Understanding the Linux Kernel, Second Edition

Filesystem Mounting

Each filesystem has its own root directory . The filesystem whose root directory is the root of the system’s directory tree is called root filesystem . Other filesystems can be mounted on the system’s directory tree; the directories on which they are inserted are called mount points . A mounted filesystem is the child of the mounted filesystem to which the mount point directory belongs. For instance, the /proc virtual filesystem is a child of the root filesystem (and the root filesystem is the parent of /proc).

In most traditional Unix-like kernels, each filesystem can be mounted only once. Suppose that an Ext2 filesystem stored in the /dev/fd0 floppy disk is mounted on /flp by issuing the command:

mount -t ext2 /dev/fd0 /flp

Until the filesystem is unmounted by issuing a umount command, any other mount command acting on /dev/fd0 fails.

However, Linux 2.4 is different: it is possible to mount the same filesystem several times. For instance, issuing the following command right after the previous one will likely succeed in Linux:

mount -t ext2 -o ro /dev/fd0 /flp-ro

As a result, the Ext2 filesystem stored in the floppy disk is mounted both on /flp and on /flp-ro; therefore, its files can be accessed through both /flp and /flp-ro (in this example, accesses through /flp-ro are read-only).

Of course, if a filesystem is mounted n times, its root directory can be accessed through n mount points, one per mount operation. Although the same filesystem can be accessed ...