1.. SPDX-License-Identifier: GPL-2.0
   2
   3=========================================
   4Overview of the Linux Virtual File System
   5=========================================
   6
   7Original author: Richard Gooch <rgooch@atnf.csiro.au>
   8
   9- Copyright (C) 1999 Richard Gooch
  10- Copyright (C) 2005 Pekka Enberg
  11
  12
  13Introduction
  14============
  15
  16The Virtual File System (also known as the Virtual Filesystem Switch) is
  17the software layer in the kernel that provides the filesystem interface
  18to userspace programs.  It also provides an abstraction within the
  19kernel which allows different filesystem implementations to coexist.
  20
  21VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so on
  22are called from a process context.  Filesystem locking is described in
  23the document Documentation/filesystems/locking.rst.
  24
  25
  26Directory Entry Cache (dcache)
  27------------------------------
  28
  29The VFS implements the open(2), stat(2), chmod(2), and similar system
  30calls.  The pathname argument that is passed to them is used by the VFS
  31to search through the directory entry cache (also known as the dentry
  32cache or dcache).  This provides a very fast look-up mechanism to
  33translate a pathname (filename) into a specific dentry.  Dentries live
  34in RAM and are never saved to disc: they exist only for performance.
  35
  36The dentry cache is meant to be a view into your entire filespace.  As
  37most computers cannot fit all dentries in the RAM at the same time, some
  38bits of the cache are missing.  In order to resolve your pathname into a
  39dentry, the VFS may have to resort to creating dentries along the way,
  40and then loading the inode.  This is done by looking up the inode.
  41
  42
  43The Inode Object
  44----------------
  45
  46An individual dentry usually has a pointer to an inode.  Inodes are
  47filesystem objects such as regular files, directories, FIFOs and other
  48beasts.  They live either on the disc (for block device filesystems) or
  49in the memory (for pseudo filesystems).  Inodes that live on the disc
  50are copied into the memory when required and changes to the inode are
  51written back to disc.  A single inode can be pointed to by multiple
  52dentries (hard links, for example, do this).
  53
  54To look up an inode requires that the VFS calls the lookup() method of
  55the parent directory inode.  This method is installed by the specific
  56filesystem implementation that the inode lives in.  Once the VFS has the
  57required dentry (and hence the inode), we can do all those boring things
  58like open(2) the file, or stat(2) it to peek at the inode data.  The
  59stat(2) operation is fairly simple: once the VFS has the dentry, it
  60peeks at the inode data and passes some of it back to userspace.
  61
  62
  63The File Object
  64---------------
  65
  66Opening a file requires another operation: allocation of a file
  67structure (this is the kernel-side implementation of file descriptors).
  68The freshly allocated file structure is initialized with a pointer to
  69the dentry and a set of file operation member functions.  These are
  70taken from the inode data.  The open() file method is then called so the
  71specific filesystem implementation can do its work.  You can see that
  72this is another switch performed by the VFS.  The file structure is
  73placed into the file descriptor table for the process.
  74
  75Reading, writing and closing files (and other assorted VFS operations)
  76is done by using the userspace file descriptor to grab the appropriate
  77file structure, and then calling the required file structure method to
  78do whatever is required.  For as long as the file is open, it keeps the
  79dentry in use, which in turn means that the VFS inode is still in use.
  80
  81
  82Registering and Mounting a Filesystem
  83=====================================
  84
  85To register and unregister a filesystem, use the following API
  86functions:
  87
  88.. code-block:: c
  89
  90	#include <linux/fs.h>
  91
  92	extern int register_filesystem(struct file_system_type *);
  93	extern int unregister_filesystem(struct file_system_type *);
  94
  95The passed struct file_system_type describes your filesystem.  When a
  96request is made to mount a filesystem onto a directory in your
  97namespace, the VFS will call the appropriate mount() method for the
  98specific filesystem.  New vfsmount referring to the tree returned by
  99->mount() will be attached to the mountpoint, so that when pathname
 100resolution reaches the mountpoint it will jump into the root of that
 101vfsmount.
 102
 103You can see all filesystems that are registered to the kernel in the
 104file /proc/filesystems.
 105
 106
 107struct file_system_type
 108-----------------------
 109
 110This describes the filesystem.  The following
 111members are defined:
 112
 113.. code-block:: c
 114
 115	struct file_system_type {
 116		const char *name;
 117		int fs_flags;
 118		int (*init_fs_context)(struct fs_context *);
 119		const struct fs_parameter_spec *parameters;
 120		struct dentry *(*mount) (struct file_system_type *, int,
 121			const char *, void *);
 122		void (*kill_sb) (struct super_block *);
 123		struct module *owner;
 124		struct file_system_type * next;
 125		struct hlist_head fs_supers;
 126
 127		struct lock_class_key s_lock_key;
 128		struct lock_class_key s_umount_key;
 129		struct lock_class_key s_vfs_rename_key;
 130		struct lock_class_key s_writers_key[SB_FREEZE_LEVELS];
 131
 132		struct lock_class_key i_lock_key;
 133		struct lock_class_key i_mutex_key;
 134		struct lock_class_key invalidate_lock_key;
 135		struct lock_class_key i_mutex_dir_key;
 136	};
 137
 138``name``
 139	the name of the filesystem type, such as "ext2", "iso9660",
 140	"msdos" and so on
 141
 142``fs_flags``
 143	various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
 144
 145``init_fs_context``
 146	Initializes 'struct fs_context' ->ops and ->fs_private fields with
 147	filesystem-specific data.
 148
 149``parameters``
 150	Pointer to the array of filesystem parameters descriptors
 151	'struct fs_parameter_spec'.
 152	More info in Documentation/filesystems/mount_api.rst.
 153
 154``mount``
 155	the method to call when a new instance of this filesystem should
 156	be mounted
 157
 158``kill_sb``
 159	the method to call when an instance of this filesystem should be
 160	shut down
 161
 162
 163``owner``
 164	for internal VFS use: you should initialize this to THIS_MODULE
 165	in most cases.
 166
 167``next``
 168	for internal VFS use: you should initialize this to NULL
 169
 170``fs_supers``
 171	for internal VFS use: hlist of filesystem instances (superblocks)
 172
 173  s_lock_key, s_umount_key, s_vfs_rename_key, s_writers_key,
 174  i_lock_key, i_mutex_key, invalidate_lock_key, i_mutex_dir_key: lockdep-specific
 175
 176The mount() method has the following arguments:
 177
 178``struct file_system_type *fs_type``
 179	describes the filesystem, partly initialized by the specific
 180	filesystem code
 181
 182``int flags``
 183	mount flags
 184
 185``const char *dev_name``
 186	the device name we are mounting.
 187
 188``void *data``
 189	arbitrary mount options, usually comes as an ASCII string (see
 190	"Mount Options" section)
 191
 192The mount() method must return the root dentry of the tree requested by
 193caller.  An active reference to its superblock must be grabbed and the
 194superblock must be locked.  On failure it should return ERR_PTR(error).
 195
 196The arguments match those of mount(2) and their interpretation depends
 197on filesystem type.  E.g. for block filesystems, dev_name is interpreted
 198as block device name, that device is opened and if it contains a
 199suitable filesystem image the method creates and initializes struct
 200super_block accordingly, returning its root dentry to caller.
 201
 202->mount() may choose to return a subtree of existing filesystem - it
 203doesn't have to create a new one.  The main result from the caller's
 204point of view is a reference to dentry at the root of (sub)tree to be
 205attached; creation of new superblock is a common side effect.
 206
 207The most interesting member of the superblock structure that the mount()
 208method fills in is the "s_op" field.  This is a pointer to a "struct
 209super_operations" which describes the next level of the filesystem
 210implementation.
 211
 212Usually, a filesystem uses one of the generic mount() implementations
 213and provides a fill_super() callback instead.  The generic variants are:
 214
 215``mount_bdev``
 216	mount a filesystem residing on a block device
 217
 218``mount_nodev``
 219	mount a filesystem that is not backed by a device
 220
 221``mount_single``
 222	mount a filesystem which shares the instance between all mounts
 223
 224A fill_super() callback implementation has the following arguments:
 225
 226``struct super_block *sb``
 227	the superblock structure.  The callback must initialize this
 228	properly.
 229
 230``void *data``
 231	arbitrary mount options, usually comes as an ASCII string (see
 232	"Mount Options" section)
 233
 234``int silent``
 235	whether or not to be silent on error
 236
 237
 238The Superblock Object
 239=====================
 240
 241A superblock object represents a mounted filesystem.
 242
 243
 244struct super_operations
 245-----------------------
 246
 247This describes how the VFS can manipulate the superblock of your
 248filesystem.  The following members are defined:
 249
 250.. code-block:: c
 251
 252	struct super_operations {
 253		struct inode *(*alloc_inode)(struct super_block *sb);
 254		void (*destroy_inode)(struct inode *);
 255		void (*free_inode)(struct inode *);
 256
 257		void (*dirty_inode) (struct inode *, int flags);
 258		int (*write_inode) (struct inode *, struct writeback_control *wbc);
 259		int (*drop_inode) (struct inode *);
 260		void (*evict_inode) (struct inode *);
 261		void (*put_super) (struct super_block *);
 262		int (*sync_fs)(struct super_block *sb, int wait);
 263		int (*freeze_super) (struct super_block *sb,
 264					enum freeze_holder who);
 265		int (*freeze_fs) (struct super_block *);
 266		int (*thaw_super) (struct super_block *sb,
 267					enum freeze_wholder who);
 268		int (*unfreeze_fs) (struct super_block *);
 269		int (*statfs) (struct dentry *, struct kstatfs *);
 270		int (*remount_fs) (struct super_block *, int *, char *);
 
