+ /*
+ * Protected by ->chunk_mutex and sb->s_umount.
+ *
+ * The reason that we use two lock to protect it is because only
+ * remount and mount operations can change it and these two operations
+ * are under sb->s_umount, but the read side (chunk allocation) can not
+ * acquire sb->s_umount or the deadlock would happen. So we use two
+ * locks to protect it. On the write side, we must acquire two locks,
+ * and on the read side, we just need acquire one of them.
+ */