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 *);
          void (*write_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
  
    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.

    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, 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 *,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 *);

  	void (*truncate) (struct inode *);

  	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 (*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: Deprecated. This will not be called if ->setsize is defined.
  	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.

  	Note: ->truncate and vmtruncate are deprecated. Do not add new
  	instances/calls of these. Filesystems should be converted to do their
  	truncate sequence via ->setattr().

    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 a situation is encountered that rcu-walk cannot handle, return
  	-ECHILD and it will be called again in ref-walk mode.

    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 filemap_fdatawait_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 write_begin/write_end 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
  written at any point after PG_Dirty is clear.  Once it is known to be
  safe, PG_Writeback is cleared.
  
  Writeback makes use of a writeback_control structure...

  
  struct address_space_operations

  -------------------------------

  
  This describes how the VFS can manipulate mapping of a file to page cache in

  your filesystem. As of kernel 2.6.22, the following members are defined:

  
  struct address_space_operations {
  	int (*writepage)(struct page *page, struct writeback_control *wbc);
  	int (*readpage)(struct file *, struct page *);
  	int (*sync_page)(struct page *);
  	int (*writepages)(struct address_space *, struct writeback_control *);
  	int (*set_page_dirty)(struct page *page);
  	int (*readpages)(struct file *filp, struct address_space *mapping,
  			struct list_head *pages, unsigned nr_pages);

  	int (*write_begin)(struct file *, struct address_space *mapping,
  				loff_t pos, unsigned len, unsigned flags,
  				struct page **pagep, void **fsdata);
  	int (*write_end)(struct file *, struct address_space *mapping,
  				loff_t pos, unsigned len, unsigned copied,
  				struct page *page, void *fsdata);

  	sector_t (*bmap)(struct address_space *, sector_t);
  	int (*invalidatepage) (struct page *, unsigned long);
  	int (*releasepage) (struct page *, int);

  	void (*freepage)(struct page *);

  	ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
  			loff_t offset, unsigned long nr_segs);
  	struct page* (*get_xip_page)(struct address_space *, sector_t,
  			int);

  	/* migrate the contents of a page to the specified target */
  	int (*migratepage) (struct page *, struct page *);

  	int (*launder_page) (struct page *);

  	int (*error_remove_page) (struct mapping *mapping, struct page *page);

  };

    writepage: called by the VM to write a dirty page to backing store.

        This may happen for data integrity reasons (i.e. 'sync'), or

        to free up memory (flush).  The difference can be seen in
        wbc->sync_mode.
        The PG_Dirty flag has been cleared and PageLocked is true.
        writepage should start writeout, should set PG_Writeback,
        and should make sure the page is unlocked, either synchronously
        or asynchronously when the write operation completes.
  
        If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to

        try too hard if there are problems, and may choose to write out
        other pages from the mapping if that is easier (e.g. due to
        internal dependencies).  If it chooses not to start writeout, it
        should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep

        calling ->writepage on that page.
  
        See the file "Locking" for more details.

  
    readpage: called by the VM to read a page from backing store.

         The page will be Locked when readpage is called, and should be
         unlocked and marked uptodate once the read completes.
         If ->readpage discovers that it needs to unlock the page for
         some reason, it can do so, and then return AOP_TRUNCATED_PAGE.

         In this case, the page will be relocated, relocked and if

         that all succeeds, ->readpage will be called again.

  
    sync_page: called by the VM to notify the backing store to perform all
    	queued I/O operations for a page. I/O operations for other pages
  	associated with this address_space object may also be performed.

  	This function is optional and is called only for pages with
    	PG_Writeback set while waiting for the writeback to complete.

    writepages: called by the VM to write out pages associated with the

    	address_space object.  If wbc->sync_mode is WBC_SYNC_ALL, then
    	the writeback_control will specify a range of pages that must be
    	written out.  If it is WBC_SYNC_NONE, then a nr_to_write is given

  	and that many pages should be written if possible.
  	If no ->writepages is given, then mpage_writepages is used

    	instead.  This will choose pages from the address space that are

    	tagged as DIRTY and will pass them to ->writepage.

  
    set_page_dirty: called by the VM to set a page dirty.

          This is particularly needed if an address space attaches
          private data to a page, and that data needs to be updated when
          a page is dirtied.  This is called, for example, when a memory
  	mapped page gets modified.
  	If defined, it should set the PageDirty flag, and the
          PAGECACHE_TAG_DIRTY tag in the radix tree.

  
    readpages: called by the VM to read pages associated with the address_space

    	object. This is essentially just a vector version of
    	readpage.  Instead of just one page, several pages are
    	requested.

  	readpages is only used for read-ahead, so read errors are

    	ignored.  If anything goes wrong, feel free to give up.

    write_begin:

  	Called by the generic buffered write code to ask the filesystem to
  	prepare to write len bytes at the given offset in the file. The
  	address_space should check that the write will be able to complete,
  	by allocating space if necessary and doing any other internal
  	housekeeping.  If the write will update parts of any basic-blocks on
  	storage, then those blocks should be pre-read (if they haven't been
  	read already) so that the updated blocks can be written out properly.
  
