Linux Device Drivers, Second Edition

How Mounting and Unmounting Works

Block devices differ from char devices and normal files in that they can be mounted on the computer’s filesystem. Mounting provides a level of indirection not seen with char devices, which are accessed through a struct file pointer that is held by a specific process. When a filesystem is mounted, there is no process holding that file structure.

When the kernel mounts a device in the filesystem, it invokes the normal open method to access the driver. However, in this case both the filp and inode arguments to open are dummy variables. In the file structure, only the f_mode and f_flags fields hold anything meaningful; in the inode structure only i_rdev may be used. The remaining fields hold random values and should not be used. The value of f_mode tells the driver whether the device is to be mounted read-only (f_mode == FMODE_READ) or read/write (f_mode == (FMODE_READ|FMODE_WRITE)).

This interface may seem a little strange; it is done this way for two reasons. First is that the open method can still be called normally by a process that accesses the device directly—the mkfs utility, for example. The other reason is a historical artifact: block devices once used the same file_operations structure as char devices, and thus had to conform to the same interface.

Other than the limitations on the arguments to the open method, the driver does not really see anything unusual when a filesystem is mounted. The device is opened, and then the request ...