 271		void (*umount_begin) (struct super_block *);
 272
 273		int (*show_options)(struct seq_file *, struct dentry *);
 274		int (*show_devname)(struct seq_file *, struct dentry *);
 275		int (*show_path)(struct seq_file *, struct dentry *);
 276		int (*show_stats)(struct seq_file *, struct dentry *);
 277
 278		ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
 279		ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
 280		struct dquot **(*get_dquots)(struct inode *);
 281
 282		long (*nr_cached_objects)(struct super_block *,
 283					struct shrink_control *);
 284		long (*free_cached_objects)(struct super_block *,
 285					struct shrink_control *);
 286	};
 287
 288All methods are called without any locks being held, unless otherwise
 289noted.  This means that most methods can block safely.  All methods are
 290only called from a process context (i.e. not from an interrupt handler
 291or bottom half).
 292
 293``alloc_inode``
 294	this method is called by alloc_inode() to allocate memory for
 295	struct inode and initialize it.  If this function is not
 296	defined, a simple 'struct inode' is allocated.  Normally
 297	alloc_inode will be used to allocate a larger structure which
 298	contains a 'struct inode' embedded within it.
 299
 300``destroy_inode``
 301	this method is called by destroy_inode() to release resources
 302	allocated for struct inode.  It is only required if
 303	->alloc_inode was defined and simply undoes anything done by
 304	->alloc_inode.
 305
 306``free_inode``
 307	this method is called from RCU callback. If you use call_rcu()
 308	in ->destroy_inode to free 'struct inode' memory, then it's
 309	better to release memory in this method.
 310
 311``dirty_inode``
 312	this method is called by the VFS when an inode is marked dirty.
 313	This is specifically for the inode itself being marked dirty,
 314	not its data.  If the update needs to be persisted by fdatasync(),
 315	then I_DIRTY_DATASYNC will be set in the flags argument.
 316	I_DIRTY_TIME will be set in the flags in case lazytime is enabled
 317	and struct inode has times updated since the last ->dirty_inode
 318	call.
 319
 320``write_inode``
 321	this method is called when the VFS needs to write an inode to
 322	disc.  The second parameter indicates whether the write should
 323	be synchronous or not, not all filesystems check this flag.
 324
 325``drop_inode``
 326	called when the last access to the inode is dropped, with the
 327	inode->i_lock spinlock held.
 328
 329	This method should be either NULL (normal UNIX filesystem
 330	semantics) or "generic_delete_inode" (for filesystems that do
 331	not want to cache inodes - causing "delete_inode" to always be
 332	called regardless of the value of i_nlink)
 333
 334	The "generic_delete_inode()" behavior is equivalent to the old
 335	practice of using "force_delete" in the put_inode() case, but
 336	does not have the races that the "force_delete()" approach had.
 337
 338``evict_inode``
 339	called when the VFS wants to evict an inode. Caller does
 340	*not* evict the pagecache or inode-associated metadata buffers;
 341	the method has to use truncate_inode_pages_final() to get rid
 342	of those. Caller makes sure async writeback cannot be running for
 343	the inode while (or after) ->evict_inode() is called. Optional.
 344
 345``put_super``
 346	called when the VFS wishes to free the superblock
 347	(i.e. unmount).  This is called with the superblock lock held
 348
 349``sync_fs``
 350	called when VFS is writing out all dirty data associated with a
 351	superblock.  The second parameter indicates whether the method
 352	should wait until the write out has been completed.  Optional.
 353
 354``freeze_super``
 355	Called instead of ->freeze_fs callback if provided.
 356	Main difference is that ->freeze_super is called without taking
 357	down_write(&sb->s_umount). If filesystem implements it and wants
 358	->freeze_fs to be called too, then it has to call ->freeze_fs
 359	explicitly from this callback. Optional.
 360
 361``freeze_fs``
 362	called when VFS is locking a filesystem and forcing it into a
 363	consistent state.  This method is currently used by the Logical
 364	Volume Manager (LVM) and ioctl(FIFREEZE). Optional.
 365
 366``thaw_super``
 367	called when VFS is unlocking a filesystem and making it writable
 368	again after ->freeze_super. Optional.
 369
 370``unfreeze_fs``
 371	called when VFS is unlocking a filesystem and making it writable
 372	again after ->freeze_fs. Optional.
 373
 374``statfs``
 375	called when the VFS needs to get filesystem statistics.
 376
 377``remount_fs``
 378	called when the filesystem is remounted.  This is called with
 379	the kernel lock held
 380
 
 
 