          The filesystem must return the locked pagecache page for the specified
  	offset, in *pagep, for the caller to write into.

  	It must be able to cope with short writes (where the length passed to
  	write_begin is greater than the number of bytes copied into the page).

  	flags is a field for AOP_FLAG_xxx flags, described in
  	include/linux/fs.h.
  
          A void * may be returned in fsdata, which then gets passed into
          write_end.
  
          Returns 0 on success; < 0 on failure (which is the error code), in
  	which case write_end is not called.
  
    write_end: After a successful write_begin, and data copy, write_end must
          be called. len is the original len passed to write_begin, and copied
          is the amount that was able to be copied (copied == len is always true
  	if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag).
  
          The filesystem must take care of unlocking the page and releasing it
          refcount, and updating i_size.
  
          Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
          that were able to be copied into pagecache.

    bmap: called by the VFS to map a logical block offset within object to

    	physical block number. This method is used by the FIBMAP

    	ioctl and for working with swap-files.  To be able to swap to

    	a file, the file must have a stable mapping to a block

    	device.  The swap system does not go through the filesystem
    	but instead uses bmap to find out where the blocks in the file
    	are and uses those addresses directly.
  
  
    invalidatepage: If a page has PagePrivate set, then invalidatepage
          will be called when part or all of the page is to be removed

  	from the address space.  This generally corresponds to either a

  	truncation or a complete invalidation of the address space
  	(in the latter case 'offset' will always be 0).
  	Any private data associated with the page should be updated
  	to reflect this truncation.  If offset is 0, then
  	the private data should be released, because the page
  	must be able to be completely discarded.  This may be done by
          calling the ->releasepage function, but in this case the
          release MUST succeed.
  
    releasepage: releasepage is called on PagePrivate pages to indicate
          that the page should be freed if possible.  ->releasepage
          should remove any private data from the page and clear the

          PagePrivate flag. If releasepage() fails for some reason, it must
  	indicate failure with a 0 return value.
  	releasepage() is used in two distinct though related cases.  The
  	first is when the VM finds a clean page with no active users and

          wants to make it a free page.  If ->releasepage succeeds, the
          page will be removed from the address_space and become free.

  	The second case is when a request has been made to invalidate

          some or all pages in an address_space.  This can happen
          through the fadvice(POSIX_FADV_DONTNEED) system call or by the
          filesystem explicitly requesting it as nfs and 9fs do (when
          they believe the cache may be out of date with storage) by
          calling invalidate_inode_pages2().
  	If the filesystem makes such a call, and needs to be certain

          that all pages are invalidated, then its releasepage will

          need to ensure this.  Possibly it can clear the PageUptodate
          bit if it cannot free private data yet.

    freepage: freepage is called once the page is no longer visible in
          the page cache in order to allow the cleanup of any private
  	data. Since it may be called by the memory reclaimer, it
  	should not assume that the original address_space mapping still
  	exists, and it should not block.

    direct_IO: called by the generic read/write routines to perform
          direct_IO - that is IO requests which bypass the page cache

          and transfer data directly between the storage and the

          application's address space.

  
    get_xip_page: called by the VM to translate a block number to a page.
  	The page is valid until the corresponding filesystem is unmounted.
  	Filesystems that want to use execute-in-place (XIP) need to implement
  	it.  An example implementation can be found in fs/ext2/xip.c.

    migrate_page:  This is used to compact the physical memory usage.
          If the VM wants to relocate a page (maybe off a memory card
          that is signalling imminent failure) it will pass a new page
  	and an old page to this function.  migrate_page should
  	transfer any private data across and update any references
          that it has to the page.

    launder_page: Called before freeing a page - it writes back the dirty page. To
    	prevent redirtying the page, it is kept locked during the whole
  	operation.

    error_remove_page: normally set to generic_error_remove_page if truncation
  	is ok for this address space. Used for memory failure handling.
  	Setting this implies you deal with pages going away under you,
  	unless you have them locked or reference counts increased.

