diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index b4cb8186035f..d5e60d25ca51 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3695,12 +3695,15 @@ static int can_overcommit(struct btrfs_root *root,
 	return 0;
 }
 
-static int writeback_inodes_sb_nr_if_idle_safe(struct super_block *sb,
-					       unsigned long nr_pages,
-					       enum wb_reason reason)
+static inline int writeback_inodes_sb_nr_if_idle_safe(struct super_block *sb,
+						      unsigned long nr_pages,
+						      enum wb_reason reason)
 {
-	if (!writeback_in_progress(sb->s_bdi) &&
-	    down_read_trylock(&sb->s_umount)) {
+	/* the flusher is dealing with the dirty inodes now. */
+	if (writeback_in_progress(sb->s_bdi))
+		return 1;
+
+	if (down_read_trylock(&sb->s_umount)) {
 		writeback_inodes_sb_nr(sb, nr_pages, reason);
 		up_read(&sb->s_umount);
 		return 1;
@@ -3709,6 +3712,28 @@ static int writeback_inodes_sb_nr_if_idle_safe(struct super_block *sb,
 	return 0;
 }
 
+void btrfs_writeback_inodes_sb_nr(struct btrfs_root *root,
+				  unsigned long nr_pages)
+{
+	struct super_block *sb = root->fs_info->sb;
+	int started;
+
+	/* If we can not start writeback, just sync all the delalloc file. */
+	started = writeback_inodes_sb_nr_if_idle_safe(sb, nr_pages,
+						      WB_REASON_FS_FREE_SPACE);
+	if (!started) {
+		/*
+		 * We needn't worry the filesystem going from r/w to r/o though
+		 * we don't acquire ->s_umount mutex, because the filesystem
+		 * should guarantee the delalloc inodes list be empty after
+		 * the filesystem is readonly(all dirty pages are written to
+		 * the disk).
+		 */
+		btrfs_start_delalloc_inodes(root, 0);
+		btrfs_wait_ordered_extents(root, 0);
+	}
+}
+
 /*
  * shrink metadata reservation for delalloc
  */
@@ -3741,10 +3766,7 @@ static void shrink_delalloc(struct btrfs_root *root, u64 to_reclaim, u64 orig,
 	while (delalloc_bytes && loops < 3) {
 		max_reclaim = min(delalloc_bytes, to_reclaim);
 		nr_pages = max_reclaim >> PAGE_CACHE_SHIFT;
-		writeback_inodes_sb_nr_if_idle_safe(root->fs_info->sb,
-						    nr_pages,
-						    WB_REASON_FS_FREE_SPACE);
-
+		btrfs_writeback_inodes_sb_nr(root, nr_pages);
 		/*
 		 * We need to wait for the async pages to actually start before
 		 * we do anything.