src/kernel/linux/v4.19/Documentation/filesystems/vfs.txt

	      Overview of the Linux Virtual File System

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

		  Last updated on June 24, 2007.

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

  This file is released under the GPLv2.


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

The Virtual File System (also known as the Virtual Filesystem Switch)
is the software layer in the kernel that provides the filesystem
interface to userspace programs. It also provides an abstraction
within the kernel which allows different filesystem implementations to
coexist.

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


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

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

The dentry cache is meant to be a view into your entire filespace. As
most computers cannot fit all dentries in the RAM at the same time,
some bits of the cache are missing. In order to resolve your pathname
into a dentry, the VFS may have to resort to creating dentries along
the way, and then loading the inode. This is done by looking up the
inode.


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

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

To look up an inode requires that the VFS calls the lookup() method of
the parent directory inode. This method is installed by the specific
filesystem implementation that the inode lives in. Once the VFS has
the required dentry (and hence the inode), we can do all those boring
things like open(2) the file, or stat(2) it to peek at the inode
data. The stat(2) operation is fairly simple: once the VFS has the
dentry, it peeks at the inode data and passes some of it back to
userspace.


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

Opening a file requires another operation: allocation of a file
structure (this is the kernel-side implementation of file
descriptors). The freshly allocated file structure is initialized with
a pointer to the dentry and a set of file operation member functions.
These are taken from the inode data. The open() file method is then
called so the specific filesystem implementation can do its work. You
can see that this is another switch performed by the VFS. The file
structure is placed into the file descriptor table for the process.

Reading, writing and closing files (and other assorted VFS operations)
is done by using the userspace file descriptor to grab the appropriate
file structure, and then calling the required file structure method to
do whatever is required. For as long as the file is open, it keeps the
dentry in use, which in turn means that the VFS inode is still in use.


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

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

   #include <linux/fs.h>

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

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

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


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

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

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

  name: the name of the filesystem type, such as "ext2", "iso9660",
	"msdos" and so on

  fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)

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

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

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

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

  s_lock_key, s_umount_key: lockdep-specific

The mount() method has the following arguments:

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

  int flags: mount flags

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

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

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

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

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

The most interesting member of the superblock structure that the
mount() method fills in is the "s_op" field. This is a pointer to
a "struct super_operations" which describes the next level of the
filesystem implementation.

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

  mount_bdev: mount a filesystem residing on a block device

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

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

A fill_super() callback implementation has the following arguments:

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

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

  int silent: whether or not to be silent on error


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

A superblock object represents a mounted filesystem.


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

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

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

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

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

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

All methods are called without any locks being held, unless otherwise
noted. This means that most methods can block safely. All methods are
only called from a process context (i.e. not from an interrupt handler
or bottom half).

  alloc_inode: this method is called by 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.

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

  write_inode: this method is called when the VFS needs to write an
	inode to disc.  The second parameter indicates whether the write
	should be synchronous or not, not all filesystems check this flag.

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

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

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

  delete_inode: called when the VFS wants to delete an inode

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

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

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

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

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

  remount_fs: called when the filesystem is remounted. This is called
	with the kernel lock held

  clear_inode: called then the VFS clears the inode. Optional

  umount_begin: called when the VFS is unmounting a filesystem.

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

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

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

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

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

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

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

Whoever sets up the inode is responsible for filling in the "i_op" field. This
is a pointer to a "struct inode_operations" which describes the methods that
can be performed on individual inodes.

struct xattr_handlers
---------------------

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 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:

struct inode_operations {
	int (*create) (struct inode *,struct dentry *, umode_t, bool);
	struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
	int (*link) (struct dentry *,struct inode *,struct dentry *);
	int (*unlink) (struct inode *,struct dentry *);
	int (*symlink) (struct inode *,struct dentry *,const char *);
	int (*mkdir) (struct inode *,struct dentry *,umode_t);
	int (*rmdir) (struct inode *,struct dentry *);
	int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
	int (*rename) (struct inode *, struct dentry *,
			struct inode *, struct dentry *, unsigned int);
	int (*readlink) (struct dentry *, char __user *,int);
	const char *(*get_link) (struct dentry *, struct inode *,
				 struct delayed_call *);
	int (*permission) (struct inode *, int);
	int (*get_acl)(struct inode *, int);
	int (*setattr) (struct dentry *, struct iattr *);
	int (*getattr) (const struct path *, struct kstat *, u32, unsigned int);
	ssize_t (*listxattr) (struct dentry *, char *, size_t);
	void (*update_time)(struct inode *, struct timespec *, int);
	int (*atomic_open)(struct inode *, struct dentry *, struct file *,
			unsigned open_flag, umode_t create_mode);
	int (*tmpfile) (struct inode *, struct dentry *, umode_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.