  The File Object
  ===============
  
  A file object represents a file opened by a process.

  struct file_operations

  ----------------------

  
  This describes how the VFS can manipulate an open file. As of kernel

  2.6.22, the following members are defined:

  
  struct file_operations {

  	struct module *owner;

  	loff_t (*llseek) (struct file *, loff_t, int);

  	ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);

  	ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);

  	ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
  	ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);

  	int (*readdir) (struct file *, void *, filldir_t);
  	unsigned int (*poll) (struct file *, struct poll_table_struct *);

  	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
  	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);

  	int (*mmap) (struct file *, struct vm_area_struct *);
  	int (*open) (struct inode *, struct file *);

  	int (*flush) (struct file *);

  	int (*release) (struct inode *, struct file *);

  	int (*fsync) (struct file *, loff_t, loff_t, int datasync);

  	int (*aio_fsync) (struct kiocb *, int datasync);
  	int (*fasync) (int, struct file *, int);

  	int (*lock) (struct file *, int, struct file_lock *);

  	ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *);
  	ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *);
  	ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *);
  	ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
  	unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
  	int (*check_flags)(int);

  	int (*flock) (struct file *, int, struct file_lock *);

  	ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned int);
  	ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int);

  };
  
  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

    aio_read: called by io_submit(2) and other asynchronous I/O operations

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

    aio_write: called by io_submit(2) and other asynchronous I/O operations

    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

    unlocked_ioctl: called by the ioctl(2) system call.

  
    compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
   	 are used on 64 bit kernels.

    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". 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 initialize the
  	"private_data" member in the file structure if you want to point
  	to a device structure
  
    flush: called by the close(2) system call to flush a file

  
    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

    lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
    	commands
  
    readv: called by the readv(2) system call
  
    writev: called by the writev(2) system call
  
    sendfile: called by the sendfile(2) system call
  
    get_unmapped_area: called by the mmap(2) system call
  
    check_flags: called by the fcntl(2) system call for F_SETFL command

    flock: called by the flock(2) system call

    splice_write: called by the VFS to splice data from a pipe to a file. This
  		method is used by the splice(2) system call
  
    splice_read: called by the VFS to splice data from file to a pipe. This
  	       method is used by the splice(2) system call

  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.

  Directory Entry Cache (dcache)
  ==============================

  
  struct dentry_operations

  ------------------------

  
  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 2.6.22, the following members are

  defined:
  
  struct dentry_operations {

  	int (*d_revalidate)(struct dentry *, struct nameidata *);

  	int (*d_hash)(const struct dentry *, const struct inode *,
  			struct qstr *);

  	int (*d_compare)(const struct dentry *, const struct inode *,
  			const struct dentry *, const struct inode *,
  			unsigned int, const char *, const struct qstr *);

  	int (*d_delete)(const struct dentry *);

  	void (*d_release)(struct dentry *);
  	void (*d_iput)(struct dentry *, struct inode *);

  	char *(*d_dname)(struct dentry *, char *, int);

  	struct vfsmount *(*d_automount)(struct path *);

  	int (*d_manage)(struct dentry *, bool);

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

  	d_revalidate may be called in rcu-walk mode (nd->flags & LOOKUP_RCU).
  	If in rcu-walk mode, the filesystem must revalidate the dentry without
  	blocking or storing to the dentry, d_parent and d_inode should not be
  	used without care (because they can go NULL), instead nd->inode should
  	be used.
  
  	If a situation is encountered that rcu-walk cannot handle, return
  	-ECHILD and it will be called again in ref-walk mode.

    d_hash: called when the VFS adds a dentry to the hash table. The first
  	dentry passed to d_hash is the parent directory that the name is

  	to be hashed into. The inode is the dentry's inode.
  
  	Same locking and synchronisation rules as d_compare regarding
  	what is safe to dereference etc.

    d_compare: called to compare a dentry name with a given name. The first
  	dentry is the parent of the dentry to be compared, the second is
  	the parent's inode, then the dentry and inode (may be NULL) of the
  	child dentry. len and name string are properties of the dentry to be
  	compared. qstr is the name to compare it with.
  
  	Must be constant and idempotent, and should not take locks if
  	possible, and should not or store into the dentry or inodes.
  	Should not dereference pointers outside the dentry or inodes without
  	lots of care (eg.  d_parent, d_inode, d_name should not be used).
  
  	However, our vfsmount is pinned, and RCU held, so the dentries and
  	inodes won't disappear, neither will our sb or filesystem module.
  	->i_sb and ->d_sb may be used.
  
  	It is a tricky calling convention because it needs to be called under
  	"rcu-walk", ie. without any locks or references on things.

    d_delete: called when the last reference to a dentry is dropped and the
  	dcache is deciding whether or not to cache it. Return 1 to delete
  	immediately, or 0 to cache the dentry. Default is NULL which means to
  	always cache a reachable dentry. d_delete must be constant and
  	idempotent.

  
    d_release: called when a dentry is really deallocated
  
    d_iput: called when a dentry loses 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

    d_dname: called when the pathname of a dentry should be generated.

