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 212For more information on mounting (and the new mount API), see 213Documentation/filesystems/mount_api.rst. 214 215The Superblock Object 216===================== 217 218A superblock object represents a mounted filesystem. 219 220 221struct super_operations 222----------------------- 223 224This describes how the VFS can manipulate the superblock of your 225filesystem. The following members are defined: 226 227.. code-block:: c 228 229 struct super_operations { 230 struct inode *(*alloc_inode)(struct super_block *sb); 231 void (*destroy_inode)(struct inode *); 232 void (*free_inode)(struct inode *); 233 234 void (*dirty_inode) (struct inode *, int flags); 235 int (*write_inode) (struct inode *, struct writeback_control *wbc); 236 int (*drop_inode) (struct inode *); 237 void (*evict_inode) (struct inode *); 238 void (*put_super) (struct super_block *); 239 int (*sync_fs)(struct super_block *sb, int wait); 240 int (*freeze_super) (struct super_block *sb, 241 enum freeze_holder who); 242 int (*freeze_fs) (struct super_block *); 243 int (*thaw_super) (struct super_block *sb, 244 enum freeze_wholder who); 245 int (*unfreeze_fs) (struct super_block *); 246 int (*statfs) (struct dentry *, struct kstatfs *); 247 int (*remount_fs) (struct super_block *, int *, char *); 248 void (*umount_begin) (struct super_block *); 249 250 int (*show_options)(struct seq_file *, struct dentry *); 251 int (*show_devname)(struct seq_file *, struct dentry *); 252 int (*show_path)(struct seq_file *, struct dentry *); 253 int (*show_stats)(struct seq_file *, struct dentry *); 254 255 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t); 256 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t); 257 struct dquot **(*get_dquots)(struct inode *); 258 259 long (*nr_cached_objects)(struct super_block *, 260 struct shrink_control *); 261 long (*free_cached_objects)(struct super_block *, 262 struct shrink_control *); 263 }; 264 265All methods are called without any locks being held, unless otherwise 266noted. This means that most methods can block safely. All methods are 267only called from a process context (i.e. not from an interrupt handler 268or bottom half). 269 270``alloc_inode`` 271 this method is called by alloc_inode() to allocate memory for 272 struct inode and initialize it. If this function is not 273 defined, a simple 'struct inode' is allocated. Normally 274 alloc_inode will be used to allocate a larger structure which 275 contains a 'struct inode' embedded within it. 276 277``destroy_inode`` 278 this method is called by destroy_inode() to release resources 279 allocated for struct inode. It is only required if 280 ->alloc_inode was defined and simply undoes anything done by 281 ->alloc_inode. 282 283``free_inode`` 284 this method is called from RCU callback. If you use call_rcu() 285 in ->destroy_inode to free 'struct inode' memory, then it's 286 better to release memory in this method. 287 288``dirty_inode`` 289 this method is called by the VFS when an inode is marked dirty. 290 This is specifically for the inode itself being marked dirty, 291 not its data. If the update needs to be persisted by fdatasync(), 292 then I_DIRTY_DATASYNC will be set in the flags argument. 293 I_DIRTY_TIME will be set in the flags in case lazytime is enabled 294 and struct inode has times updated since the last ->dirty_inode 295 call. 296 297``write_inode`` 298 this method is called when the VFS needs to write an inode to 299 disc. The second parameter indicates whether the write should 300 be synchronous or not, not all filesystems check this flag. 301 302``drop_inode`` 303 called when the last access to the inode is dropped, with the 304 inode->i_lock spinlock held. 305 306 This method should be either NULL (normal UNIX filesystem 307 semantics) or "inode_just_drop" (for filesystems that do 308 not want to cache inodes - causing "delete_inode" to always be 309 called regardless of the value of i_nlink) 310 311 The "inode_just_drop()" behavior is equivalent to the old 312 practice of using "force_delete" in the put_inode() case, but 313 does not have the races that the "force_delete()" approach had. 314 315``evict_inode`` 316 called when the VFS wants to evict an inode. Caller does 317 *not* evict the pagecache or inode-associated metadata buffers; 318 the method has to use truncate_inode_pages_final() to get rid 319 of those. Caller makes sure async writeback cannot be running for 320 the inode while (or after) ->evict_inode() is called. Optional. 