diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 9a6bc09..4fc87de 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -63,10 +63,31 @@
 	 * so it has a mismatch name.
 	 */
 	WB_REASON_FORKER_THREAD,
+	WB_REASON_FOREIGN_FLUSH,
 
 	WB_REASON_MAX,
 };
 
+struct wb_completion {
+	atomic_t		cnt;
+	wait_queue_head_t	*waitq;
+};
+
+#define __WB_COMPLETION_INIT(_waitq)	\
+	(struct wb_completion){ .cnt = ATOMIC_INIT(1), .waitq = (_waitq) }
+
+/*
+ * If one wants to wait for one or more wb_writeback_works, each work's
+ * ->done should be set to a wb_completion defined using the following
+ * macro.  Once all work items are issued with wb_queue_work(), the caller
+ * can wait for the completion of all using wb_wait_for_completion().  Work
+ * items which are waited upon aren't freed automatically on completion.
+ */
+#define WB_COMPLETION_INIT(bdi)		__WB_COMPLETION_INIT(&(bdi)->wb_waitq)
+
+#define DEFINE_WB_COMPLETION(cmpl, bdi)	\
+	struct wb_completion cmpl = WB_COMPLETION_INIT(bdi)
+
 /*
  * For cgroup writeback, multiple wb's may map to the same blkcg.  Those
  * wb's can operate mostly independently but should share the congested
@@ -165,6 +186,8 @@
 };
 
 struct backing_dev_info {
+	u64 id;
+	struct rb_node rb_node; /* keyed by ->id */
 	struct list_head bdi_list;
 	unsigned long ra_pages;	/* max readahead in PAGE_SIZE units */
 	unsigned long io_pages;	/* max allowed IO size */
@@ -190,6 +213,7 @@
 	struct radix_tree_root cgwb_tree; /* radix tree of active cgroup wbs */
 	struct rb_root cgwb_congested_tree; /* their congested states */
 	struct mutex cgwb_release_mutex;  /* protect shutdown of wb structs */
+	struct rw_semaphore wb_switch_rwsem; /* no cgwb switch while syncing */
 #else
 	struct bdi_writeback_congested *wb_congested;
 #endif
@@ -202,7 +226,6 @@
 
 #ifdef CONFIG_DEBUG_FS
 	struct dentry *debug_dir;
-	struct dentry *debug_stats;
 #endif
 };
 
@@ -258,6 +281,14 @@
  */
 static inline void wb_put(struct bdi_writeback *wb)
 {
+	if (WARN_ON_ONCE(!wb->bdi)) {
+		/*
+		 * A driver bug might cause a file to be removed before bdi was
+		 * initialized.
+		 */
+		return;
+	}
+
 	if (wb != &wb->bdi->wb)
 		percpu_ref_put(&wb->refcnt);
 }