xref: /linux/Documentation/filesystems/vfs.rst (revision 3a2c4d55e32ad65efebdb6de44eef3bfa08bb49d)

.. SPDX-License-Identifier: GPL-2.0

=========================================
Overview of the Linux Virtual File System
=========================================

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

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


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.rst.


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:

.. code-block:: c

	#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 get_tree() method for the
specific filesystem.  See Documentation/filesystems/mount_api.rst
for more details.

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


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

This describes the filesystem.  The following
members are defined:

.. code-block:: c

	struct file_system_type {
		const char *name;
		int fs_flags;
		int (*init_fs_context)(struct fs_context *);
		const struct fs_parameter_spec *parameters;
		void (*kill_sb) (struct super_block *);
		struct module *owner;
		struct file_system_type * next;
		struct hlist_head fs_supers;

		struct lock_class_key s_lock_key;
		struct lock_class_key s_umount_key;
		struct lock_class_key s_vfs_rename_key;
		struct lock_class_key s_writers_key[SB_FREEZE_LEVELS];

		struct lock_class_key i_lock_key;
		struct lock_class_key i_mutex_key;
		struct lock_class_key invalidate_lock_key;
		struct lock_class_key i_mutex_dir_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.)

``init_fs_context``
	Initializes 'struct fs_context' ->ops and ->fs_private fields with
	filesystem-specific data.

``parameters``
	Pointer to the array of filesystem parameters descriptors
	'struct fs_parameter_spec'.
	More info in Documentation/filesystems/mount_api.rst.

``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

``fs_supers``
	for internal VFS use: hlist of filesystem instances (superblocks)

  s_lock_key, s_umount_key, s_vfs_rename_key, s_writers_key,
  i_lock_key, i_mutex_key, invalidate_lock_key, i_mutex_dir_key: lockdep-specific

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.  The following members are defined:

.. code-block:: c

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

		void (*dirty_inode) (struct inode *, int flags);
		int (*write_inode) (struct inode *, struct writeback_control *wbc);
		int (*drop_inode) (struct inode *);
		void (*evict_inode) (struct inode *);
		void (*put_super) (struct super_block *);
		int (*sync_fs)(struct super_block *sb, int wait);
		int (*freeze_super) (struct super_block *sb,
					enum freeze_holder who);
		int (*freeze_fs) (struct super_block *);
		int (*thaw_super) (struct super_block *sb,
					enum freeze_wholder who);
		int (*unfreeze_fs) (struct super_block *);
		int (*statfs) (struct dentry *, struct kstatfs *);
		void (*umount_begin) (struct super_block *);

		int (*show_options)(struct seq_file *, struct dentry *);
		int (*show_devname)(struct seq_file *, struct dentry *);
		int (*show_path)(struct seq_file *, struct dentry *);
		int (*show_stats)(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);
		struct dquot **(*get_dquots)(struct inode *);

		long (*nr_cached_objects)(struct super_block *,
					struct shrink_control *);
		long (*free_cached_objects)(struct super_block *,
					struct shrink_control *);
	};

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 alloc_inode() 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.

``free_inode``
	this method is called from RCU callback. If you use call_rcu()
	in ->destroy_inode to free 'struct inode' memory, then it's
	better to release memory in this method.

``dirty_inode``
	this method is called by the VFS when an inode is marked dirty.
	This is specifically for the inode itself being marked dirty,
	not its data.  If the update needs to be persisted by fdatasync(),
	then I_DIRTY_DATASYNC will be set in the flags argument.
	I_DIRTY_TIME will be set in the flags in case lazytime is enabled
	and struct inode has times updated since the last ->dirty_inode
	call.

``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 "inode_just_drop" (for filesystems that do
	not want to cache inodes - causing "delete_inode" to always be
	called regardless of the value of i_nlink)

	The "inode_just_drop()" 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.

``evict_inode``
	called when the VFS wants to evict an inode. Caller does
	*not* evict the pagecache or inode-associated metadata buffers;
	the method has to use truncate_inode_pages_final() to get rid
	of those. Caller makes sure async writeback cannot be running for
	the inode while (or after) ->evict_inode() is called. Optional.

