diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 69321013683c..169baa817d96 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1297,6 +1297,12 @@ struct btrfs_fs_info {
 	u64 last_trans_log_full_commit;
 	unsigned long mount_opt;
 	unsigned long compress_type:4;
+	/*
+	 * It is a suggestive number, the read side is safe even it gets a
+	 * wrong number because we will write out the data into a regular
+	 * extent. The write side(mount/remount) is under ->s_umount lock,
+	 * so it is also safe.
+	 */
 	u64 max_inline;
 	u64 alloc_start;
 	struct btrfs_transaction *running_transaction;