path: root/Documentation/filesystems/vfs.txt

	      Overview of the Linux Virtual File System

	Original author: Richard Gooch <rgooch@atnf.csiro.au>

		  Last updated on June 24, 2007.

  Copyright (C) 1999 Richard Gooch
  Copyright (C) 2005 Pekka Enberg

  This file is released under the GPLv2.


Introduction
============

The Virtual File System (also 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
coexist.

VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so
on are called from a process context. Filesystem locking is described
in the document Documentation/filesystems/Locking.


Directory Entry Cache (dcache)
------------------------------

The VFS implements the open(2), stat(2), chmod(2), and similar system
calls. The pathname argument that is passed to them is used by the VFS
to search through the directory entry cache (also known as the dentry
cache or dcache). This provides a very fast look-up mechanism to
translate a pathname (filename) into a specific dentry. Dentries live
in RAM and are never saved to disc: they exist only for performance.

The dentry cache is meant to be a view into your entire filespace. As
most computers cannot fit all dentries in the RAM at the same time,
some bits of the cache are 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.


The Inode Object
----------------

An individual dentry usually has a pointer to an inode. Inodes are
filesystem objects such as regular files, directories, FIFOs and other
beasts.  They live either on the disc (for block device filesystems)
or in the memory (for pseudo filesystems). Inodes that live on the
disc are copied into the memory when required and changes to the inode
are written back to disc. A single inode can be pointed to by multiple
dentries (hard links, for example, do this).

To look up an inode 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. 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.


The File Object
---------------

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 initialized 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 its 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 to
do whatever is required. For as long as the file is open, it keeps the
dentry in use, which in turn means that the VFS inode is still in use.


Registering and Mounting a Filesystem
=====================================

To register and unregister a filesystem, use the following API
functions:

   #include <linux/fs.h>

   extern int register_filesystem(struct file_system_type *);
   extern int unregister_filesystem(struct file_system_type *);

The passed struct file_system_type describes your filesystem. When a
request is made to mount a filesystem onto a directory in your namespace,
the VFS will call the appropriate mount() method for the specific
filesystem.  New vfsmount referring to the tree returned by ->mount()
will be attached to the mountpoint, so that when pathname resolution
reaches the mountpoint it will jump into the root of that vfsmount.

You can see all filesystems that are registered to the kernel in the
file /proc/filesystems.


struct file_system_type
-----------------------

This describes the filesystem. As of kernel 2.6.39, the following
members are defined:

struct file_system_type {
	const char *name;
	int fs_flags;
        struct dentry *(*mount) (struct file_system_type *, int,
                       const char *, void *);
        void (*kill_sb) (struct super_block *);
        struct module *owner;
        struct file_system_type * next;
        struct list_head fs_supers;
	struct lock_class_key s_lock_key;
	struct lock_class_key s_umount_key;
};

  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.)

  mount: the method to call when a new instance of this
	filesystem should be mounted

  kill_sb: the method to call when an instance of this filesystem
	should be shut down

  owner: for internal VFS use: you should initialize this to THIS_MODULE in
  	most cases.

  next: for internal VFS use: you should initialize this to NULL

  s_lock_key, s_umount_key: lockdep-specific

The mount() method has the following arguments:

  struct file_system_type *fs_type: describes the filesystem, partly initialized
  	by the specific filesystem code

  int flags: mount flags

  const char *dev_name: the device name we are mounting.

  void *data: arbitrary mount options, usually comes as an ASCII
	string (see "Mount Options" section)

The mount() method must return the root dentry of the tree requested by
caller.  An active reference to its superblock must be grabbed and the
superblock must be locked.  On failure it should return ERR_PTR(error).

The arguments match those of mount(2) and their interpretation
depends on filesystem type.  E.g. for block filesystems, dev_name is
interpreted as block device name, that device is opened and if it
contains a suitable filesystem image the method creates and initializes
struct super_block accordingly, returning its root dentry to caller.

->mount() may choose to return a subtree of existing filesystem - it
doesn't have to create a new one.  The main result from the caller's
point of view is a reference to dentry at the root of (sub)tree to
be attached; creation of new superblock is a common side effect.

The most interesting member of the superblock structure that the
mount() 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.

Usually, a filesystem uses one of the generic mount() implementations
and provides a fill_super() callback instead. The generic variants are:

  mount_bdev: mount a filesystem residing on a block device

  mount_nodev: mount a filesystem that is not backed by a device

  mount_single: mount a filesystem which shares the instance between
  	all mounts

A fill_super() callback implementation has the following arguments:

  struct super_block *sb: the superblock structure. The callback
  	must initialize this properly.

  void *data: arbitrary mount options, usually comes as an ASCII
	string (see "Mount Options" section)

  int silent: whether or not to be silent on error


The Superblock Object
=====================

A superblock object represents a mounted filesystem.


struct super_operations
-----------------------

This describes how the VFS can manipulate the superblock of your
filesystem. As of kernel 2.6.22, the following members are defined:

struct super_operations {
        struct inode *(*alloc_inode)(struct super_block *sb);
        void (*destroy_inode)(struct inode *);

        void (*dirty_inode) (struct inode *, int flags);
        int (*write_inode) (struct inode *, int);
        void (*drop_inode) (struct inode *);
        void (*delete_inode) (struct inode *);
        void (*put_super) (struct super_block *);
        int (*sync_fs)(struct super_block *sb, int wait);
        int (*freeze_fs) (struct super_block *);
        int (*unfreeze_fs) (struct super_block *);
        int (*statfs) (struct dentry *, struct kstatfs *);
        int (*remount_fs) (struct super_block *, int *, char *);
        void (*clear_inode) (struct inode *);
        void (*umount_begin) (struct super_block *);

        int (*show_options)(struct seq_file *, struct dentry *);

        ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
        ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
	int (*nr_cached_objects)(struct super_block *);
	void (*free_cached_objects)(struct super_block *, int);
};

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).

  alloc_inode: this method is called by inode_alloc() to allocate memory
 	for struct inode and initialize it.  If this function is not
 	defined, a simple 'struct inode' is allocated.  Normally
 	alloc_inode will be used to allocate a larger structure which
 	contains a 'struct inode' embedded within it.

  destroy_inode: this method is called by destroy_inode() to release
  	resources allocated for struct inode.  It is only required if
  	->alloc_inode was defined and simply undoes anything done by
	->alloc_inode.

  dirty_inode: this method is called by the VFS to mark an inode dirty.

  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.

  drop_inode: called when the last access to the inode is dropped,
	with the inode->i_lock spinlock held.

	This method should be either NULL (normal UNIX filesystem
	semantics) or "generic_delete_inode" (for filesystems that do not
	want to cache inodes - causing "delete_inode" to always be
	called regardless of the value of i_nlink)

	The "generic_delete_inode()" behavior is equivalent to the
	old practice of using "force_delete" in the put_inode() case,
	but does not have the races that the "force_delete()" approach
	had. 

  delete_inode: called when the VFS wants to delete an inode

  put_super: called when the VFS wishes to free the superblock
	(i.e. unmount). This is called with the superblock lock held

  sync_fs: called when VFS is writing out all dirty data associated with
  	a superblock. The second parameter indicates whether the method
	should wait until the write out has been completed. Optional.

  freeze_fs: called when VFS is locking a filesystem and
  	forcing it into a consistent state.  This method is currently
  	used by the Logical Volume Manager (LVM).

  unfreeze_fs: called when VFS is unlocking a filesystem and making it writable
  	again.

  statfs: called when the VFS needs to get filesystem statistics.

  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

  umount_begin: called when the VFS is unmounting a filesystem.

  show_options: called by the VFS to show mount options for
	/proc/<pid>/mounts.  (see "Mount Options" section)

  quota_read: called by the VFS to read from filesystem quota file.

  quota_write: called by the VFS to write to filesystem quota file.

  nr_cached_objects: called by the sb cache shrinking function for the
	filesystem to return the number of freeable cached objects it contains.
	Optional.

  free_cache_objects: called by the sb cache shrinking function for the
	filesystem to scan the number of objects indicated to try to free them.
	Optional, but any filesystem implementing this method needs to also
	implement ->nr_cached_objects for it to be called correctly.

	We can't do anything with any errors that the filesystem might
	encountered, hence the void return type. This will never be called if
	the VM is trying to reclaim under GFP_NOFS conditions, hence this
	method does not need to handle that situation itself.

	Implementations must include conditional reschedule calls inside any
	scanning loop that is done. This allows the VFS to determine
	appropriate scan batch sizes without having to worry about whether
	implementations will cause holdoff problems due to large scan batch
	sizes.

Whoever sets up the inode 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.


The Inode Object
================

An inode object represents an object within the filesystem.


struct inode_operations
-----------------------

This describes how the VFS can manipulate an inode in your
filesystem. As of kernel 2.6.22, the following members are defined:

struct inode_operations {
	int (*create) (struct inode *,struct dentry *, umode_t, bool);
	struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
	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 *,umode_t);
	int (*rmdir) (struct inode *,struct dentry *);
	int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
	int (*rename) (struct inode *, struct dentry *,
			struct inode *, struct dentry *);
	int (*readlink) (struct dentry *, char __user *,int);
        void * (*follow_link) (struct dentry *, struct nameidata *);
        void (*put_link) (struct dentry *, struct nameidata *, void *);
	int (*permission) (struct inode *, int);
	int (*get_acl)(struct inode *, int);
	int (*setattr) (struct dentry *, struct iattr *);
	int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);
	int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
	ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
	ssize_t (*listxattr) (struct dentry *, char *, size_t);
	int (*removexattr) (struct dentry *, const char *);
	void (*update_time)(struct inode *, struct timespec *, int);
	int (*atomic_open)(struct inode *, struct dentry *,
				struct file *, unsigned open_flag,
				umode_t create_mode, int *opened);
};

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

  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 look up 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

  rename: called by the rename(2) system call to rename the object to
	have the parent and name given by the second inode and dentry.

  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.  This method returns a void pointer cookie
	that is passed to put_link().

  put_link: called by the VFS to release resources allocated by
  	follow_link().  The cookie returned by follow_link() is passed
  	to this method as the last parameter.  It is used by
  	filesystems such as NFS where page cache is not stable
  	(i.e. page that was installed when the symbolic link walk
  	started might not be in the page cache at the end of the
  	walk).

  permission: called by the VFS to check for access rights on a POSIX-like
  	filesystem.

	May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
        mode, the filesystem must check the permission without blocking or
	storing to the inode.

	If