/* -*- auto-fill -*- */
Overview of the Virtual File System
Richard Gooch
5-JUL-1999
Conventions used in this document
=================================
Each section in this document will have the string "" at the
right-hand side of the section title. Each subsection will have
"" at the right-hand side. These strings are meant to make
it easier to search through the document.
NOTE that the master copy of this document is available online at:
~rgooch/linux/docs/vfs.txt
What is it?
===========
The Virtual File System (otherwise known as the Virtual Filesystem
Switch) is the software layer in the kernel that provides the
filesystem interface to userspace programmes. It also provides an
abstraction within the kernel which allows different filesystem
implementations to co-exist.
A Quick Look At How It Works
============================
In this section I'll briefly describe how things work, before
launching into the details. I'll start with describing what happens
when user programmes open and manipulate files, and then look from the
other view which is how a filesystem is supported and subsequently
mounted.
Opening a File
--------------
The VFS implements the open(2), stat(2), chmod(2) and similar system
calls. The pathname argument is used by the VFS to search through the
directory entry cache (dentry cache or "dcache"). This provides a very
fast lookup mechanism to translate a pathname (filename) into a
specific dentry.
An individual dentry usually has a pointer to an inode. Inodes are the
things that live on disc drives, and can be regular files (you know:
those things that you write data into), directories, FIFOs and other
beasts. Dentries live in RAM and are never saved to disc: they exist
only for performance. Inodes live on disc and are copied into memory
when required. Later any changes are written back to disc. The inode
that lives in RAM is a VFS inode, and it is this which the dentry
points to. A single inode can be pointed to by multiple dentries
(think about hardlinks).
The dcache is meant to be a view into your entire filespace. Unlike
Linus, most of us losers can't fit enough dentries into RAM to cover
all of our filespace, so the dcache has bits missing. In order to
resolve your pathname into a dentry, the VFS may have to resort to
creating dentries along the way, and then loading the inode. This is
done by looking up the inode.
To lookup an inode (usually read from disc) requires that the VFS
calls the lookup() method of the parent directory inode. This method
is installed by the specific filesystem implementation that the inode
lives in. There will be more on this later.
Once the VFS has the required dentry (and hence the inode), we can do
all those boring things like open(2) the file, or stat(2) it to peek
at the inode data. The stat(2) operation is fairly simple: once the
VFS has the dentry, it peeks at the inode data and passes some of it
back to userspace.
Opening a file requires another operation: allocation of a file
structure (this is the kernel-side implementation of file
descriptors). The freshly allocated file structure is initialised with
a pointer to the dentry and a set of file operation member functions.
These are taken from the inode data. The open() file method is then
called so the specific filesystem implementation can do it's work. You
can see that this is another switch performed by the VFS.
The file structure is placed into the file descriptor table for the
process.
Reading, writing and closing files (and other assorted VFS operations)
is done by using the userspace file descriptor to grab the appropriate
file structure, and then calling the required file structure method
function to do whatever is required.
For as long as the file is open, it keeps the dentry "open" (in use),
which in turn means that the VFS inode is still in use.
All VFS system calls (i.e. open(2), stat(2), read(2), write(2),
chmod(2) and so on) are called from a process context. You should
assume that these calls are made without any kernel locks being
held. This means that the processes may be executing the same piece of
filesystem or driver code at the same time, on different
processors. You should ensure that access to shared resources is
protected by appropriate locks.
Registering and Mounting a Filesystem
-------------------------------------
If you want to support a new kind of filesystem in the kernel, all you
need to do is call register_filesystem(). You pass a structure
describing the filesystem implementation (struct file_system_type)
which is then added to an internal table of supported filesystems. You
can do:
% cat /proc/filesystems
to see what filesystems are currently available on your system.
When a request is made to mount a block device onto a directory in
your filespace the VFS will call the appropriate method for the
specific filesystem. The dentry for the mount point will then be
updated to point to the root inode for the new filesystem.
It's now time to look at things in more detail.
struct file_system_type
=======================
This describes the filesystem. As of kernel 2.1.99, the following
members are defined:
struct file_system_type {
const char *name;
int fs_flags;
struct super_block *(*read_super) (struct super_block *, void *, int);
struct file_system_type * next;
};
name: the name of the filesystem type, such as "ext2", "iso9660",
"msdos" and so on
fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
read_super: the method to call when a new instance of this
filesystem should be mounted
next: for internal VFS use: you should initialise this to NULL
The read_super() method has the following arguments:
struct super_block *sb: the superblock structure. This is partially
initialised by the VFS and the rest must be initialised by the
read_super() method
void *data: arbitrary mount options, usually comes as an ASCII
string
int silent: whether or not to be silent on error
The read_super() method must determine if the block device specified
in the superblock contains a filesystem of the type the method
supports. On success the method returns the superblock pointer, on
failure it returns NULL.
The most interesting member of the superblock structure that the
read_super() method fills in is the "s_op" field. This is a pointer to
a "struct super_operations" which describes the next level of the
filesystem implementation.
struct super_operations
=======================
This describes how the VFS can manipulate the superblock of your
filesystem. As of kernel 2.1.99, the following members are defined:
struct super_operations {
void (*read_inode) (struct inode *);
void (*write_inode) (struct inode *);
void (*put_inode) (struct inode *);
void (*delete_inode) (struct inode *);
int (*notify_change) (struct dentry *, struct iattr *);
void (*put_super) (struct super_block *);
void (*write_super) (struct super_block *);
int (*statfs) (struct super_block *, struct statfs *, int);
int (*remount_fs) (struct super_block *, int *, char *);
void (*clear_inode) (struct inode *);
};
All methods are called without any locks being held, unless otherwise
noted. This means that most methods can block safely. All methods are
only called from a process context (i.e. not from an interrupt handler
or bottom half).
read_inode: this method is called to read a specific inode from the
mounted filesystem. The "i_ino" member in the "struct inode"
will be initialised by the VFS to indicate which inode to
read. Other members are filled in by this method
write_inode: this method is called when the VFS needs to write an
inode to disc
put_inode: called when the VFS inode is removed from the inode
cache. This method is optional
delete_inode: called when the VFS wants to delete an inode
notify_change: called when VFS inode attributes are changed. If this
is NULL the VFS falls back to the write_inode() method. This
is called with the kernel lock held
put_super: called when the VFS wishes to free the superblock
(i.e. unmount). This is called with the superblock lock held
write_super: called when the VFS superblock needs to be written to
disc. This method is optional
statfs: called when t