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

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

``freeze_super``
	Called instead of ->freeze_fs callback if provided.
	Main difference is that ->freeze_super is called without taking
	down_write(&sb->s_umount). If filesystem implements it and wants
	->freeze_fs to be called too, then it has to call ->freeze_fs
	explicitly from this callback. 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) and ioctl(FIFREEZE). Optional.

``thaw_super``
	called when VFS is unlocking a filesystem and making it writable
	again after ->freeze_super. Optional.

``unfreeze_fs``
	called when VFS is unlocking a filesystem and making it writable
	again after ->freeze_fs. Optional.

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

``umount_begin``
	called when the VFS is unmounting a filesystem.

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

``show_devname``
	Optional. Called by the VFS to show device name for
	/proc/<pid>/{mounts,mountinfo,mountstats}. If not provided then
	'(struct mount).mnt_devname' will be used.

``show_path``
	Optional. Called by the VFS (for /proc/<pid>/mountinfo) to show
	the mount root dentry path relative to the filesystem root.

``show_stats``
	Optional. Called by the VFS (for /proc/<pid>/mountstats) to show
	filesystem-specific mount statistics.

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

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

``get_dquots``
	called by quota to get 'struct dquot' array for a particular inode.
	Optional.

``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.


struct xattr_handler
---------------------

On filesystems that support extended attributes (xattrs), the s_xattr
superblock field points to a NULL-terminated array of xattr handlers.
Extended attributes are name:value pairs.

``name``
	Indicates that the handler matches attributes with the specified
	name (such as "system.posix_acl_access"); the prefix field must
	be NULL.

``prefix``
	Indicates that the handler matches all attributes with the
	specified name prefix (such as "user."); the name field must be
	NULL.

``list``
	Determine if attributes matching this xattr handler should be
	listed for a particular dentry.  Used by some listxattr
	implementations like generic_listxattr.

``get``
	Called by the VFS to get the value of a particular extended
	attribute.  This method is called by the getxattr(2) system
	call.

``set``
	Called by the VFS to set the value of a particular extended
	attribute.  When the new value is NULL, called to remove a
	particular extended attribute.  This method is called by the
	setxattr(2) and removexattr(2) system calls.

When none of the xattr handlers of a filesystem match the specified
attribute name or when a filesystem doesn't support extended attributes,
the various ``*xattr(2)`` system calls return -EOPNOTSUPP.


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:

.. code-block:: c

	struct inode_operations {
		int (*create) (struct mnt_idmap *, struct inode *,struct dentry *, umode_t);
		struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
		int (*link) (struct dentry *,struct inode *,struct dentry *);
		int (*unlink) (struct inode *,struct dentry *);
		int (*symlink) (struct mnt_idmap *, struct inode *,struct dentry *,const char *);
		struct dentry *(*mkdir) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t);
		int (*rmdir) (struct inode *,struct dentry *);
		int (*mknod) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t,dev_t);
		int (*rename) (struct mnt_idmap *, struct inode *, struct dentry *,
			       struct inode *, struct dentry *, unsigned int);
		int (*readlink) (struct dentry *, char __user *,int);
		const char *(*get_link) (struct dentry *, struct inode *,
					 struct delayed_call *);
		int (*permission) (struct mnt_idmap *, struct inode *, int);
		struct posix_acl * (*get_inode_acl)(struct inode *, int, bool);
		int (*setattr) (struct mnt_idmap *, struct dentry *, struct iattr *);
		int (*getattr) (struct mnt_idmap *, const struct path *, struct kstat *, u32, unsigned int);
		ssize_t (*listxattr) (struct dentry *, char *, size_t);
		void (*update_time)(struct inode *inode, enum fs_update_time type,
				    int flags);
		void (*sync_lazytime)(struct inode *inode);
		int (*atomic_open)(struct inode *, struct dentry *, struct file *,
				   unsigned open_flag, umode_t create_mode);
		int (*tmpfile) (struct mnt_idmap *, struct inode *, struct file *, umode_t);
		struct posix_acl * (*get_acl)(struct mnt_idmap *, struct dentry *, int);
	        int (*set_acl)(struct mnt_idmap *, struct dentry *, struct posix_acl *, int);
		int (*fileattr_set)(struct mnt_idmap *idmap,
				    struct dentry *dentry, struct file_kattr *fa);
		int (*fileattr_get)(struct dentry *dentry, struct file_kattr *fa);
	        struct offset_ctx *(*get_offset_ctx)(struct inode *inode);
	};

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_new() just as you would in the create() method.