 381``umount_begin``
 382	called when the VFS is unmounting a filesystem.
 383
 384``show_options``
 385	called by the VFS to show mount options for /proc/<pid>/mounts
 386	and /proc/<pid>/mountinfo.
 387	(see "Mount Options" section)
 388
 389``show_devname``
 390	Optional. Called by the VFS to show device name for
 391	/proc/<pid>/{mounts,mountinfo,mountstats}. If not provided then
 392	'(struct mount).mnt_devname' will be used.
 393
 394``show_path``
 395	Optional. Called by the VFS (for /proc/<pid>/mountinfo) to show
 396	the mount root dentry path relative to the filesystem root.
 397
 398``show_stats``
 399	Optional. Called by the VFS (for /proc/<pid>/mountstats) to show
 400	filesystem-specific mount statistics.
 401
 402``quota_read``
 403	called by the VFS to read from filesystem quota file.
 404
 405``quota_write``
 406	called by the VFS to write to filesystem quota file.
 407
 408``get_dquots``
 409	called by quota to get 'struct dquot' array for a particular inode.
 410	Optional.
 411
 412``nr_cached_objects``
 413	called by the sb cache shrinking function for the filesystem to
 414	return the number of freeable cached objects it contains.
 415	Optional.
 416
 417``free_cache_objects``
 418	called by the sb cache shrinking function for the filesystem to
 419	scan the number of objects indicated to try to free them.
 420	Optional, but any filesystem implementing this method needs to
 421	also implement ->nr_cached_objects for it to be called
 422	correctly.
 423
 424	We can't do anything with any errors that the filesystem might
 425	encountered, hence the void return type.  This will never be
 426	called if the VM is trying to reclaim under GFP_NOFS conditions,
 427	hence this method does not need to handle that situation itself.
 428
 429	Implementations must include conditional reschedule calls inside
 430	any scanning loop that is done.  This allows the VFS to
 431	determine appropriate scan batch sizes without having to worry
 432	about whether implementations will cause holdoff problems due to
 433	large scan batch sizes.
 434
 435Whoever sets up the inode is responsible for filling in the "i_op"
 436field.  This is a pointer to a "struct inode_operations" which describes
 437the methods that can be performed on individual inodes.
 438
 439
 440struct xattr_handler
 441---------------------
 442
 443On filesystems that support extended attributes (xattrs), the s_xattr
 444superblock field points to a NULL-terminated array of xattr handlers.
 445Extended attributes are name:value pairs.
 446
 447``name``
 448	Indicates that the handler matches attributes with the specified
 449	name (such as "system.posix_acl_access"); the prefix field must
 450	be NULL.
 451
 452``prefix``
 453	Indicates that the handler matches all attributes with the
 454	specified name prefix (such as "user."); the name field must be
 455	NULL.
 456
 457``list``
 458	Determine if attributes matching this xattr handler should be
 459	listed for a particular dentry.  Used by some listxattr
 460	implementations like generic_listxattr.
 461
 462``get``
 463	Called by the VFS to get the value of a particular extended
 464	attribute.  This method is called by the getxattr(2) system
 465	call.
 466
 467``set``
 468	Called by the VFS to set the value of a particular extended
 469	attribute.  When the new value is NULL, called to remove a
 470	particular extended attribute.  This method is called by the
 471	setxattr(2) and removexattr(2) system calls.
 472
 473When none of the xattr handlers of a filesystem match the specified
 474attribute name or when a filesystem doesn't support extended attributes,
 475the various ``*xattr(2)`` system calls return -EOPNOTSUPP.
 476
 477
 478The Inode Object
 479================
 480
 481An inode object represents an object within the filesystem.
 482
 483
 484struct inode_operations
 485-----------------------
 486
 487This describes how the VFS can manipulate an inode in your filesystem.
 488As of kernel 2.6.22, the following members are defined:
 489
 490.. code-block:: c
 491
 492	struct inode_operations {
 493		int (*create) (struct mnt_idmap *, struct inode *,struct dentry *, umode_t, bool);
 494		struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
 495		int (*link) (struct dentry *,struct inode *,struct dentry *);
 496		int (*unlink) (struct inode *,struct dentry *);
 497		int (*symlink) (struct mnt_idmap *, struct inode *,struct dentry *,const char *);
 498		int (*mkdir) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t);
 499		int (*rmdir) (struct inode *,struct dentry *);
 500		int (*mknod) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t,dev_t);
 501		int (*rename) (struct mnt_idmap *, struct inode *, struct dentry *,
 502			       struct inode *, struct dentry *, unsigned int);
 503		int (*readlink) (struct dentry *, char __user *,int);
 504		const char *(*get_link) (struct dentry *, struct inode *,
 505					 struct delayed_call *);
 506		int (*permission) (struct mnt_idmap *, struct inode *, int);
 507		struct posix_acl * (*get_inode_acl)(struct inode *, int, bool);
 508		int (*setattr) (struct mnt_idmap *, struct dentry *, struct iattr *);
 509		int (*getattr) (struct mnt_idmap *, const struct path *, struct kstat *, u32, unsigned int);
 510		ssize_t (*listxattr) (struct dentry *, char *, size_t);
 511		void (*update_time)(struct inode *, struct timespec *, int);
 512		int (*atomic_open)(struct inode *, struct dentry *, struct file *,
 513				   unsigned open_flag, umode_t create_mode);
 514		int (*tmpfile) (struct mnt_idmap *, struct inode *, struct file *, umode_t);
 515		struct posix_acl * (*get_acl)(struct mnt_idmap *, struct dentry *, int);
 516	        int (*set_acl)(struct mnt_idmap *, struct dentry *, struct posix_acl *, int);
 517		int (*fileattr_set)(struct mnt_idmap *idmap,
 518				    struct dentry *dentry, struct fileattr *fa);
 519		int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
 520	        struct offset_ctx *(*get_offset_ctx)(struct inode *inode);
 521	};
 522
 523Again, all methods are called without any locks being held, unless
 524otherwise noted.
 525
 526``create``
 527	called by the open(2) and creat(2) system calls.  Only required
 528	if you want to support regular files.  The dentry you get should
 529	not have an inode (i.e. it should be a negative dentry).  Here
 530	you will probably call d_instantiate() with the dentry and the
 531	newly created inode
 532
 533``lookup``
 534	called when the VFS needs to look up an inode in a parent
 535	directory.  The name to look for is found in the dentry.  This
 536	method must call d_add() to insert the found inode into the
 537	dentry.  The "i_count" field in the inode structure should be
 538	incremented.  If the named inode does not exist a NULL inode
 539	should be inserted into the dentry (this is called a negative
 540	dentry).  Returning an error code from this routine must only be
 541	done on a real error, otherwise creating inodes with system
 542	calls like create(2), mknod(2), mkdir(2) and so on will fail.
 543	If you wish to overload the dentry methods then you should
 544	initialise the "d_dop" field in the dentry; this is a pointer to
 545	a struct "dentry_operations".  This method is called with the
 546	directory inode semaphore held
 547
 548``link``
 549	called by the link(2) system call.  Only required if you want to
 550	support hard links.  You will probably need to call
 551	d_instantiate() just as you would in the create() method
 552
 553``unlink``
 554	called by the unlink(2) system call.  Only required if you want
 555	to support deleting inodes
 556
 557``symlink``
 558	called by the symlink(2) system call.  Only required if you want
 559	to support symlinks.  You will probably need to call
 560	d_instantiate() just as you would in the create() method
 561
 562``mkdir``
 563	called by the mkdir(2) system call.  Only required if you want
 564	to support creating subdirectories.  You will probably need to
 565	call d_instantiate() just as you would in the create() method
 566
 567``rmdir``
 568	called by the rmdir(2) system call.  Only required if you want
 569	to support deleting subdirectories
 570
 571``mknod``
 572	called by the mknod(2) system call to create a device (char,
 573	block) inode or a named pipe (FIFO) or socket.  Only required if
 574	you want to support creating these types of inodes.  You will
 575	probably need to call d_instantiate() just as you would in the
 576	create() method
 577
 578``rename``
 579	called by the rename(2) system call to rename the object to have
 580	the parent and name given by the second inode and dentry.
 581
 582	The filesystem must return -EINVAL for any unsupported or
 583	unknown flags.  Currently the following flags are implemented:
 584	(1) RENAME_NOREPLACE: this flag indicates that if the target of
 585	the rename exists the rename should fail with -EEXIST instead of
 586	replacing the target.  The VFS already checks for existence, so
 587	for local filesystems the RENAME_NOREPLACE implementation is
 588	equivalent to plain rename.
 589	(2) RENAME_EXCHANGE: exchange source and target.  Both must
 590	exist; this is checked by the VFS.  Unlike plain rename, source
 591	and target may be of different type.
 592
 593``get_link``
 594	called by the VFS to follow a symbolic link to the inode it
 595	points to.  Only required if you want to support symbolic links.
 596	This method returns the symlink body to traverse (and possibly
 597	resets the current position with nd_jump_link()).  If the body
 598	won't go away until the inode is gone, nothing else is needed;
 599	if it needs to be otherwise pinned, arrange for its release by
 600	having get_link(..., ..., done) do set_delayed_call(done,
 601	destructor, argument).  In that case destructor(argument) will
 602	be called once VFS is done with the body you've returned.  May
 603	be called in RCU mode; that is indicated by NULL dentry
 604	argument.  If request can't be handled without leaving RCU mode,