321 322``put_super`` 323 called when the VFS wishes to free the superblock 324 (i.e. unmount). This is called with the superblock lock held 325 326``sync_fs`` 327 called when VFS is writing out all dirty data associated with a 328 superblock. The second parameter indicates whether the method 329 should wait until the write out has been completed. Optional. 330 331``freeze_super`` 332 Called instead of ->freeze_fs callback if provided. 333 Main difference is that ->freeze_super is called without taking 334 down_write(&sb->s_umount). If filesystem implements it and wants 335 ->freeze_fs to be called too, then it has to call ->freeze_fs 336 explicitly from this callback. Optional. 337 338``freeze_fs`` 339 called when VFS is locking a filesystem and forcing it into a 340 consistent state. This method is currently used by the Logical 341 Volume Manager (LVM) and ioctl(FIFREEZE). Optional. 342 343``thaw_super`` 344 called when VFS is unlocking a filesystem and making it writable 345 again after ->freeze_super. Optional. 346 347``unfreeze_fs`` 348 called when VFS is unlocking a filesystem and making it writable 349 again after ->freeze_fs. Optional. 350 351``statfs`` 352 called when the VFS needs to get filesystem statistics. 353 354``remount_fs`` 355 called when the filesystem is remounted. This is called with 356 the kernel lock held 357 358``umount_begin`` 359 called when the VFS is unmounting a filesystem. 360 361``show_options`` 362 called by the VFS to show mount options for /proc/<pid>/mounts 363 and /proc/<pid>/mountinfo. 364 (see "Mount Options" section) 365 366``show_devname`` 367 Optional. Called by the VFS to show device name for 368 /proc/<pid>/{mounts,mountinfo,mountstats}. If not provided then 369 '(struct mount).mnt_devname' will be used. 370 371``show_path`` 372 Optional. Called by the VFS (for /proc/<pid>/mountinfo) to show 373 the mount root dentry path relative to the filesystem root. 374 375``show_stats`` 376 Optional. Called by the VFS (for /proc/<pid>/mountstats) to show 377 filesystem-specific mount statistics. 378 379``quota_read`` 380 called by the VFS to read from filesystem quota file. 381 382``quota_write`` 383 called by the VFS to write to filesystem quota file. 384 385``get_dquots`` 386 called by quota to get 'struct dquot' array for a particular inode. 387 Optional. 388 389``nr_cached_objects`` 390 called by the sb cache shrinking function for the filesystem to 391 return the number of freeable cached objects it contains. 392 Optional. 393 394``free_cache_objects`` 395 called by the sb cache shrinking function for the filesystem to 396 scan the number of objects indicated to try to free them. 397 Optional, but any filesystem implementing this method needs to 398 also implement ->nr_cached_objects for it to be called 399 correctly. 400 401 We can't do anything with any errors that the filesystem might 402 encountered, hence the void return type. This will never be 403 called if the VM is trying to reclaim under GFP_NOFS conditions, 404 hence this method does not need to handle that situation itself. 405 406 Implementations must include conditional reschedule calls inside 407 any scanning loop that is done. This allows the VFS to 408 determine appropriate scan batch sizes without having to worry 409 about whether implementations will cause holdoff problems due to 410 large scan batch sizes. 411 412Whoever sets up the inode is responsible for filling in the "i_op" 413field. This is a pointer to a "struct inode_operations" which describes 414the methods that can be performed on individual inodes. 415 416 417struct xattr_handler 418--------------------- 419 420On filesystems that support extended attributes (xattrs), the s_xattr 421superblock field points to a NULL-terminated array of xattr handlers. 422Extended attributes are name:value pairs. 423 424``name`` 425 Indicates that the handler matches attributes with the specified 426 name (such as "system.posix_acl_access"); the prefix field must 427 be NULL. 428 429``prefix`` 430 Indicates that the handler matches all attributes with the 431 specified name prefix (such as "user."); the name field must be 432 NULL. 433 434``list`` 435 Determine if attributes matching this xattr handler should be 436 listed for a particular dentry. Used by some listxattr 437 implementations like generic_listxattr. 