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/* -*- auto-fill -*-                                                         */

                Overview of the Virtual File System

                Richard Gooch <rgooch@atnf.csiro.au>

                              5-JUL-1999


Conventions used in this document                                     <section>
=================================

Each section in this document will have the string "<section>" at the
right-hand side of the section title. Each subsection will have
"<subsection>" 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:
http://www.atnf.csiro.au/~rgooch/linux/docs/vfs.txt


What is it?                                                           <section>
===========

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 programs. It also provides an
abstraction within the kernel which allows different filesystem
implementations to co-exist.


A Quick Look At How It Works                                          <section>
============================

In this section I'll briefly describe how things work, before
launching into the details. I'll start with describing what happens
when user programs open and manipulate files, and then look from the
other view which is how a filesystem is supported and subsequently
mounted.

Opening a File                                                     <subsection>
--------------

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                              <subsection>
-------------------------------------

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                                               <section>
=======================

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                                               <section>
=======================

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 *, int);
        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.  The second parameter indicates whether the write
        should be synchronous or not, not all filesystems check this flag.

  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 the VFS needs to get filesystem statistics. This
        is called with the kernel lock held

  remount_fs: called when the filesystem is remounted. This is called
        with the kernel lock held

  clear_inode: called then the VFS clears the inode. Optional

The read_inode() method is responsible for filling in the "i_op"
field. This is a pointer to a "struct inode_operations" which
describes the methods that can be performed on individual inodes.


struct inode_operations                                               <section>
=======================

This describes how the VFS can manipulate an inode in your
filesystem. As of kernel 2.1.99, the following members are defined:

struct inode_operations {
        struct file_operations * default_file_ops;
        int (*create) (struct inode *,struct dentry *,int);
        int (*lookup) (struct inode *,struct dentry *);
        int (*link) (struct dentry *,struct inode *,struct dentry *);
        int (*unlink) (struct inode *,struct dentry *);
        int (*symlink) (struct inode *,struct dentry *,const char *);
        int (*mkdir) (struct inode *,struct dentry *,int);
        int (*rmdir) (struct inode *,struct dentry *);
        int (*mknod) (struct inode *,struct dentry *,int,int);
        int (*rename) (struct inode *, struct dentry *,
                        struct inode *, struct dentry *);
        int (*readlink) (struct dentry *, char *,int);
        struct dentry * (*follow_link) (struct dentry *, struct dentry *);
        int (*readpage) (struct file *, struct page *);
        int (*writepage) (struct file *, struct page *);
        int (*bmap) (struct inode *,int);
        void (*truncate) (struct inode *);
        int (*permission) (struct inode *, int);
        int (*smap) (struct inode *,int);
        int (*updatepage) (struct file *, struct page *, const char *,
                                unsigned long, unsigned int, int);
        int (*revalidate) (struct dentry *);
};

Again, all methods are called without any locks being held, unless
otherwise noted.

  default_file_ops: this is a pointer to a "struct file_operations"
        which describes how to open and then manipulate open files

  create: called by the open(2) and creat(2) system calls. Only
        required if you want to support regular files. The dentry you
        get should not have an inode (i.e. it should be a negative
        dentry). Here you will probably call d_instantiate() with the
        dentry and the newly created inode

  lookup: called when the VFS needs to lookup an inode in a parent
        directory. The name to look for is found in the dentry. This
        method must call d_add() to insert the found inode into the
        dentry. The "i_count" field in the inode structure should be
        incremented. If the named inode does not exist a NULL inode
        should be inserted into the dentry (this is called a negative
        dentry). Returning an error code from this routine must only
        be done on a real error, otherwise creating inodes with system
        calls like create(2), mknod(2), mkdir(2) and so on will fail.
        If you wish to overload the dentry methods then you should
        initialise the "d_dop" field in the dentry; this is a pointer
        to a struct "dentry_operations".
        This method is called with the directory inode semaphore held

  link: called by the link(2) system call. Only required if you want
        to support hard links. You will probably need to call
        d_instantiate() just as you would in the create() method

  unlink: called by the unlink(2) system call. Only required if you
        want to support deleting inodes

  symlink: called by the symlink(2) system call. Only required if you
        want to support symlinks. You will probably need to call
        d_instantiate() just as you would in the create() method

  mkdir: called by the mkdir(2) system call. Only required if you want
        to support creating subdirectories. You will probably need to
        call d_instantiate() just as you would in the create() method

  rmdir: called by the rmdir(2) system call. Only required if you want
        to support deleting subdirectories

  mknod: called by the mknod(2) system call to create a device (char,
        block) inode or a named pipe (FIFO) or socket. Only required
        if you want to support creating these types of inodes. You
        will probably need to call d_instantiate() just as you would
        in the create() method

  readlink: called by the readlink(2) system call. Only required if
        you want to support reading symbolic links

  follow_link: called by the VFS to follow a symbolic link to the
        inode it points to. Only required if you want to support
        symbolic links


struct file_operations                                                <section>
======================