 605	have it return ERR_PTR(-ECHILD).
 606
 607	If the filesystem stores the symlink target in ->i_link, the
 608	VFS may use it directly without calling ->get_link(); however,
 609	->get_link() must still be provided.  ->i_link must not be
 610	freed until after an RCU grace period.  Writing to ->i_link
 611	post-iget() time requires a 'release' memory barrier.
 612
 613``readlink``
 614	this is now just an override for use by readlink(2) for the
 615	cases when ->get_link uses nd_jump_link() or object is not in
 616	fact a symlink.  Normally filesystems should only implement
 617	->get_link for symlinks and readlink(2) will automatically use
 618	that.
 619
 620``permission``
 621	called by the VFS to check for access rights on a POSIX-like
 622	filesystem.
 623
 624	May be called in rcu-walk mode (mask & MAY_NOT_BLOCK).  If in
 625	rcu-walk mode, the filesystem must check the permission without
 626	blocking or storing to the inode.
 627
 628	If a situation is encountered that rcu-walk cannot handle,
 629	return
 630	-ECHILD and it will be called again in ref-walk mode.
 631
 632``setattr``
 633	called by the VFS to set attributes for a file.  This method is
 634	called by chmod(2) and related system calls.
 635
 636``getattr``
 637	called by the VFS to get attributes of a file.  This method is
 638	called by stat(2) and related system calls.
 639
 640``listxattr``
 641	called by the VFS to list all extended attributes for a given
 642	file.  This method is called by the listxattr(2) system call.
 643
 644``update_time``
 645	called by the VFS to update a specific time or the i_version of
 646	an inode.  If this is not defined the VFS will update the inode
 647	itself and call mark_inode_dirty_sync.
 648
 649``atomic_open``
 650	called on the last component of an open.  Using this optional
 651	method the filesystem can look up, possibly create and open the
 652	file in one atomic operation.  If it wants to leave actual
 653	opening to the caller (e.g. if the file turned out to be a
 654	symlink, device, or just something filesystem won't do atomic
 655	open for), it may signal this by returning finish_no_open(file,
 656	dentry).  This method is only called if the last component is
 657	negative or needs lookup.  Cached positive dentries are still
 658	handled by f_op->open().  If the file was created, FMODE_CREATED
 659	flag should be set in file->f_mode.  In case of O_EXCL the
 660	method must only succeed if the file didn't exist and hence
 661	FMODE_CREATED shall always be set on success.
 662
 663``tmpfile``
 664	called in the end of O_TMPFILE open().  Optional, equivalent to
 665	atomically creating, opening and unlinking a file in given
 666	directory.  On success needs to return with the file already
 667	open; this can be done by calling finish_open_simple() right at
 668	the end.
 669
 670``fileattr_get``
 671	called on ioctl(FS_IOC_GETFLAGS) and ioctl(FS_IOC_FSGETXATTR) to
 672	retrieve miscellaneous file flags and attributes.  Also called
 673	before the relevant SET operation to check what is being changed
 674	(in this case with i_rwsem locked exclusive).  If unset, then
 675	fall back to f_op->ioctl().
 676
 677``fileattr_set``
 678	called on ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_FSSETXATTR) to
 679	change miscellaneous file flags and attributes.  Callers hold
 680	i_rwsem exclusive.  If unset, then fall back to f_op->ioctl().
 681``get_offset_ctx``
 682	called to get the offset context for a directory inode. A
 683        filesystem must define this operation to use
 684        simple_offset_dir_operations.
 685
 686The Address Space Object
 687========================
 688
 689The address space object is used to group and manage pages in the page
 690cache.  It can be used to keep track of the pages in a file (or anything
 691else) and also track the mapping of sections of the file into process
 692address spaces.
 693
 694There are a number of distinct yet related services that an
 695address-space can provide.  These include communicating memory pressure,
 696page lookup by address, and keeping track of pages tagged as Dirty or
 697Writeback.
 698
 699The first can be used independently to the others.  The VM can try to
 700either write dirty pages in order to clean them, or release clean pages
 701in order to reuse them.  To do this it can call the ->writepage method
 702on dirty pages, and ->release_folio on clean folios with the private
 703flag set.  Clean pages without PagePrivate and with no external references
 704will be released without notice being given to the address_space.
 705
 706To achieve this functionality, pages need to be placed on an LRU with
 707lru_cache_add and mark_page_active needs to be called whenever the page
 708is used.
 709
 710Pages are normally kept in a radix tree index by ->index.  This tree
 711maintains information about the PG_Dirty and PG_Writeback status of each
 712page, so that pages with either of these flags can be found quickly.
 713
 714The Dirty tag is primarily used by mpage_writepages - the default
 715->writepages method.  It uses the tag to find dirty pages to call
 716->writepage on.  If mpage_writepages is not used (i.e. the address
 717provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is almost
 718unused.  write_inode_now and sync_inode do use it (through
 719__sync_single_inode) to check if ->writepages has been successful in
 720writing out the whole address_space.
 721
 722The Writeback tag is used by filemap*wait* and sync_page* functions, via
 723filemap_fdatawait_range, to wait for all writeback to complete.
 724
 725An address_space handler may attach extra information to a page,
 726typically using the 'private' field in the 'struct page'.  If such
 727information is attached, the PG_Private flag should be set.  This will
 728cause various VM routines to make extra calls into the address_space
 729handler to deal with that data.
 730
 731An address space acts as an intermediate between storage and
 732application.  Data is read into the address space a whole page at a
 733time, and provided to the application either by copying of the page, or
 734by memory-mapping the page.  Data is written into the address space by
 735the application, and then written-back to storage typically in whole
 736pages, however the address_space has finer control of write sizes.
 737
 738The read process essentially only requires 'read_folio'.  The write
 739process is more complicated and uses write_begin/write_end or
 740dirty_folio to write data into the address_space, and writepage and
 741writepages to writeback data to storage.
 742
 743Adding and removing pages to/from an address_space is protected by the
 744inode's i_mutex.
 745
 746When data is written to a page, the PG_Dirty flag should be set.  It
 747typically remains set until writepage asks for it to be written.  This
 748should clear PG_Dirty and set PG_Writeback.  It can be actually written
 749at any point after PG_Dirty is clear.  Once it is known to be safe,
 750PG_Writeback is cleared.
 751
 752Writeback makes use of a writeback_control structure to direct the
 753operations.  This gives the writepage and writepages operations some
 754information about the nature of and reason for the writeback request,
 755and the constraints under which it is being done.  It is also used to
 756return information back to the caller about the result of a writepage or
 757writepages request.
 758
 759
 760Handling errors during writeback
 761--------------------------------
 762
 763Most applications that do buffered I/O will periodically call a file
 764synchronization call (fsync, fdatasync, msync or sync_file_range) to
 765ensure that data written has made it to the backing store.  When there
 766is an error during writeback, they expect that error to be reported when
 767a file sync request is made.  After an error has been reported on one
 768request, subsequent requests on the same file descriptor should return
 7690, unless further writeback errors have occurred since the previous file
 770synchronization.
 771
 772Ideally, the kernel would report errors only on file descriptions on
 773which writes were done that subsequently failed to be written back.  The
 774generic pagecache infrastructure does not track the file descriptions
 775that have dirtied each individual page however, so determining which
 776file descriptors should get back an error is not possible.
 777
 778Instead, the generic writeback error tracking infrastructure in the
 779kernel settles for reporting errors to fsync on all file descriptions
 780that were open at the time that the error occurred.  In a situation with
 781multiple writers, all of them will get back an error on a subsequent
 782fsync, even if all of the writes done through that particular file
 783descriptor succeeded (or even if there were no writes on that file
 784descriptor at all).
 785
 786Filesystems that wish to use this infrastructure should call
 787mapping_set_error to record the error in the address_space when it
 788occurs.  Then, after writing back data from the pagecache in their
 789file->fsync operation, they should call file_check_and_advance_wb_err to
 790ensure that the struct file's error cursor has advanced to the correct
 791point in the stream of errors emitted by the backing device(s).
 792
 793
 794struct address_space_operations
 795-------------------------------
 796
 797This describes how the VFS can manipulate mapping of a file to page
 798cache in your filesystem.  The following members are defined:
 799
 800.. code-block:: c
 801
 802	struct address_space_operations {
 803		int (*writepage)(struct page *page, struct writeback_control *wbc);
 804		int (*read_folio)(struct file *, struct folio *);
 805		int (*writepages)(struct address_space *, struct writeback_control *);
 806		bool (*dirty_folio)(struct address_space *, struct folio *);
 807		void (*readahead)(struct readahead_control *);
 