438 439``get`` 440 Called by the VFS to get the value of a particular extended 441 attribute. This method is called by the getxattr(2) system 442 call. 443 444``set`` 445 Called by the VFS to set the value of a particular extended 446 attribute. When the new value is NULL, called to remove a 447 particular extended attribute. This method is called by the 448 setxattr(2) and removexattr(2) system calls. 449 450When none of the xattr handlers of a filesystem match the specified 451attribute name or when a filesystem doesn't support extended attributes, 452the various ``*xattr(2)`` system calls return -EOPNOTSUPP. 453 454 455The Inode Object 456================ 457 458An inode object represents an object within the filesystem. 459 460 461struct inode_operations 462----------------------- 463 464This describes how the VFS can manipulate an inode in your filesystem. 465As of kernel 2.6.22, the following members are defined: 466 467.. code-block:: c 468 469 struct inode_operations { 470 int (*create) (struct mnt_idmap *, struct inode *,struct dentry *, umode_t, bool); 471 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int); 472 int (*link) (struct dentry *,struct inode *,struct dentry *); 473 int (*unlink) (struct inode *,struct dentry *); 474 int (*symlink) (struct mnt_idmap *, struct inode *,struct dentry *,const char *); 475 struct dentry *(*mkdir) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t); 476 int (*rmdir) (struct inode *,struct dentry *); 477 int (*mknod) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t,dev_t); 478 int (*rename) (struct mnt_idmap *, struct inode *, struct dentry *, 479 struct inode *, struct dentry *, unsigned int); 480 int (*readlink) (struct dentry *, char __user *,int); 481 const char *(*get_link) (struct dentry *, struct inode *, 482 struct delayed_call *); 483 int (*permission) (struct mnt_idmap *, struct inode *, int); 484 struct posix_acl * (*get_inode_acl)(struct inode *, int, bool); 485 int (*setattr) (struct mnt_idmap *, struct dentry *, struct iattr *); 486 int (*getattr) (struct mnt_idmap *, const struct path *, struct kstat *, u32, unsigned int); 487 ssize_t (*listxattr) (struct dentry *, char *, size_t); 488 void (*update_time)(struct inode *, struct timespec *, int); 489 int (*atomic_open)(struct inode *, struct dentry *, struct file *, 490 unsigned open_flag, umode_t create_mode); 491 int (*tmpfile) (struct mnt_idmap *, struct inode *, struct file *, umode_t); 492 struct posix_acl * (*get_acl)(struct mnt_idmap *, struct dentry *, int); 493 int (*set_acl)(struct mnt_idmap *, struct dentry *, struct posix_acl *, int); 494 int (*fileattr_set)(struct mnt_idmap *idmap, 495 struct dentry *dentry, struct file_kattr *fa); 496 int (*fileattr_get)(struct dentry *dentry, struct file_kattr *fa); 497 struct offset_ctx *(*get_offset_ctx)(struct inode *inode); 498 }; 499 500Again, all methods are called without any locks being held, unless 501otherwise noted. 502 503``create`` 504 called by the open(2) and creat(2) system calls. Only required 505 if you want to support regular files. The dentry you get should 506 not have an inode (i.e. it should be a negative dentry). Here 507 you will probably call d_instantiate() with the dentry and the 508 newly created inode 509 510``lookup`` 511 called when the VFS needs to look up an inode in a parent 512 directory. The name to look for is found in the dentry. This 513 method must call d_add() to insert the found inode into the 514 dentry. The "i_count" field in the inode structure should be 515 incremented. If the named inode does not exist a NULL inode 516 should be inserted into the dentry (this is called a negative 517 dentry). Returning an error code from this routine must only be 518 done on a real error, otherwise creating inodes with system 519 calls like create(2), mknod(2), mkdir(2) and so on will fail. 520 If you wish to overload the dentry methods then you should 521 initialise the "d_dop" field in the dentry; this is a pointer to 522 a struct "dentry_operations". This method is called with the 523 directory inode semaphore held 524 525``link`` 526 called by the link(2) system call. Only required if you want to 527 support hard links. You will probably need to call 528 d_instantiate() just as you would in the create() method 529 530``unlink`` 531 called by the unlink(2) system call. Only required if you want 532 to support deleting inodes 533 534``symlink`` 535 called by the symlink(2) system call. Only required if you want 536 to support symlinks. You will probably need to call 537 d_instantiate() just as you would in the create() method 538 539``mkdir`` 540 called by the mkdir(2) system call. Only required if you want 541 to support creating subdirectories. You will probably need to 542 call d_instantiate_new() just as you would in the create() method. 