下面是从Undertanding the linux kernel 3rd摘录的
12.4. Filesystem Handling
Like every traditional Unix system, Linux makes use of
a system 's root filesystem : it is the filesystem
that is directly mounted by the kernel during the
booting phase and that holds the system initialization
scripts and the most essential system programs.
Other filesystems can be mountedeither by the
initialization scripts or directly by the userson
directories of already mounted filesystems. Being a tree
of directories, every filesystem has its own root
directory. The directory on which a filesystem is
mounted is called the mount point. A mounted filesystem
is a child of the mounted filesystem to which the
mount point directory belongs. For instance, the /proc
virtual filesystem is a child of the system 's root
filesystem (and the system 's root filesystem is the
parent of /proc). The root directory of a mounted
filesystem hides the content of the mount point
directory of the parent filesystem, as well as the
whole subtree of the parent filesystem below the mount
point.