WQ_UNBOUND | WQ_SYSFS, 0);
+	bdi_wq = alloc_workqueue("writeback", WQ_MEM_RECLAIM | WQ_UNBOUND |
+				 WQ_SYSFS, 0);
 	if (!bdi_wq)
 		return -ENOMEM;
 
@@ -628,13 +618,12 @@
 }
 
 /**
- * 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
@@ -651,6 +640,39 @@
  * 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)
@@ -663,20 +685,7 @@
 		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;
@@ -689,6 +698,7 @@
 	INIT_RADIX_TREE(&bdi->cgwb_tree, GFP_ATOMIC);
 	bdi->cgwb_congested_tree = RB_ROOT;
 	mutex_init(&bdi->cgwb_release_mutex);
+	init_rwsem(&bdi->wb_switch_rwsem);
 
 	ret = wb_init(&bdi->wb, bdi, 1, GFP_KERNEL);
 	if (!ret) {
@@ -871,9 +881,58 @@
 }
 EXPORT_SYMBOL(bdi_alloc_node);
 
+static struct rb_node **bdi_lookup_rb_node(u64 id, struct rb_node **parentp)
+{
+	struct rb_node **p = &bdi_tree.rb_node;
+	struct rb_node *parent = NULL;
+	struct backing_dev_info *bdi;
+
+	lockdep_assert_held(&bdi_lock);
+
+	while (*p) {
+		parent = *p;
+		bdi = rb_entry(parent, struct backing_dev_info, rb_node);
+
+		if (bdi->id > id)
+			p = &(*p)->rb_left;
+		else if (bdi->id < id)
+			p = &(*p)->rb_right;
+		else
+			break;
+	}
+
+	if (parentp)
+		*parentp = parent;
+	return p;
+}
+
+/**
+ * bdi_get_by_id - lookup and get bdi from its id
+ * @id: bdi id to lookup
+ *
+ * Find bdi matching @id and get it.  Returns NULL if the matching bdi
+ * doesn't exist or is already unregistered.
+ */
+struct backing_dev_info *bdi_get_by_id(u64 id)
+{
+	struct backing_dev_info *bdi = NULL;
+	struct rb_node **p;
+
+	spin_lock_bh(&bdi_lock);
+	p = bdi_lookup_rb_node(id, NULL);
+	if (*p) {
+		bdi = rb_entry(*p, struct backing_dev_info, rb_node);
+		bdi_get(bdi);
+	}
+	spin_unlock_bh(&bdi_lock);
+
+	return bdi;
+}
+
 int bdi_register_va(struct backing_dev_info *bdi, const char *fmt, va_list args)
 {
 	struct device *dev;
+	struct rb_node *parent, **p;
 
 	if (bdi->dev)	/* The driver needs to use separate queues per device */
 		return 0;
@@ -889,7 +948,15 @@
 	set_bit(WB_registered, &bdi->wb.state);
 
 	spin_lock_bh(&bdi_lock);
+
+	bdi->id = ++bdi_id_cursor;
+
+	p = bdi_lookup_rb_node(bdi->id, &parent);
+	rb_link_node(&bdi->rb_node, parent, p);
+	rb_insert_color(&bdi->rb_node, &bdi_tree);
+
 	list_add_tail_rcu(&bdi->bdi_list, &bdi_list);
+
 	spin_unlock_bh(&bdi_lock);
 
 	trace_writeback_bdi_register(bdi);
@@ -930,6 +997,7 @@
 static void bdi_remove_from_list(struct backing_dev_info *bdi)
 {
 	spin_lock_bh(&bdi_lock);
+	rb_erase(&bdi->rb_node, &bdi_tree);
 	list_del_rcu(&bdi->bdi_list);
 	spin_unlock_bh(&bdi_lock);