543 544 If d_instantiate_new() is not used and if the fh_to_dentry() 545 export operation is provided, or if the storage might be 546 accessible by another path (e.g. with a network filesystem) 547 then more care may be needed. Importantly d_instantate() 548 should not be used with an inode that is no longer I_NEW if there 549 any chance that the inode could already be attached to a dentry. 550 This is because of a hard rule in the VFS that a directory must 551 only ever have one dentry. 552 553 For example, if an NFS filesystem is mounted twice the new directory 554 could be visible on the other mount before it is on the original 555 mount, and a pair of name_to_handle_at(), open_by_handle_at() 556 calls could instantiate the directory inode with an IS_ROOT() 557 dentry before the first mkdir returns. 558 559 If there is any chance this could happen, then the new inode 560 should be d_drop()ed and attached with d_splice_alias(). The 561 returned dentry (if any) should be returned by ->mkdir(). 562 563``rmdir`` 564 called by the rmdir(2) system call. Only required if you want 565 to support deleting subdirectories 566 567``mknod`` 568 called by the mknod(2) system call to create a device (char, 569 block) inode or a named pipe (FIFO) or socket. Only required if 570 you want to support creating these types of inodes. You will 571 probably need to call d_instantiate() just as you would in the 572 create() method 573 574``rename`` 575 called by the rename(2) system call to rename the object to have 576 the parent and name given by the second inode and dentry. 577 578 The filesystem must return -EINVAL for any unsupported or 579 unknown flags. Currently the following flags are implemented: 580 (1) RENAME_NOREPLACE: this flag indicates that if the target of 581 the rename exists the rename should fail with -EEXIST instead of 582 replacing the target. The VFS already checks for existence, so 583 for local filesystems the RENAME_NOREPLACE implementation is 584 equivalent to plain rename. 585 (2) RENAME_EXCHANGE: exchange source and target. Both must 586 exist; this is checked by the VFS. Unlike plain rename, source 587 and target may be of different type. 588 589``get_link`` 590 called by the VFS to follow a symbolic link to the inode it 591 points to. Only required if you want to support symbolic links. 592 This method returns the symlink body to traverse (and possibly 593 resets the current position with nd_jump_link()). If the body 594 won't go away until the inode is gone, nothing else is needed; 595 if it needs to be otherwise pinned, arrange for its release by 596 having get_link(..., ..., done) do set_delayed_call(done, 597 destructor, argument). In that case destructor(argument) will 598 be called once VFS is done with the body you've returned. May 599 be called in RCU mode; that is indicated by NULL dentry 600 argument. If request can't be handled without leaving RCU mode, 601 have it return ERR_PTR(-ECHILD). 602 603 If the filesystem stores the symlink target in ->i_link, the 604 VFS may use it directly without calling ->get_link(); however, 605 ->get_link() must still be provided. ->i_link must not be 606 freed until after an RCU grace period. Writing to ->i_link 607 post-iget() time requires a 'release' memory barrier. 608 609``readlink`` 610 this is now just an override for use by readlink(2) for the 611 cases when ->get_link uses nd_jump_link() or object is not in 612 fact a symlink. Normally filesystems should only implement 613 ->get_link for symlinks and readlink(2) will automatically use 614 that. 615 616``permission`` 617 called by the VFS to check for access rights on a POSIX-like 618 filesystem. 619 620 May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in 621 rcu-walk mode, the filesystem must check the permission without 622 blocking or storing to the inode. 623 624 If a situation is encountered that rcu-walk cannot handle, 625 return 626 -ECHILD and it will be called again in ref-walk mode. 627 628``setattr`` 629 called by the VFS to set attributes for a file. This method is 630 called by chmod(2) and related system calls. 