This describes how the VFS can manipulate an open file. As of kernel
2.1.99, the following members are defined:

struct file_operations {
        loff_t (*llseek) (struct file *, loff_t, int);
        ssize_t (*read) (struct file *, char *, size_t, loff_t *);
        ssize_t (*write) (struct file *, const char *, size_t, loff_t *);
        int (*readdir) (struct file *, void *, filldir_t);
        unsigned int (*poll) (struct file *, struct poll_table_struct *);
        int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
        int (*mmap) (struct file *, struct vm_area_struct *);
        int (*open) (struct inode *, struct file *);
        int (*release) (struct inode *, struct file *);
        int (*fsync) (struct file *, struct dentry *);
        int (*fasync) (struct file *, int);
        int (*check_media_change) (kdev_t dev);
        int (*revalidate) (kdev_t dev);
        int (*lock) (struct file *, int, struct file_lock *);
};

Again, all methods are called without any locks being held, unless
otherwise noted.

  llseek: called when the VFS needs to move the file position index

  read: called by read(2) and related system calls

  write: called by write(2) and related system calls

  readdir: called when the VFS needs to read the directory contents

  poll: called by the VFS when a process wants to check if there is
        activity on this file and (optionally) go to sleep until there
        is activity. Called by the select(2) and poll(2) system calls

  ioctl: called by the ioctl(2) system call

  mmap: called by the mmap(2) system call

  open: called by the VFS when an inode should be opened. When the VFS
        opens a file, it creates a new "struct file" and initialises
        the "f_op" file operations member with the "default_file_ops"
        field in the inode structure. It then calls the open method
        for the newly allocated file structure. You might think that
        the open method really belongs in "struct inode_operations",
        and you may be right. I think it's done the way it is because
        it makes filesystems simpler to implement. The open() method
        is a good place to initialise the "private_data" member in the
        file structure if you want to point to a device structure

  release: called when the last reference to an open file is closed

  fsync: called by the fsync(2) system call

  fasync: called by the fcntl(2) system call when asynchronous
        (non-blocking) mode is enabled for a file

Note that the file operations are implemented by the specific
filesystem in which the inode resides. When opening a device node
(character or block special) most filesystems will call special
support routines in the VFS which will locate the required device
driver information. These support routines replace the filesystem file
operations with those for the device driver, and then proceed to call
the new open() method for the file. This is how opening a device file
in the filesystem eventually ends up calling the device driver open()
method. Note the devfs (the Device FileSystem) has a more direct path
from device node to device driver (this is an unofficial kernel
patch).


struct dentry_operations                                              <section>
========================

This describes how a filesystem can overload the standard dentry
operations. Dentries and the dcache are the domain of the VFS and the
individual filesystem implementations. Device drivers have no business
here. These methods may be set to NULL, as they are either optional or
the VFS uses a default. As of kernel 2.1.99, the following members are
defined:

struct dentry_operations {
        int (*d_revalidate)(struct dentry *);
        int (*d_hash) (struct dentry *, struct qstr *);
        int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
        void (*d_delete)(struct dentry *);
        void (*d_release)(struct dentry *);
        void (*d_iput)(struct dentry *, struct inode *);
};

  d_revalidate: called when the VFS needs to revalidate a dentry. This
        is called whenever a name lookup finds a dentry in the
        dcache. Most filesystems leave this as NULL, because all their
        dentries in the dcache are valid

  d_hash: called when the VFS adds a dentry to the hash table

  d_compare: called when a dentry should be compared with another

  d_delete: called when the last reference to a dentry is
        deleted. This means no-one is using the dentry, however it is
        still valid and in the dcache

  d_release: called when a dentry is really deallocated

  d_iput: called when a dentry looses its inode (just prior to its
        being deallocated). The default when this is NULL is that the
        VFS calls iput(). If you define this method, you must call
        iput() yourself

Each dentry has a pointer to its parent dentry, as well as a hash list
of child dentries. Child dentries are basically like files in a
directory.

There are a number of functions defined which permit a filesystem to
manipulate dentries:

  dget: open a new handle for an existing dentry (this just increments
        the usage count)

  dput: close a handle for a dentry (decrements the usage count). If
        the usage count drops to 0, the "d_delete" method is called
        and the dentry is placed on the unused list if the dentry is
        still in its parents hash list. Putting the dentry on the
        unused list just means that if the system needs some RAM, it
        goes through the unused list of dentries and deallocates them.
        If the dentry has already been unhashed and the usage count
        drops to 0, in this case the dentry is deallocated after the
        "d_delete" method is called

  d_drop: this unhashes a dentry from its parents hash list. A
        subsequent call to dput() will dellocate the dentry if its
        usage count drops to 0

  d_delete: delete a dentry. If there are no other open references to
        the dentry then the dentry is turned into a negative dentry
        (the d_iput() method is called). If there are other
        references, then d_drop() is called instead

  d_add: add a dentry to its parents hash list and then calls
        d_instantiate()

  d_instantiate: add a dentry to the alias hash list for the inode and
        updates the "d_inode" member. The "i_count" member in the
        inode structure should be set/incremented. If the inode
        pointer is NULL, the dentry is called a "negative
        dentry". This function is commonly called when an inode is
        created for an existing negative dentry