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Based on kernel version 3.2. Page generated on 2012-01-05 23:28 EST.

	
		      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 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);
		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 *,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);
		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>

• [ filesystems ]
• 00-INDEX
• 9p.txt
• adfs.txt
• affs.txt
• afs.txt
• autofs4-mount-control.txt
• automount-support.txt
• befs.txt
• bfs.txt
• btrfs.txt
• [ caching ]
• ceph.txt
• cifs.txt
• coda.txt
• [ configfs ]
• cramfs.txt
• debugfs.txt
• devpts.txt
• directory-locking
• dlmfs.txt
• dnotify.txt
• dnotify_test.c
• ecryptfs.txt
• exofs.txt
• ext2.txt
• ext3.txt
• ext4.txt
• fiemap.txt
• files.txt
• fuse.txt
• gfs2-glocks.txt
• gfs2-uevents.txt
• gfs2.txt
• hfs.txt
• hfsplus.txt
• hpfs.txt
• inotify.txt
• isofs.txt
• jfs.txt
• Locking
• locks.txt
• logfs.txt
• Makefile
• mandatory-locking.txt
• ncpfs.txt
• [ nfs ]
• nilfs2.txt
• ntfs.txt
• ocfs2.txt
• omfs.txt
• path-lookup.txt
• [ pohmelfs ]
• porting
• proc.txt
• quota.txt
• ramfs-rootfs-initramfs.txt
• relay.txt
• romfs.txt
• seq_file.txt
• sharedsubtree.txt
• spufs.txt
• squashfs.txt
• sysfs-pci.txt
• sysfs-tagging.txt
• sysfs.txt
• sysv-fs.txt
• tmpfs.txt
• ubifs.txt
• udf.txt
• ufs.txt
• vfat.txt
• vfs.txt
• xfs-delayed-logging-design.txt
• xfs.txt
• xip.txt
•