 
 808		int (*write_begin)(struct file *, struct address_space *mapping,
 809				   loff_t pos, unsigned len,
 810				struct page **pagep, void **fsdata);
 811		int (*write_end)(struct file *, struct address_space *mapping,
 812				 loff_t pos, unsigned len, unsigned copied,
 813				 struct folio *folio, void *fsdata);
 814		sector_t (*bmap)(struct address_space *, sector_t);
 815		void (*invalidate_folio) (struct folio *, size_t start, size_t len);
 816		bool (*release_folio)(struct folio *, gfp_t);
 817		void (*free_folio)(struct folio *);
 818		ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
 819		int (*migrate_folio)(struct mapping *, struct folio *dst,
 820				struct folio *src, enum migrate_mode);
 821		int (*launder_folio) (struct folio *);
 822
 823		bool (*is_partially_uptodate) (struct folio *, size_t from,
 824					       size_t count);
 825		void (*is_dirty_writeback)(struct folio *, bool *, bool *);
 826		int (*error_remove_folio)(struct mapping *mapping, struct folio *);
 827		int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span)
 
 
 
 
 828		int (*swap_deactivate)(struct file *);
 829		int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
 830	};
 831
 832``writepage``
 833	called by the VM to write a dirty page to backing store.  This
 834	may happen for data integrity reasons (i.e. 'sync'), or to free
 835	up memory (flush).  The difference can be seen in
 836	wbc->sync_mode.  The PG_Dirty flag has been cleared and
 837	PageLocked is true.  writepage should start writeout, should set
 838	PG_Writeback, and should make sure the page is unlocked, either
 839	synchronously or asynchronously when the write operation
 840	completes.
 841
 842	If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
 843	try too hard if there are problems, and may choose to write out
 844	other pages from the mapping if that is easier (e.g. due to
 845	internal dependencies).  If it chooses not to start writeout, it
 846	should return AOP_WRITEPAGE_ACTIVATE so that the VM will not
 847	keep calling ->writepage on that page.
 848
 849	See the file "Locking" for more details.
 850
 851``read_folio``
 852	Called by the page cache to read a folio from the backing store.
 853	The 'file' argument supplies authentication information to network
 854	filesystems, and is generally not used by block based filesystems.
 855	It may be NULL if the caller does not have an open file (eg if
 856	the kernel is performing a read for itself rather than on behalf
 857	of a userspace process with an open file).
 858
 859	If the mapping does not support large folios, the folio will
 860	contain a single page.	The folio will be locked when read_folio
 861	is called.  If the read completes successfully, the folio should
 862	be marked uptodate.  The filesystem should unlock the folio
 863	once the read has completed, whether it was successful or not.
 864	The filesystem does not need to modify the refcount on the folio;
 865	the page cache holds a reference count and that will not be
 866	released until the folio is unlocked.
 867
 868	Filesystems may implement ->read_folio() synchronously.
 869	In normal operation, folios are read through the ->readahead()
 870	method.  Only if this fails, or if the caller needs to wait for
 871	the read to complete will the page cache call ->read_folio().
 872	Filesystems should not attempt to perform their own readahead
 873	in the ->read_folio() operation.
 874
 875	If the filesystem cannot perform the read at this time, it can
 876	unlock the folio, do whatever action it needs to ensure that the
 877	read will succeed in the future and return AOP_TRUNCATED_PAGE.
 878	In this case, the caller should look up the folio, lock it,
 879	and call ->read_folio again.
 880
 881	Callers may invoke the ->read_folio() method directly, but using
 882	read_mapping_folio() will take care of locking, waiting for the
 883	read to complete and handle cases such as AOP_TRUNCATED_PAGE.
 884
 885``writepages``
 886	called by the VM to write out pages associated with the
 887	address_space object.  If wbc->sync_mode is WB_SYNC_ALL, then
 888	the writeback_control will specify a range of pages that must be
 889	written out.  If it is WB_SYNC_NONE, then a nr_to_write is
 890	given and that many pages should be written if possible.  If no
 891	->writepages is given, then mpage_writepages is used instead.
 892	This will choose pages from the address space that are tagged as
 893	DIRTY and will pass them to ->writepage.
 894
 895``dirty_folio``
 896	called by the VM to mark a folio as dirty.  This is particularly
 897	needed if an address space attaches private data to a folio, and
 898	that data needs to be updated when a folio is dirtied.  This is
 899	called, for example, when a memory mapped page gets modified.
 900	If defined, it should set the folio dirty flag, and the
 901	PAGECACHE_TAG_DIRTY search mark in i_pages.
 902
 903``readahead``
 904	Called by the VM to read pages associated with the address_space
 905	object.  The pages are consecutive in the page cache and are
 906	locked.  The implementation should decrement the page refcount
 907	after starting I/O on each page.  Usually the page will be
 908	unlocked by the I/O completion handler.  The set of pages are
 909	divided into some sync pages followed by some async pages,
 910	rac->ra->async_size gives the number of async pages.  The
 911	filesystem should attempt to read all sync pages but may decide
 912	to stop once it reaches the async pages.  If it does decide to
 913	stop attempting I/O, it can simply return.  The caller will
 914	remove the remaining pages from the address space, unlock them
 915	and decrement the page refcount.  Set PageUptodate if the I/O
 916	completes successfully.
 
 
 
 
 
 
 917
 918``write_begin``
 919	Called by the generic buffered write code to ask the filesystem
 920	to prepare to write len bytes at the given offset in the file.
 921	The address_space should check that the write will be able to
 922	complete, by allocating space if necessary and doing any other
 923	internal housekeeping.  If the write will update parts of any
 924	basic-blocks on storage, then those blocks should be pre-read
 925	(if they haven't been read already) so that the updated blocks
 926	can be written out properly.
 927
 928	The filesystem must return the locked pagecache folio for the
 929	specified offset, in ``*foliop``, for the caller to write into.
 930
 931	It must be able to cope with short writes (where the length
 932	passed to write_begin is greater than the number of bytes copied
 933	into the folio).
 