	If d_instantiate_new() is not used and if the fh_to_dentry()
	export operation is provided, or if the storage might be
	accessible by another path (e.g. with a network filesystem)
	then more care may be needed.  Importantly d_instantate()
	should not be used with an inode that is no longer I_NEW if there
	any chance that the inode could already be attached to a dentry.
	This is because of a hard rule in the VFS that a directory must
	only ever have one dentry.

	For example, if an NFS filesystem is mounted twice the new directory
	could be visible on the other mount before it is on the original
	mount, and a pair of name_to_handle_at(), open_by_handle_at()
	calls could instantiate the directory inode with an IS_ROOT()
	dentry before the first mkdir returns.

	If there is any chance this could happen, then the new inode
	should be d_drop()ed and attached with d_splice_alias().  The
	returned dentry (if any) should be returned by ->mkdir().

``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.

	The filesystem must return -EINVAL for any unsupported or
	unknown flags.  Currently the following flags are implemented:
	(1) RENAME_NOREPLACE: this flag indicates that if the target of
	the rename exists the rename should fail with -EEXIST instead of
	replacing the target.  The VFS already checks for existence, so
	for local filesystems the RENAME_NOREPLACE implementation is
	equivalent to plain rename.
	(2) RENAME_EXCHANGE: exchange source and target.  Both must
	exist; this is checked by the VFS.  Unlike plain rename, source
	and target may be of different type.

``get_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 the symlink body to traverse (and possibly
	resets the current position with nd_jump_link()).  If the body
	won't go away until the inode is gone, nothing else is needed;
	if it needs to be otherwise pinned, arrange for its release by
	having get_link(..., ..., done) do set_delayed_call(done,
	destructor, argument).  In that case destructor(argument) will
	be called once VFS is done with the body you've returned.  May
	be called in RCU mode; that is indicated by NULL dentry
	argument.  If request can't be handled without leaving RCU mode,
	have it return ERR_PTR(-ECHILD).

	If the filesystem stores the symlink target in ->i_link, the
	VFS may use it directly without calling ->get_link(); however,
	->get_link() must still be provided.  ->i_link must not be
	freed until after an RCU grace period.  Writing to ->i_link
	post-iget() time requires a 'release' memory barrier.

``readlink``
	this is now just an override for use by readlink(2) for the
	cases when ->get_link uses nd_jump_link() or object is not in
	fact a symlink.  Normally filesystems should only implement
	->get_link for symlinks and readlink(2) will automatically use
	that.

``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.

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

``update_time``
	called by the VFS to update a specific time or the i_version of
	an inode.  If this is not defined the VFS will update the inode
	itself and call mark_inode_dirty_sync.

``sync_lazytime``:
	called by the writeback code to update the lazy time stamps to
	regular time stamp updates that get syncing into the on-disk
	inode.

``atomic_open``
	called on the last component of an open.  Using this optional
	method the filesystem can look up, possibly create and open the
	file in one atomic operation.  If it wants to leave actual
	opening to the caller (e.g. if the file turned out to be a
	symlink, device, or just something filesystem won't do atomic
	open for), it may signal this by returning finish_no_open(file,
	dentry).  This method is only called if the last component is
	negative or needs lookup.  Cached positive dentries are still
	handled by f_op->open().  If the file was created, FMODE_CREATED
	flag should be set in file->f_mode.  In case of O_EXCL the
	method must only succeed if the file didn't exist and hence
	FMODE_CREATED shall always be set on success.

``tmpfile``
	called in the end of O_TMPFILE open().  Optional, equivalent to
	atomically creating, opening and unlinking a file in given
	directory.  On success needs to return with the file already
	open; this can be done by calling finish_open_simple() right at
	the end.

``fileattr_get``
	called on ioctl(FS_IOC_GETFLAGS) and ioctl(FS_IOC_FSGETXATTR) to
	retrieve miscellaneous file flags and attributes.  Also called
	before the relevant SET operation to check what is being changed
	(in this case with i_rwsem locked exclusive).  If unset, then
	fall back to f_op->ioctl().