  	Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay

  	pathname generation. (Instead of doing it when dentry is created,

  	it's done only when the path is needed.). Real filesystems probably

  	dont want to use it, because their dentries are present in global
  	dcache hash, so their hash should be an invariant. As no lock is
  	held, d_dname() should not try to modify the dentry itself, unless
  	appropriate SMP safety is used. CAUTION : d_path() logic is quite
  	tricky. The correct way to return for example "Hello" is to put it
  	at the end of the buffer, and returns a pointer to the first char.
  	dynamic_dname() helper function is provided to take care of this.

    d_automount: called when an automount dentry is to be traversed (optional).

  	This should create a new VFS mount record and return the record to the
  	caller.  The caller is supplied with a path parameter giving the
  	automount directory to describe the automount target and the parent
  	VFS mount record to provide inheritable mount parameters.  NULL should
  	be returned if someone else managed to make the automount first.  If
  	the vfsmount creation failed, then an error code should be returned.
  	If -EISDIR is returned, then the directory will be treated as an
  	ordinary directory and returned to pathwalk to continue walking.
  
  	If a vfsmount is returned, the caller will attempt to mount it on the
  	mountpoint and will remove the vfsmount from its expiration list in
  	the case of failure.  The vfsmount should be returned with 2 refs on
  	it to prevent automatic expiration - the caller will clean up the
  	additional ref.

  
  	This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
  	dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is set on the
  	inode being added.

    d_manage: called to allow the filesystem to manage the transition from a
  	dentry (optional).  This allows autofs, for example, to hold up clients
  	waiting to explore behind a 'mountpoint' whilst letting the daemon go
  	past and construct the subtree there.  0 should be returned to let the
  	calling process continue.  -EISDIR can be returned to tell pathwalk to
  	use this directory as an ordinary directory and to ignore anything
  	mounted on it and not to check the automount flag.  Any other error
  	code will abort pathwalk completely.

  	If the 'rcu_walk' parameter is true, then the caller is doing a
  	pathwalk in RCU-walk mode.  Sleeping is not permitted in this mode,
  	and the caller can be asked to leave it and call again by returing
  	-ECHILD.

  	This function is only used if DCACHE_MANAGE_TRANSIT is set on the
  	dentry being transited from.

  Example :
  
  static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
  {
  	return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
  				dentry->d_inode->i_ino);
  }

  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.

  Directory Entry Cache API

  --------------------------
  
  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, and the dentry is still in its
  	parent's hash, the "d_delete" method is called to check whether
  	it should be cached. If it should not be cached, or if the dentry
  	is not hashed, it is deleted. Otherwise cached dentries are put
  	into an LRU list to be reclaimed on memory shortage.

  
    d_drop: this unhashes a dentry from its parents hash list. A

  	subsequent call to dput() will deallocate 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
  
    d_lookup: look up a dentry given its parent and path name component
  	It looks up the child of that given name from the dcache
  	hash table. If it is found, the reference count is incremented

  	and the dentry is returned. The caller must use dput()

  	to free the dentry when it finishes using it.

  Mount Options
  =============
  
  Parsing options
  ---------------
  
  On mount and remount the filesystem is passed a string containing a
  comma separated list of mount options.  The options can have either of
  these forms:
  
    option
    option=value
  
  The <linux/parser.h> header defines an API that helps parse these
  options.  There are plenty of examples on how to use it in existing
  filesystems.
  
  Showing options
  ---------------
  
  If a filesystem accepts mount options, it must define show_options()
  to show all the currently active options.  The rules are:
  
    - options MUST be shown which are not default or their values differ
      from the default
  
    - options MAY be shown which are enabled by default or have their
      default value
  
  Options used only internally between a mount helper and the kernel
  (such as file descriptors), or which only have an effect during the
  mounting (such as ones controlling the creation of a journal) are exempt
  from the above rules.
  
  The underlying reason for the above rules is to make sure, that a
  mount can be accurately replicated (e.g. umounting and mounting again)
  based on the information found in /proc/mounts.
  
  A simple method of saving options at mount/remount time and showing
  them is provided with the save_mount_options() and
  generic_show_options() helper functions.  Please note, that using
  these may have drawbacks.  For more info see header comments for these
  functions in fs/namespace.c.

  
  Resources
  =========
  
  (Note some of these resources are not up-to-date with the latest kernel
   version.)
  
  Creating Linux virtual filesystems. 2002
      <http://lwn.net/Articles/13325/>
  
  The Linux Virtual File-system Layer by Neil Brown. 1999
      <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
  
  A tour of the Linux VFS by Michael K. Johnson. 1996
      <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
  
  A small trail through the Linux kernel by Andries Brouwer. 2001
      <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>