 
 
 934
 935	A void * may be returned in fsdata, which then gets passed into
 936	write_end.
 937
 938	Returns 0 on success; < 0 on failure (which is the error code),
 939	in which case write_end is not called.
 940
 941``write_end``
 942	After a successful write_begin, and data copy, write_end must be
 943	called.  len is the original len passed to write_begin, and
 944	copied is the amount that was able to be copied.
 945
 946	The filesystem must take care of unlocking the folio,
 947	decrementing its refcount, and updating i_size.
 948
 949	Returns < 0 on failure, otherwise the number of bytes (<=
 950	'copied') that were able to be copied into pagecache.
 951
 952``bmap``
 953	called by the VFS to map a logical block offset within object to
 954	physical block number.  This method is used by the FIBMAP ioctl
 955	and for working with swap-files.  To be able to swap to a file,
 956	the file must have a stable mapping to a block device.  The swap
 957	system does not go through the filesystem but instead uses bmap
 958	to find out where the blocks in the file are and uses those
 959	addresses directly.
 960
 961``invalidate_folio``
 962	If a folio has private data, then invalidate_folio will be
 963	called when part or all of the folio is to be removed from the
 964	address space.  This generally corresponds to either a
 965	truncation, punch hole or a complete invalidation of the address
 966	space (in the latter case 'offset' will always be 0 and 'length'
 967	will be folio_size()).  Any private data associated with the folio
 968	should be updated to reflect this truncation.  If offset is 0
 969	and length is folio_size(), then the private data should be
 970	released, because the folio must be able to be completely
 971	discarded.  This may be done by calling the ->release_folio
 972	function, but in this case the release MUST succeed.
 973
 974``release_folio``
 975	release_folio is called on folios with private data to tell the
 976	filesystem that the folio is about to be freed.  ->release_folio
 977	should remove any private data from the folio and clear the
 978	private flag.  If release_folio() fails, it should return false.
 979	release_folio() is used in two distinct though related cases.
 980	The first is when the VM wants to free a clean folio with no
 981	active users.  If ->release_folio succeeds, the folio will be
 982	removed from the address_space and be freed.
 
 983
 984	The second case is when a request has been made to invalidate
 985	some or all folios in an address_space.  This can happen
 986	through the fadvise(POSIX_FADV_DONTNEED) system call or by the
 987	filesystem explicitly requesting it as nfs and 9p do (when they
 988	believe the cache may be out of date with storage) by calling
 989	invalidate_inode_pages2().  If the filesystem makes such a call,
 990	and needs to be certain that all folios are invalidated, then
 991	its release_folio will need to ensure this.  Possibly it can
 992	clear the uptodate flag if it cannot free private data yet.
 993
 994``free_folio``
 995	free_folio is called once the folio is no longer visible in the
 996	page cache in order to allow the cleanup of any private data.
 997	Since it may be called by the memory reclaimer, it should not
 998	assume that the original address_space mapping still exists, and
 999	it should not block.
1000
1001``direct_IO``
1002	called by the generic read/write routines to perform direct_IO -
1003	that is IO requests which bypass the page cache and transfer
1004	data directly between the storage and the application's address
1005	space.
1006
1007``migrate_folio``
 
 
 
 
 
1008	This is used to compact the physical memory usage.  If the VM
1009	wants to relocate a folio (maybe from a memory device that is
1010	signalling imminent failure) it will pass a new folio and an old
1011	folio to this function.  migrate_folio should transfer any private
1012	data across and update any references that it has to the folio.
1013
1014``launder_folio``
1015	Called before freeing a folio - it writes back the dirty folio.
1016	To prevent redirtying the folio, it is kept locked during the
 
 
 
1017	whole operation.
1018
1019``is_partially_uptodate``
1020	Called by the VM when reading a file through the pagecache when
1021	the underlying blocksize is smaller than the size of the folio.
1022	If the required block is up to date then the read can complete
1023	without needing I/O to bring the whole page up to date.
1024
1025``is_dirty_writeback``
1026	Called by the VM when attempting to reclaim a folio.  The VM uses
1027	dirty and writeback information to determine if it needs to
1028	stall to allow flushers a chance to complete some IO.
1029	Ordinarily it can use folio_test_dirty and folio_test_writeback but
1030	some filesystems have more complex state (unstable folios in NFS
1031	prevent reclaim) or do not set those flags due to locking
1032	problems.  This callback allows a filesystem to indicate to the
1033	VM if a folio should be treated as dirty or writeback for the
1034	purposes of stalling.
1035
1036``error_remove_folio``
1037	normally set to generic_error_remove_folio if truncation is ok
1038	for this address space.  Used for memory failure handling.
1039	Setting this implies you deal with pages going away under you,
1040	unless you have them locked or reference counts increased.
1041
1042``swap_activate``
1043
1044	Called to prepare the given file for swap.  It should perform
1045	any validation and preparation necessary to ensure that writes
1046	can be performed with minimal memory allocation.  It should call
1047	add_swap_extent(), or the helper iomap_swapfile_activate(), and
1048	return the number of extents added.  If IO should be submitted
1049	through ->swap_rw(), it should set SWP_FS_OPS, otherwise IO will
1050	be submitted directly to the block device ``sis->bdev``.
1051
1052``swap_deactivate``
1053	Called during swapoff on files where swap_activate was
1054	successful.
1055
1056``swap_rw``
1057	Called to read or write swap pages when SWP_FS_OPS is set.
1058
1059The File Object
1060===============
1061
1062A file object represents a file opened by a process.  This is also known
1063as an "open file description" in POSIX parlance.
1064
1065
1066struct file_operations
1067----------------------
1068
1069This describes how the VFS can manipulate an open file.  As of kernel
10704.18, the following members are defined:
1071
1072.. code-block:: c
1073
1074	struct file_operations {
1075		struct module *owner;
1076		loff_t (*llseek) (struct file *, loff_t, int);
1077		ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
1078		ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
1079		ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
1080		ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
1081		int (*iopoll)(struct kiocb *kiocb, bool spin);
 
1082		int (*iterate_shared) (struct file *, struct dir_context *);
1083		__poll_t (*poll) (struct file *, struct poll_table_struct *);
1084		long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
1085		long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
1086		int (*mmap) (struct file *, struct vm_area_struct *);
1087		int (*open) (struct inode *, struct file *);
1088		int (*flush) (struct file *, fl_owner_t id);
1089		int (*release) (struct inode *, struct file *);
1090		int (*fsync) (struct file *, loff_t, loff_t, int datasync);
1091		int (*fasync) (int, struct file *, int);
1092		int (*lock) (struct file *, int, struct file_lock *);
 
1093		unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
1094		int (*check_flags)(int);
1095		int (*flock) (struct file *, int, struct file_lock *);
1096		ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
1097		ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
1098		int (*setlease)(struct file *, long, struct file_lock **, void **);
1099		long (*fallocate)(struct file *file, int mode, loff_t offset,
1100				  loff_t len);
1101		void (*show_fdinfo)(struct seq_file *m, struct file *f);
1102	#ifndef CONFIG_MMU
1103		unsigned (*mmap_capabilities)(struct file *);
1104	#endif
1105		ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
1106		loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
1107					   struct file *file_out, loff_t pos_out,
1108					   loff_t len, unsigned int remap_flags);
1109		int (*fadvise)(struct file *, loff_t, loff_t, int);
1110	};
1111
1112Again, all methods are called without any locks being held, unless
1113otherwise noted.
1114
1115``llseek``
1116	called when the VFS needs to move the file position index
1117
1118``read``
1119	called by read(2) and related system calls
1120
1121``read_iter``
1122	possibly asynchronous read with iov_iter as destination
1123
1124``write``
1125	called by write(2) and related system calls
1126
1127``write_iter``
1128	possibly asynchronous write with iov_iter as source
1129
1130``iopoll``
1131	called when aio wants to poll for completions on HIPRI iocbs
1132
1133``iterate_shared``
1134	called when the VFS needs to read the directory contents
1135
 