``fileattr_set``
	called on ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_FSSETXATTR) to
	change miscellaneous file flags and attributes.  Callers hold
	i_rwsem exclusive.  If unset, then fall back to f_op->ioctl().
``get_offset_ctx``
	called to get the offset context for a directory inode. A
        filesystem must define this operation to use
        simple_offset_dir_operations.

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
release clean pages in order to reuse them.  To do this it can call
->release_folio on clean folios with the private
flag 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
write back.  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.

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 'read_folio'.  The write
process is more complicated and uses write_begin/write_end or
dirty_folio to write data into the address_space, and
writepages to writeback data to storage.

Removing pages from an address_space requires holding the inode's i_rwsem
exclusively, while adding pages to the address_space requires holding the
inode's i_mapping->invalidate_lock exclusively.

When data is written to a page, the PG_Dirty flag should be set.  It
typically remains set until writepages 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 to direct the
operations.  This gives the writepages operation some
information about the nature of and reason for the writeback request,
and the constraints under which it is being done.  It is also used to
return information back to the caller about the result of a
writepages request.


Handling errors during writeback
--------------------------------

Most applications that do buffered I/O will periodically call a file
synchronization call (fsync, fdatasync, msync or sync_file_range) to
ensure that data written has made it to the backing store.  When there
is an error during writeback, they expect that error to be reported when
a file sync request is made.  After an error has been reported on one
request, subsequent requests on the same file descriptor should return
0, unless further writeback errors have occurred since the previous file
synchronization.

Ideally, the kernel would report errors only on file descriptions on
which writes were done that subsequently failed to be written back.  The
generic pagecache infrastructure does not track the file descriptions
that have dirtied each individual page however, so determining which
file descriptors should get back an error is not possible.

Instead, the generic writeback error tracking infrastructure in the
kernel settles for reporting errors to fsync on all file descriptions
that were open at the time that the error occurred.  In a situation with
multiple writers, all of them will get back an error on a subsequent
fsync, even if all of the writes done through that particular file
descriptor succeeded (or even if there were no writes on that file
descriptor at all).

Filesystems that wish to use this infrastructure should call
mapping_set_error to record the error in the address_space when it
occurs.  Then, after writing back data from the pagecache in their
file->fsync operation, they should call file_check_and_advance_wb_err to
ensure that the struct file's error cursor has advanced to the correct
point in the stream of errors emitted by the backing device(s).


struct address_space_operations
-------------------------------

This describes how the VFS can manipulate mapping of a file to page
cache in your filesystem.  The following members are defined:

.. code-block:: c

	struct address_space_operations {
		int (*read_folio)(struct file *, struct folio *);
		int (*writepages)(struct address_space *, struct writeback_control *);
		bool (*dirty_folio)(struct address_space *, struct folio *);
		void (*readahead)(struct readahead_control *);
		int (*write_begin)(const struct kiocb *, struct address_space *mapping,
				   loff_t pos, unsigned len,
				   struct page **pagep, void **fsdata);
		int (*write_end)(const struct kiocb *, struct address_space *mapping,
				 loff_t pos, unsigned len, unsigned copied,
				 struct folio *folio, void *fsdata);
		sector_t (*bmap)(struct address_space *, sector_t);
		void (*invalidate_folio) (struct folio *, size_t start, size_t len);
		bool (*release_folio)(struct folio *, gfp_t);
		void (*free_folio)(struct folio *);
		ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
		int (*migrate_folio)(struct mapping *, struct folio *dst,
				struct folio *src, enum migrate_mode);
		int (*launder_folio) (struct folio *);

		bool (*is_partially_uptodate) (struct folio *, size_t from,
					       size_t count);
		void (*is_dirty_writeback)(struct folio *, bool *, bool *);
		int (*error_remove_folio)(struct mapping *mapping, struct folio *);
		int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span)
		int (*swap_deactivate)(struct file *);
	};

``read_folio``
	Called by the page cache to read a folio from the backing store.
	The 'file' argument supplies authentication information to network
	filesystems, and is generally not used by block based filesystems.
	It may be NULL if the caller does not have an open file (eg if
	the kernel is performing a read for itself rather than on behalf
	of a userspace process with an open file).

