The “Virtual File System” in Linux

This article outlines the VFS structure and gives an overview of how the Linux kernel accesses its file hierarchy. The information herein refers to Linux 2.0.x (for any x) and 2.1.y (with y up to at least 18).
Interesting Examples

My discussion is now finished, but there are many places where interesting source code is available for viewing. Implementations of file system types worth examining:

  • Obviously, the “/proc” file system: it is quite easy to look at, because it is neither performance-critical nor particularly fully featured (except the sysctl idea). Enough said.

  • The “UMSDOS” file system: it is part of the mainstream kernel and runs piggy-back on the “Ms-DOS” file system. It implements only a few of the operations of the VFS to add new capabilities to an old-fashioned file system format.

  • The “userfs” module: it is available from both tsx-11 and sunsite under ALPHA/userfs; version 0.9.3 will load to Linux 2.0. This module defines a new file system type which uses external programs to retrieve data; interesting applications are the ftp file system and a read-only file system to mount compressed tar files. Even though reverting to user programs to get file system data is dangerous and might lead to unexpected deadlocks, the idea is quite interesting.

  • “supermount”: the file system is available on sunsite and mirrors. This file system type is able to mount removable devices like floppies or CD-ROMs and handle device removal without forcing the user to umount/mount the device. The module works by controlling another file system type while arranging to keep the device unmounted when it is not used; the operation is transparent to the user.

  • “ext2”: the extended-2 file system has been the standard Linux file system for a few years now. It is difficult code, but worth reading for those interested in seeing how a real file system is implemented. It also has hooks for interesting security features like the immutable-flag and the append-only-flag. Files marked as immutable or append-only can only be deleted when the system is in single-user mode, and are therefore secured from network intruders.

  • “romfs”: this is the smallest file system I've ever seen. It was introduced in Linux-2.1.21. It's a single source file, and it's quite enjoyable to browse. As its name asserts, it is read-only.

is a wild soul with an attraction for source code. He is a fan of Linus Torvalds and Baden Powell and enjoys the two communities of volunteer workers they have attracted. He can be reached at rubini@linux.it.

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great info, but having problems to build a module

Antony Valieff's picture

I found the information about /proc because I am looking to interact with some hardware with GPL release code. Great introduction, thanks.
The only problem I have is that in my test module code, when I build it, proc_register_dynamic is not found.
I am clueless, working on MIPS based system, kernel version 2.4.19.
It's a wireless device being sold by Linksys.

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