 
 
 
1136``poll``
1137	called by the VFS when a process wants to check if there is
1138	activity on this file and (optionally) go to sleep until there
1139	is activity.  Called by the select(2) and poll(2) system calls
1140
1141``unlocked_ioctl``
1142	called by the ioctl(2) system call.
1143
1144``compat_ioctl``
1145	called by the ioctl(2) system call when 32 bit system calls are
1146	 used on 64 bit kernels.
1147
1148``mmap``
1149	called by the mmap(2) system call
1150
1151``open``
1152	called by the VFS when an inode should be opened.  When the VFS
1153	opens a file, it creates a new "struct file".  It then calls the
1154	open method for the newly allocated file structure.  You might
1155	think that the open method really belongs in "struct
1156	inode_operations", and you may be right.  I think it's done the
1157	way it is because it makes filesystems simpler to implement.
1158	The open() method is a good place to initialize the
1159	"private_data" member in the file structure if you want to point
1160	to a device structure
1161
1162``flush``
1163	called by the close(2) system call to flush a file
1164
1165``release``
1166	called when the last reference to an open file is closed
1167
1168``fsync``
1169	called by the fsync(2) system call.  Also see the section above
1170	entitled "Handling errors during writeback".
1171
1172``fasync``
1173	called by the fcntl(2) system call when asynchronous
1174	(non-blocking) mode is enabled for a file
1175
1176``lock``
1177	called by the fcntl(2) system call for F_GETLK, F_SETLK, and
1178	F_SETLKW commands
1179
1180``get_unmapped_area``
1181	called by the mmap(2) system call
1182
1183``check_flags``
1184	called by the fcntl(2) system call for F_SETFL command
1185
1186``flock``
1187	called by the flock(2) system call
1188
1189``splice_write``
1190	called by the VFS to splice data from a pipe to a file.  This
1191	method is used by the splice(2) system call
1192
1193``splice_read``
1194	called by the VFS to splice data from file to a pipe.  This
1195	method is used by the splice(2) system call
1196
1197``setlease``
1198	called by the VFS to set or release a file lock lease.  setlease
1199	implementations should call generic_setlease to record or remove
1200	the lease in the inode after setting it.
1201
1202``fallocate``
1203	called by the VFS to preallocate blocks or punch a hole.
1204
1205``copy_file_range``
1206	called by the copy_file_range(2) system call.
1207
1208``remap_file_range``
1209	called by the ioctl(2) system call for FICLONERANGE and FICLONE
1210	and FIDEDUPERANGE commands to remap file ranges.  An
1211	implementation should remap len bytes at pos_in of the source
1212	file into the dest file at pos_out.  Implementations must handle
1213	callers passing in len == 0; this means "remap to the end of the
1214	source file".  The return value should the number of bytes
1215	remapped, or the usual negative error code if errors occurred
1216	before any bytes were remapped.  The remap_flags parameter
1217	accepts REMAP_FILE_* flags.  If REMAP_FILE_DEDUP is set then the
1218	implementation must only remap if the requested file ranges have
1219	identical contents.  If REMAP_FILE_CAN_SHORTEN is set, the caller is
1220	ok with the implementation shortening the request length to
1221	satisfy alignment or EOF requirements (or any other reason).
1222
1223``fadvise``
1224	possibly called by the fadvise64() system call.
1225
1226Note that the file operations are implemented by the specific
1227filesystem in which the inode resides.  When opening a device node
1228(character or block special) most filesystems will call special
1229support routines in the VFS which will locate the required device
1230driver information.  These support routines replace the filesystem file
1231operations with those for the device driver, and then proceed to call
1232the new open() method for the file.  This is how opening a device file
1233in the filesystem eventually ends up calling the device driver open()
1234method.
1235
1236
1237Directory Entry Cache (dcache)
1238==============================
1239
1240
1241struct dentry_operations
1242------------------------
1243
1244This describes how a filesystem can overload the standard dentry
1245operations.  Dentries and the dcache are the domain of the VFS and the
1246individual filesystem implementations.  Device drivers have no business
1247here.  These methods may be set to NULL, as they are either optional or
1248the VFS uses a default.  As of kernel 2.6.22, the following members are
1249defined:
1250
1251.. code-block:: c
1252
1253	struct dentry_operations {
1254		int (*d_revalidate)(struct dentry *, unsigned int);
1255		int (*d_weak_revalidate)(struct dentry *, unsigned int);
1256		int (*d_hash)(const struct dentry *, struct qstr *);
1257		int (*d_compare)(const struct dentry *,
1258				 unsigned int, const char *, const struct qstr *);
1259		int (*d_delete)(const struct dentry *);
1260		int (*d_init)(struct dentry *);
1261		void (*d_release)(struct dentry *);
1262		void (*d_iput)(struct dentry *, struct inode *);
1263		char *(*d_dname)(struct dentry *, char *, int);
1264		struct vfsmount *(*d_automount)(struct path *);
1265		int (*d_manage)(const struct path *, bool);
1266		struct dentry *(*d_real)(struct dentry *, enum d_real_type type);
1267	};
1268
1269``d_revalidate``
1270	called when the VFS needs to revalidate a dentry.  This is
1271	called whenever a name look-up finds a dentry in the dcache.
1272	Most local filesystems leave this as NULL, because all their
1273	dentries in the dcache are valid.  Network filesystems are
1274	different since things can change on the server without the
1275	client necessarily being aware of it.
1276
1277	This function should return a positive value if the dentry is
1278	still valid, and zero or a negative error code if it isn't.
1279
1280	d_revalidate may be called in rcu-walk mode (flags &
1281	LOOKUP_RCU).  If in rcu-walk mode, the filesystem must
1282	revalidate the dentry without blocking or storing to the dentry,
1283	d_parent and d_inode should not be used without care (because
1284	they can change and, in d_inode case, even become NULL under
1285	us).
1286
1287	If a situation is encountered that rcu-walk cannot handle,
1288	return
1289	-ECHILD and it will be called again in ref-walk mode.
1290
1291``d_weak_revalidate``
1292	called when the VFS needs to revalidate a "jumped" dentry.  This
1293	is called when a path-walk ends at dentry that was not acquired
1294	by doing a lookup in the parent directory.  This includes "/",
1295	"." and "..", as well as procfs-style symlinks and mountpoint
1296	traversal.
1297
1298	In this case, we are less concerned with whether the dentry is
1299	still fully correct, but rather that the inode is still valid.
1300	As with d_revalidate, most local filesystems will set this to
1301	NULL since their dcache entries are always valid.
1302
1303	This function has the same return code semantics as
1304	d_revalidate.
1305
1306	d_weak_revalidate is only called after leaving rcu-walk mode.
1307
1308``d_hash``
1309	called when the VFS adds a dentry to the hash table.  The first
1310	dentry passed to d_hash is the parent directory that the name is
1311	to be hashed into.
1312
1313	Same locking and synchronisation rules as d_compare regarding
1314	what is safe to dereference etc.
1315
1316``d_compare``
1317	called to compare a dentry name with a given name.  The first
1318	dentry is the parent of the dentry to be compared, the second is
1319	the child dentry.  len and name string are properties of the
1320	dentry to be compared.  qstr is the name to compare it with.
1321
1322	Must be constant and idempotent, and should not take locks if
1323	possible, and should not or store into the dentry.  Should not
1324	dereference pointers outside the dentry without lots of care
1325	(eg.  d_parent, d_inode, d_name should not be used).
1326
1327	However, our vfsmount is pinned, and RCU held, so the dentries
1328	and inodes won't disappear, neither will our sb or filesystem
1329	module.  ->d_sb may be used.
1330
1331	It is a tricky calling convention because it needs to be called
1332	under "rcu-walk", ie. without any locks or references on things.
1333
1334``d_delete``
1335	called when the last reference to a dentry is dropped and the
1336	dcache is deciding whether or not to cache it.  Return 1 to
1337	delete immediately, or 0 to cache the dentry.  Default is NULL
1338	which means to always cache a reachable dentry.  d_delete must
1339	be constant and idempotent.
1340
1341``d_init``
1342	called when a dentry is allocated
1343
1344``d_release``
1345	called when a dentry is really deallocated
1346
1347``d_iput``
1348	called when a dentry loses its inode (just prior to its being
1349	deallocated).  The default when this is NULL is that the VFS
1350	calls iput().  If you define this method, you must call iput()
1351	yourself
1352
1353``d_dname``
1354	called when the pathname of a dentry should be generated.
1355	Useful for some pseudo filesystems (sockfs, pipefs, ...) to
1356	delay pathname generation.  (Instead of doing it when dentry is
1357	created, it's done only when the path is needed.).  Real
1358	filesystems probably dont want to use it, because their dentries
1359	are present in global dcache hash, so their hash should be an
1360	invariant.  As no lock is held, d_dname() should not try to
1361	modify the dentry itself, unless appropriate SMP safety is used.
1362	CAUTION : d_path() logic is quite tricky.  The correct way to
1363	return for example "Hello" is to put it at the end of the
1364	buffer, and returns a pointer to the first char.
1365	dynamic_dname() helper function is provided to take care of
1366	this.
1367
1368	Example :
1369
1370.. code-block:: c
1371
1372	static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1373	{
1374		return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1375				dentry->d_inode->i_ino);
1376	}
1377
1378``d_automount``
1379	called when an automount dentry is to be traversed (optional).
1380	This should create a new VFS mount record and return the record
1381	to the caller.  The caller is supplied with a path parameter
1382	giving the automount directory to describe the automount target
1383	and the parent VFS mount record to provide inheritable mount
1384	parameters.  NULL should be returned if someone else managed to
1385	make the automount first.  If the vfsmount creation failed, then
1386	an error code should be returned.  If -EISDIR is returned, then
1387	the directory will be treated as an ordinary directory and
1388	returned to pathwalk to continue walking.
1389
1390	If a vfsmount is returned, the caller will attempt to mount it
1391	on the mountpoint and will remove the vfsmount from its
1392	expiration list in the case of failure.  The vfsmount should be
1393	returned with 2 refs on it to prevent automatic expiration - the
1394	caller will clean up the additional ref.
1395
1396	This function is only used if DCACHE_NEED_AUTOMOUNT is set on
1397	the dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is
1398	set on the inode being added.
1399
1400``d_manage``
1401	called to allow the filesystem to manage the transition from a
1402	dentry (optional).  This allows autofs, for example, to hold up
1403	clients waiting to explore behind a 'mountpoint' while letting
1404	the daemon go past and construct the subtree there.  0 should be
1405	returned to let the calling process continue.  -EISDIR can be
1406	returned to tell pathwalk to use this directory as an ordinary
1407	directory and to ignore anything mounted on it and not to check
1408	the automount flag.  Any other error code will abort pathwalk
1409	completely.
1410
1411	If the 'rcu_walk' parameter is true, then the caller is doing a
1412	pathwalk in RCU-walk mode.  Sleeping is not permitted in this
1413	mode, and the caller can be asked to leave it and call again by
1414	returning -ECHILD.  -EISDIR may also be returned to tell
1415	pathwalk to ignore d_automount or any mounts.
1416
1417	This function is only used if DCACHE_MANAGE_TRANSIT is set on
1418	the dentry being transited from.
1419
1420``d_real``
1421	overlay/union type filesystems implement this method to return one
1422	of the underlying dentries of a regular file hidden by the overlay.
1423
1424	The 'type' argument takes the values D_REAL_DATA or D_REAL_METADATA
1425	for returning the real underlying dentry that refers to the inode
1426	hosting the file's data or metadata respectively.
 
