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 mounted—either by the initialization scripts or directly by the users—on 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.[*]
In a traditional Unix system, there is only one tree of mounted filesystems: starting from the system's root filesystem, each process can potentially access every file in a mounted filesystem by specifying the proper pathname. In this respect, Linux 2.6 is more refined: every process might have its own tree of mounted filesystems—the so-called namespace of the process.
Usually most processes share the same namespace, which is the tree of mounted filesystems that is rooted ...