	If the mapping does not support large folios, the folio will
	contain a single page.	The folio will be locked when read_folio
	is called.  If the read completes successfully, the folio should
	be marked uptodate.  The filesystem should unlock the folio
	once the read has completed, whether it was successful or not.
	The filesystem does not need to modify the refcount on the folio;
	the page cache holds a reference count and that will not be
	released until the folio is unlocked.

	Filesystems may implement ->read_folio() synchronously.
	In normal operation, folios are read through the ->readahead()
	method.  Only if this fails, or if the caller needs to wait for
	the read to complete will the page cache call ->read_folio().
	Filesystems should not attempt to perform their own readahead
	in the ->read_folio() operation.

	If the filesystem cannot perform the read at this time, it can
	unlock the folio, do whatever action it needs to ensure that the
	read will succeed in the future and return AOP_TRUNCATED_PAGE.
	In this case, the caller should look up the folio, lock it,
	and call ->read_folio again.

	Callers may invoke the ->read_folio() method directly, but using
	read_mapping_folio() will take care of locking, waiting for the
	read to complete and handle cases such as AOP_TRUNCATED_PAGE.

``writepages``
	called by the VM to write out pages associated with the
	address_space object.  If wbc->sync_mode is WB_SYNC_ALL, then
	the writeback_control will specify a range of pages that must be
	written out.  If it is WB_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 write them back.

``dirty_folio``
	called by the VM to mark a folio as dirty.  This is particularly
	needed if an address space attaches private data to a folio, and
	that data needs to be updated when a folio is dirtied.  This is
	called, for example, when a memory mapped page gets modified.
	If defined, it should set the folio dirty flag, and the
	PAGECACHE_TAG_DIRTY search mark in i_pages.

``readahead``
	Called by the VM to read pages associated with the address_space
	object.  The pages are consecutive in the page cache and are
	locked.  The implementation should decrement the page refcount
	after starting I/O on each page.  Usually the page will be
	unlocked by the I/O completion handler.  The set of pages are
	divided into some sync pages followed by some async pages,
	rac->ra->async_size gives the number of async pages.  The
	filesystem should attempt to read all sync pages but may decide
	to stop once it reaches the async pages.  If it does decide to
	stop attempting I/O, it can simply return.  The caller will
	remove the remaining pages from the address space, unlock them
	and decrement the page refcount.  Set PageUptodate if the I/O
	completes successfully.

``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 folio for the
	specified offset, in ``*foliop``, 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 folio).

	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.

	The filesystem must take care of unlocking the folio,
	decrementing its 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.

``invalidate_folio``
	If a folio has private data, then invalidate_folio will be
	called when part or all of the folio is to be removed from the
	address space.  This generally corresponds to either a
	truncation, punch hole or a complete invalidation of the address
	space (in the latter case 'offset' will always be 0 and 'length'
	will be folio_size()).  Any private data associated with the folio
	should be updated to reflect this truncation.  If offset is 0
	and length is folio_size(), then the private data should be
	released, because the folio must be able to be completely
	discarded.  This may be done by calling the ->release_folio
	function, but in this case the release MUST succeed.

``release_folio``
	release_folio is called on folios with private data to tell the
	filesystem that the folio is about to be freed.  ->release_folio
	should remove any private data from the folio and clear the
	private flag.  If release_folio() fails, it should return false.
	release_folio() is used in two distinct though related cases.
	The first is when the VM wants to free a clean folio with no
	active users.  If ->release_folio succeeds, the folio will be
	removed from the address_space and be freed.

	The second case is when a request has been made to invalidate
	some or all folios in an address_space.  This can happen
	through the fadvise(POSIX_FADV_DONTNEED) system call or by the
	filesystem explicitly requesting it as nfs and 9p 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 folios are invalidated, then
	its release_folio will need to ensure this.  Possibly it can
	clear the uptodate flag if it cannot free private data yet.

``free_folio``
	free_folio is called once the folio 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.

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

``launder_folio``
	Called before freeing a folio - it writes back the dirty folio.
	To prevent redirtying the folio, it is kept locked during the
	whole operation.

``is_partially_uptodate``
	Called by the VM when reading a file through the pagecache when
	the underlying blocksize is smaller than the size of the folio.
	If the required block is up to date then the read can complete
	without needing I/O to bring the whole page up to date.

