samples/Makefile


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# Makefile for Linux samples code

obj-$(CONFIG_SAMPLES)	+= kobject/ kprobes/ trace_events/ \
			   hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/ seccomp/

	      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 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 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 device onto a directory in your filespace,
the VFS will call the appropriate get_sb() 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.

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 2.6.22, the following
members are defined:

struct file_system_type {
	const char *name;
	int fs_flags;
        int (*get_sb) (struct file_system_type *, int,
                       const char *, void *, struct vfsmount *);
        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.)

  get_sb: 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 unmounted

  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 get_sb() method has the following arguments:

  struct file_system_type *fs_type: decribes 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

  struct vfsmount *mnt: a vfs-internal representation of a mount point

The get_sb() method must determine if the block device specified
in the dev_name and fs_type contains a filesystem of the type the method
supports. If it succeeds in opening the named block device, it initializes a
struct super_block descriptor for the filesystem contained by the block device.
On failure it returns an error.

The most interesting member of the superblock structure that the
get_sb() 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 get_sb() implementations
and provides a fill_super() method instead. The generic methods are:

  get_sb_bdev: mount a filesystem residing on a block device

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

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

A fill_super() method implementation has the following arguments:

  struct super_block *sb: the superblock structure. The method fill_super()
  	must initialize this properly.

  void *data: arbitrary mount options, usually comes as an ASCII
	string

  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 (*read_inode) (struct inode *);

        void (*dirty_inode) (struct inode *);
        int (*write_inode) (struct inode *, int);
        void (*put_inode) (struct inode *);
        void (*drop_inode) (struct inode *);
        void (*delete_inode) (struct inode *);
        void (*put_super) (struct super_block *);
        void (*write_super) (struct super_block *);
        int (*sync_fs)(struct super_block *sb, int wait);
        void (*write_super_lockfs) (struct super_block *);
        void (*unlockfs) (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 vfsmount *);

        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);
};

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.

  read_inode: this method is called to read a specific inode from the
        mounted filesystem.  The i_ino member in the struct inode is
	initialized by the VFS to indicate which inode to read. Other
	members are filled in by this method.

	You can set this to NULL and use iget5_locked() instead of iget()
	to read inodes.  This is necessary for filesystems for which the
	inode number is not sufficient to identify an 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.

  put_inode: called when the VFS inode is removed from the inode
	cache.

  drop_inode: called when the last access to the inode is dropped,
	with the inode_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

  write_super: called when the VFS superblock needs to be written to
	disc. This method is optional

  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.

  write_super_lockfs: 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).

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

  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

  umount_begin: called when the VFS is unmounting a filesystem.

  show_options: called by the VFS to show mount options for /proc/<pid>/mounts.

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

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

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.


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 *,int, struct nameidata *);
	struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameidata *);
	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,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 *);
	void (*truncate) (struct inode *);
	int (*permission) (struct inode *, int, struct nameidata *);
	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 (*truncate_range)(struct inode *, loff_t, loff_t);
};

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).

  truncate: called by the VFS to change the size of a file.  The
 	i_size field of the inode is set to the desired size by the
 	VFS before this method is called.  This method is called by
 	the truncate(2) system call and related functionality.

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

  setattr: called by the VFS to set attributes for a file. This method
  	is called by chmod(2) and related system calls.

  getattr: called by the VFS to get attributes of a file. This method
  	is called by stat(2) and related system calls.

  setxattr: called by the VFS to set an extended attribute for a file.
  	Extended attribute is a name:value pair associated with an
  	inode. This method is called by setxattr(2) system call.

  getxattr: called by the VFS to retrieve the value of an extended
  	attribute name. This method is called by getxattr(2) function
  	call.

  listxattr: called by the VFS to list all extended attributes for a
  	given file. This method is called by listxattr(2) system call.

  removexattr: called by the VFS to remove an extended attribute from
  	a file. This method is called by removexattr(2) system call.

  truncate_range: a method provided by the underlying filesystem to truncate a
  	range of blocks , i.e. punch a hole somewhere in a file.


The Address Space Object
========================

The address space object is used to group and manage pages in the page
cache.  It can be used to keep track of the pages in a file (or
anything else) and also track the mapping of sections of the file into
process address spaces.

There are a number of distinct yet related services that an
address-space can provide.  These include communicating memory
pressure, page lookup by address, and keeping track of pages tagged as
Dirty or Writeback.

The first can be used independently to the others.  The VM can try to
either write dirty pages in order to clean them, or release clean
pages in order to reuse them.  To do this it can call the ->writepage
method on dirty pages, and ->releasepage on clean pages with
PagePrivate set. Clean pages without PagePrivate and with no external
references will be released without notice being given to the
address_space.

To achieve this functionality, pages need to be placed on an LRU with
lru_cache_add and mark_page_active needs to be called whenever the
page is used.

Pages are normally kept in a radix tree index by ->index. This tree
maintains information about the PG_Dirty and PG_Writeback status of
each page, so that pages with either of these flags can be found
quickly.

The Dirty tag is primarily used by mpage_writepages - the default
->writepages method.  It uses the tag to find dirty pages to call
->writepage on.  If mpage_writepages is not used (i.e. the address
provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
almost unused.  write_inode_now and sync_inode do use it (through
__sync_single_inode) to check if ->writepages has been successful in
writing out the whole address_space.

The Writeback tag is used by filemap*wait* and sync_page* functions,
via wait_on_page_writeback_range, to wait for all writeback to
complete.  While waiting ->sync_page (if defined) will be called on
each page that is found to require writeback.

An address_space handler may attach extra information to a page,
typically using the 'private' field in the 'struct page'.  If such
information is attached, the PG_Private flag should be set.  This will
cause various VM routines to make extra calls into the address_space
handler to deal with that data.

An address space acts as an intermediate between storage and
application.  Data is read into the address space a whole page at a
time, and provided to the application either by copying of the page,
or by memory-mapping the page.
Data is written into the address space by the application, and then
written-back to storage typically in whole pages, however the
address_space has finer control of write sizes.

The read process essentially only requires 'readpage'.  The write
process is more complicated and uses prepare_write/commit_write or
set_page_dirty to write data into the address_space, and writepage,
sync_page, and writepages to writeback data to storage.

Adding and removing pages to/from an address_space is protected by the
inode's i_mutex.

When data is written to a page, the PG_Dirty flag should be set.  It
typically remains set until writepage asks for it to be written.  This
should clear PG_Dirty and set PG_Writeback.  It can be actually
wri