631 632``getattr`` 633 called by the VFS to get attributes of a file. This method is 634 called by stat(2) and related system calls. 635 636``listxattr`` 637 called by the VFS to list all extended attributes for a given 638 file. This method is called by the listxattr(2) system call. 639 640``update_time`` 641 called by the VFS to update a specific time or the i_version of 642 an inode. If this is not defined the VFS will update the inode 643 itself and call mark_inode_dirty_sync. 644 645``atomic_open`` 646 called on the last component of an open. Using this optional 647 method the filesystem can look up, possibly create and open the 648 file in one atomic operation. If it wants to leave actual 649 opening to the caller (e.g. if the file turned out to be a 650 symlink, device, or just something filesystem won't do atomic 651 open for), it may signal this by returning finish_no_open(file, 652 dentry). This method is only called if the last component is 653 negative or needs lookup. Cached positive dentries are still 654 handled by f_op->open(). If the file was created, FMODE_CREATED 655 flag should be set in file->f_mode. In case of O_EXCL the 656 method must only succeed if the file didn't exist and hence 657 FMODE_CREATED shall always be set on success. 658 659``tmpfile`` 660 called in the end of O_TMPFILE open(). Optional, equivalent to 661 atomically creating, opening and unlinking a file in given 662 directory. On success needs to return with the file already 663 open; this can be done by calling finish_open_simple() right at 664 the end. 665 666``fileattr_get`` 667 called on ioctl(FS_IOC_GETFLAGS) and ioctl(FS_IOC_FSGETXATTR) to 668 retrieve miscellaneous file flags and attributes. Also called 669 before the relevant SET operation to check what is being changed 670 (in this case with i_rwsem locked exclusive). If unset, then 671 fall back to f_op->ioctl(). 672 673``fileattr_set`` 674 called on ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_FSSETXATTR) to 675 change miscellaneous file flags and attributes. Callers hold 676 i_rwsem exclusive. If unset, then fall back to f_op->ioctl(). 677``get_offset_ctx`` 678 called to get the offset context for a directory inode. A 679 filesystem must define this operation to use 680 simple_offset_dir_operations. 681 682The Address Space Object 683======================== 684 685The address space object is used to group and manage pages in the page 686cache. It can be used to keep track of the pages in a file (or anything 687else) and also track the mapping of sections of the file into process 688address spaces. 689 690There are a number of distinct yet related services that an 691address-space can provide. These include communicating memory pressure, 692page lookup by address, and keeping track of pages tagged as Dirty or 693Writeback. 694 695The first can be used independently to the others. The VM can try to 696release clean pages in order to reuse them. To do this it can call 697->release_folio on clean folios with the private 698flag set. Clean pages without PagePrivate and with no external references 699will be released without notice being given to the address_space. 700 701To achieve this functionality, pages need to be placed on an LRU with 702lru_cache_add and mark_page_active needs to be called whenever the page 703is used. 704 705Pages are normally kept in a radix tree index by ->index. This tree 706maintains information about the PG_Dirty and PG_Writeback status of each 707page, so that pages with either of these flags can be found quickly. 708 709The Dirty tag is primarily used by mpage_writepages - the default 710->writepages method. It uses the tag to find dirty pages to 711write back. If mpage_writepages is not used (i.e. the address 712provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is almost 713unused. write_inode_now and sync_inode do use it (through 714__sync_single_inode) to check if ->writepages has been successful in 715writing out the whole address_space. 716 717The Writeback tag is used by filemap*wait* and sync_page* functions, via 718filemap_fdatawait_range, to wait for all writeback to complete. 