	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).

  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.

  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.

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.

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
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 to direct the
operations.  This gives the the writepage and writepages operations 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 writepage or
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
syncronization.

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:

struct address_space_operations {
	int (*writepage)(struct page *page, struct writeback_control *wbc);
	int (*readpage)(struct file *, 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);
	void (*invalidatepage) (struct page *, unsigned int, unsigned int);
	int (*releasepage) (struct page *, int);
	void (*freepage)(struct page *);
	ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
	/* isolate a page for migration */
	bool (*isolate_page) (struct page *, isolate_mode_t);
	/* migrate the contents of a page to the specified target */
	int (*migratepage) (struct page *, struct page *);
	/* put migration-failed page back to right list */
	void (*putback_page) (struct page *);
	int (*launder_page) (struct page *);

	int (*is_partially_uptodate) (struct page *, unsigned long,
					unsigned long);
	void (*is_dirty_writeback) (struct page *, bool *, bool *);
	int (*error_remove_page) (struct mapping *mapping, struct page *page);
	int (*swap_activate)(struct file *);
	int (*swap_deactivate)(struct file *);
};

  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.

  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.

        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, punch hole  or a complete invalidation of the address
	space (in the latter case 'offset' will always be 0 and 'length'
	will be PAGE_SIZE). Any private data associated with the page
	should be updated to reflect this truncation.  If offset is 0 and
	length is PAGE_SIZE, 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 fadvise(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.

  isolate_page: Called by the VM when isolating a movable non-lru page.
	If page is successfully isolated, VM marks the page as PG_isolated
	via __SetPageIsolated.

  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.

  putback_page: Called by the VM when isolated page's migration fails.

  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.

  is_partially_uptodate: Called by the VM when reading a file through the
	pagecache when the underlying blocksize != pagesize. If the required
	block is up to date then the read can complete without needing the IO
	to bring the whole page up to date.

  is_dirty_writeback: Called by the VM when attempting to reclaim a page.
	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 PageDirty and PageWriteback but some filesystems have
	more complex state (unstable pages 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 page should be
	treated as dirty or writeback for the purposes of stalling.

  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.

  swap_activate: Called when swapon is used on a file to allocate
	space if necessary and pin the block lookup information in
	memory. A return value of zero indicates success,
	in which case this file can be used to back swapspace.

  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:

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 (*read_iter) (struct kiocb *, struct iov_iter *);
	ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
	int (*iterate) (struct file *, struct dir_context *);
	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 *);
	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 *, loff_t *, size_t, unsigned int);
	ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
	int (*setlease)(struct file *, long, struct file_lock **, 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);
	int (*clone_file_range)(struct file *, loff_t, struct file *, loff_t, u64);
	int (*dedupe_file_range)(struct file *, loff_t, struct file *, loff_t, u64);
	int (*fadvise)(struct file *, loff_t, loff_t, 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

  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

  iterate: called when the VFS needs to read the directory contents

  iterate_shared: called when the VFS needs to read the directory contents
	when filesystem supports concurrent dir iterators

  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. 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. setlease
	    implementations should call generic_setlease to record or remove
	    the lease in the inode after setting it.

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

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

  clone_file_range: called by the ioctl(2) system call for FICLONERANGE and
	FICLONE commands.

  dedupe_file_range: called by the ioctl(2) system call for FIDEDUPERANGE
	command.

  fadvise: possibly called by the fadvise64() 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 *, 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 *, const struct inode *);
};

  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 :

	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.  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 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 hidden by the overlay.  It is used in two
	different modes:

	Called from file_dentry() it returns the real dentry matching the inode
	argument.  The real dentry may be from a lower layer already copied up,
	but still referenced from the file.  This mode is selected with a
	non-NULL inode argument.

	With NULL inode the topmost real underlying dentry is returned.

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
    <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>