``is_dirty_writeback``
	Called by the VM when attempting to reclaim a folio.  The VM uses
	dirty and writeback information to determine if it needs to
	stall to allow flushers a chance to complete some IO.
	Ordinarily it can use folio_test_dirty and folio_test_writeback but
	some filesystems have more complex state (unstable folios in NFS
	prevent reclaim) or do not set those flags due to locking
	problems.  This callback allows a filesystem to indicate to the
	VM if a folio should be treated as dirty or writeback for the
	purposes of stalling.

``error_remove_folio``
	normally set to generic_error_remove_folio 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.

``swap_activate``

	Called to prepare the given file for swap.  It should perform
	any validation and preparation necessary to ensure that writes
	can be performed with minimal memory allocation.  It should call
	add_swap_extent(), or the helper iomap_swapfile_activate(), and
	return the number of extents added.  If IO should be submitted
	through the file system it should call swap_fs_activate, otherwise IO
	will be submitted directly to the block device ``sis->bdev``.

``swap_deactivate``
	Called during swapoff on files where swap_activate was
	successful.

The File Object
===============

A file object represents a file opened by a process.  This is also known
as an "open file description" in POSIX parlance.


struct file_operations
----------------------

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

.. code-block:: c

	struct file_operations {
		struct module *owner;
		fop_flags_t fop_flags;
		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 (*read_iter) (struct kiocb *, struct iov_iter *);
		ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
		int (*iopoll)(struct kiocb *kiocb, struct io_comp_batch *,
				unsigned int flags);
		int (*iterate_shared) (struct file *, struct dir_context *);
		__poll_t (*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 *, fl_owner_t id);
		int (*release) (struct inode *, struct file *);
		int (*fsync) (struct file *, loff_t, loff_t, int datasync);
		int (*fasync) (int, struct file *, int);
		int (*lock) (struct file *, int, struct file_lock *);
		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 *, loff_t *, size_t, unsigned int);
		ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
		void (*splice_eof)(struct file *file);
		int (*setlease)(struct file *, int, struct file_lease **, void **);
		long (*fallocate)(struct file *file, int mode, loff_t offset,
				  loff_t len);
		void (*show_fdinfo)(struct seq_file *m, struct file *f);
	#ifndef CONFIG_MMU
		unsigned (*mmap_capabilities)(struct file *);
	#endif
		ssize_t (*copy_file_range)(struct file *, loff_t, struct file *,
				loff_t, size_t, unsigned int);
		loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
					   struct file *file_out, loff_t pos_out,
					   loff_t len, unsigned int remap_flags);
		int (*fadvise)(struct file *, loff_t, loff_t, int);
		int (*uring_cmd)(struct io_uring_cmd *ioucmd, unsigned int issue_flags);
		int (*uring_cmd_iopoll)(struct io_uring_cmd *, struct io_comp_batch *,
					unsigned int poll_flags);
		int (*mmap_prepare)(struct vm_area_desc *);
	};

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

``read_iter``
	possibly asynchronous read with iov_iter as destination

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

``write_iter``
	possibly asynchronous write with iov_iter as source

``iopoll``
	called when aio wants to poll for completions on HIPRI iocbs

``iterate_shared``
	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. Deprecated in favour of
	``mmap_prepare``.

``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.  Also see the section above
	entitled "Handling errors during writeback".

``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

``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

``setlease``
	called by the VFS to set or release a file lock lease.  Local
	filesystems that wish to use the kernel-internal lease implementation
	should set this to generic_setlease(). Other setlease implementations
	should call generic_setlease() to record or remove the lease in the inode
	after setting it. When set to NULL, attempts to set or remove a lease will
	return -EINVAL.

``fallocate``
	called by the VFS to preallocate blocks or punch a hole.

``copy_file_range``
	called by the copy_file_range(2) system call.