 
1427
1428	For non-regular files, the 'dentry' argument is returned.
1429
1430Each dentry has a pointer to its parent dentry, as well as a hash list
1431of child dentries.  Child dentries are basically like files in a
1432directory.
1433
1434
1435Directory Entry Cache API
1436--------------------------
1437
1438There are a number of functions defined which permit a filesystem to
1439manipulate dentries:
1440
1441``dget``
1442	open a new handle for an existing dentry (this just increments
1443	the usage count)
1444
1445``dput``
1446	close a handle for a dentry (decrements the usage count).  If
1447	the usage count drops to 0, and the dentry is still in its
1448	parent's hash, the "d_delete" method is called to check whether
1449	it should be cached.  If it should not be cached, or if the
1450	dentry is not hashed, it is deleted.  Otherwise cached dentries
1451	are put into an LRU list to be reclaimed on memory shortage.
1452
1453``d_drop``
1454	this unhashes a dentry from its parents hash list.  A subsequent
1455	call to dput() will deallocate the dentry if its usage count
1456	drops to 0
1457
1458``d_delete``
1459	delete a dentry.  If there are no other open references to the
1460	dentry then the dentry is turned into a negative dentry (the
1461	d_iput() method is called).  If there are other references, then
1462	d_drop() is called instead
1463
1464``d_add``
1465	add a dentry to its parents hash list and then calls
1466	d_instantiate()
1467
1468``d_instantiate``
1469	add a dentry to the alias hash list for the inode and updates
1470	the "d_inode" member.  The "i_count" member in the inode
1471	structure should be set/incremented.  If the inode pointer is
1472	NULL, the dentry is called a "negative dentry".  This function
1473	is commonly called when an inode is created for an existing
1474	negative dentry
1475
1476``d_lookup``
1477	look up a dentry given its parent and path name component It
1478	looks up the child of that given name from the dcache hash
1479	table.  If it is found, the reference count is incremented and
1480	the dentry is returned.  The caller must use dput() to free the
1481	dentry when it finishes using it.
1482
1483
1484Mount Options
1485=============
1486
1487
1488Parsing options
1489---------------
1490
1491On mount and remount the filesystem is passed a string containing a
1492comma separated list of mount options.  The options can have either of
1493these forms:
1494
1495  option
1496  option=value
1497
1498The <linux/parser.h> header defines an API that helps parse these
1499options.  There are plenty of examples on how to use it in existing
1500filesystems.
1501
1502
1503Showing options
1504---------------
1505
1506If a filesystem accepts mount options, it must define show_options() to
1507show all the currently active options.  The rules are:
1508
1509  - options MUST be shown which are not default or their values differ
1510    from the default
1511
1512  - options MAY be shown which are enabled by default or have their
1513    default value
1514
1515Options used only internally between a mount helper and the kernel (such
1516as file descriptors), or which only have an effect during the mounting
1517(such as ones controlling the creation of a journal) are exempt from the
1518above rules.
1519
1520The underlying reason for the above rules is to make sure, that a mount
1521can be accurately replicated (e.g. umounting and mounting again) based
1522on the information found in /proc/mounts.
1523
1524
1525Resources
1526=========
1527
1528(Note some of these resources are not up-to-date with the latest kernel
1529 version.)
1530
1531Creating Linux virtual filesystems. 2002
1532    <https://lwn.net/Articles/13325/>
1533
1534The Linux Virtual File-system Layer by Neil Brown. 1999
1535    <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1536
1537A tour of the Linux VFS by Michael K. Johnson. 1996
1538    <https://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1539
1540A small trail through the Linux kernel by Andries Brouwer. 2001
1541    <https://www.win.tue.nl/~aeb/linux/vfs/trail.html>