sys_mount что есть флаг MS_REC?

man 2 mount

Since Linux 2.4.0 it is possible to remount part of the file hierarchy somewhere else. The call is
mount --bind olddir newdir
After this call the same contents is accessible in two places. One can also remount a single file (on a single file).

This call attaches only (part of) a single filesystem, not possible submounts. The entire file hierarchy including submounts is attached a second place using
mount --rbind olddir newdir


Вам второе

>Чем клонирование дерева отличается от копирования?

может синхронизацией монтирования в дереве?