``remap_file_range``
	called by the ioctl(2) system call for FICLONERANGE and FICLONE
	and FIDEDUPERANGE commands to remap file ranges.  An
	implementation should remap len bytes at pos_in of the source
	file into the dest file at pos_out.  Implementations must handle
	callers passing in len == 0; this means "remap to the end of the
	source file".  The return value should the number of bytes
	remapped, or the usual negative error code if errors occurred
	before any bytes were remapped.  The remap_flags parameter
	accepts REMAP_FILE_* flags.  If REMAP_FILE_DEDUP is set then the
	implementation must only remap if the requested file ranges have
	identical contents.  If REMAP_FILE_CAN_SHORTEN is set, the caller is
	ok with the implementation shortening the request length to
	satisfy alignment or EOF requirements (or any other reason).

``fadvise``
	possibly called by the fadvise64() system call.

``mmap_prepare``
	Called by the mmap(2) system call. Allows a VFS to set up a
	file-backed memory mapping, most notably establishing relevant
	private state and VMA callbacks.

	If further action such as pre-population of page tables is required,
	this can be specified by the vm_area_desc->action field and related
	parameters.

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:

.. code-block:: c

	struct dentry_operations {
		int (*d_revalidate)(struct inode *, const struct qstr *,
				    struct dentry *, unsigned int);
		int (*d_weak_revalidate)(struct dentry *, unsigned int);
		int (*d_hash)(const struct dentry *, struct qstr *);
		int (*d_compare)(const struct dentry *,
				 unsigned int, const char *, const struct qstr *);
		int (*d_delete)(const struct dentry *);
		int (*d_init)(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)(const struct path *, bool);
		struct dentry *(*d_real)(struct dentry *, enum d_real_type type);
		bool (*d_unalias_trylock)(const struct dentry *);
		void (*d_unalias_unlock)(const struct dentry *);
	};

``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 local filesystems leave this as NULL, because all their
	dentries in the dcache are valid.  Network filesystems are
	different since things can change on the server without the
	client necessarily being aware of it.

	This function should return a positive value if the dentry is
	still valid, and zero or a negative error code if it isn't.

	d_revalidate may be called in rcu-walk mode (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 change and, in d_inode case, even become NULL under
	us).

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

``d_weak_revalidate``
	called when the VFS needs to revalidate a "jumped" dentry.  This
	is called when a path-walk ends at dentry that was not acquired
	by doing a lookup in the parent directory.  This includes "/",
	"." and "..", as well as procfs-style symlinks and mountpoint
	traversal.

	In this case, we are less concerned with whether the dentry is
	still fully correct, but rather that the inode is still valid.
	As with d_revalidate, most local filesystems will set this to
	NULL since their dcache entries are always valid.

	This function has the same return code semantics as
	d_revalidate.

	d_weak_revalidate is only called after leaving rcu-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.

	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 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.  Should not
	dereference pointers outside the dentry 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.  ->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_init``
	called when a dentry is allocated

``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.

	Example :

.. code-block:: c

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

``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.

	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' while 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
	returning -ECHILD.  -EISDIR may also be returned to tell
	pathwalk to ignore d_automount or any mounts.

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

``d_real``
	overlay/union type filesystems implement this method to return one
	of the underlying dentries of a regular file hidden by the overlay.

	The 'type' argument takes the values D_REAL_DATA or D_REAL_METADATA
	for returning the real underlying dentry that refers to the inode
	hosting the file's data or metadata respectively.

	For non-regular files, the 'dentry' argument is returned.

``d_unalias_trylock``
	if present, will be called by d_splice_alias() before moving a
	preexisting attached alias.  Returning false prevents __d_move(),
	making d_splice_alias() fail with -ESTALE.

	Rationale: setting FS_RENAME_DOES_D_MOVE will prevent d_move()
	and d_exchange() calls from the outside of filesystem methods;
	however, it does not guarantee that attached dentries won't
	be renamed or moved by d_splice_alias() finding a preexisting
	alias for a directory inode.  Normally we would not care;
	however, something that wants to stabilize the entire path to
	root over a blocking operation might need that.  See 9p for one
	(and hopefully only) example.

``d_unalias_unlock``
	should be paired with ``d_unalias_trylock``; that one is called after
	__d_move() call in __d_unalias().


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.


Resources
=========

(Note some of these resources are not up-to-date with the latest kernel
 version.)

Creating Linux virtual filesystems. 2002
    <https://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
    <https://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>

A small trail through the Linux kernel by Andries Brouwer. 2001
    <https://www.win.tue.nl/~aeb/linux/vfs/trail.html>