719 720An address_space handler may attach extra information to a page, 721typically using the 'private' field in the 'struct page'. If such 722information is attached, the PG_Private flag should be set. This will 723cause various VM routines to make extra calls into the address_space 724handler to deal with that data. 725 726An address space acts as an intermediate between storage and 727application. Data is read into the address space a whole page at a 728time, and provided to the application either by copying of the page, or 729by memory-mapping the page. Data is written into the address space by 730the application, and then written-back to storage typically in whole 731pages, however the address_space has finer control of write sizes. 732 733The read process essentially only requires 'read_folio'. The write 734process is more complicated and uses write_begin/write_end or 735dirty_folio to write data into the address_space, and 736writepages to writeback data to storage. 737 738Removing pages from an address_space requires holding the inode's i_rwsem 739exclusively, while adding pages to the address_space requires holding the 740inode's i_mapping->invalidate_lock exclusively. 741 742When data is written to a page, the PG_Dirty flag should be set. It 743typically remains set until writepages asks for it to be written. This 744should clear PG_Dirty and set PG_Writeback. It can be actually written 745at any point after PG_Dirty is clear. Once it is known to be safe, 746PG_Writeback is cleared. 747 748Writeback makes use of a writeback_control structure to direct the 749operations. This gives the writepages operation some 750information about the nature of and reason for the writeback request, 751and the constraints under which it is being done. It is also used to 752return information back to the caller about the result of a 753writepages request. 754 755 756Handling errors during writeback 757-------------------------------- 758 759Most applications that do buffered I/O will periodically call a file 760synchronization call (fsync, fdatasync, msync or sync_file_range) to 761ensure that data written has made it to the backing store. When there 762is an error during writeback, they expect that error to be reported when 763a file sync request is made. After an error has been reported on one 764request, subsequent requests on the same file descriptor should return 7650, unless further writeback errors have occurred since the previous file 766synchronization. 767 768Ideally, the kernel would report errors only on file descriptions on 769which writes were done that subsequently failed to be written back. The 770generic pagecache infrastructure does not track the file descriptions 771that have dirtied each individual page however, so determining which 772file descriptors should get back an error is not possible. 773 774Instead, the generic writeback error tracking infrastructure in the 775kernel settles for reporting errors to fsync on all file descriptions 776that were open at the time that the error occurred. In a situation with 777multiple writers, all of them will get back an error on a subsequent 778fsync, even if all of the writes done through that particular file 779descriptor succeeded (or even if there were no writes on that file 780descriptor at all). 781 782Filesystems that wish to use this infrastructure should call 783mapping_set_error to record the error in the address_space when it 784occurs. Then, after writing back data from the pagecache in their 785file->fsync operation, they should call file_check_and_advance_wb_err to 786ensure that the struct file's error cursor has advanced to the correct 787point in the stream of errors emitted by the backing device(s). 788 789 790struct address_space_operations 791------------------------------- 792 793This describes how the VFS can manipulate mapping of a file to page 794cache in your filesystem. The following members are defined: 795 796.. code-block:: c 797 798 struct address_space_operations { 799 int (*read_folio)(struct file *, struct folio *); 800 int (*writepages)(struct address_space *, struct writeback_control *); 801 bool (*dirty_folio)(struct address_space *, struct folio *); 802 void (*readahead)(struct readahead_control *); 803 int (*write_begin)(const struct kiocb *, struct address_space *mapping, 804 loff_t pos, unsigned len, 805 struct page **pagep, void **fsdata); 806 int (*write_end)(const struct kiocb *, struct address_space *mapping, 807 loff_t pos, unsigned len, unsigned copied, 808 struct folio *folio, void *fsdata); 809 sector_t (*bmap)(struct address_space *, sector_t); 810 void (*invalidate_folio) (struct folio *, size_t start, size_t len); 811 bool (*release_folio)(struct folio *, gfp_t); 812 void (*free_folio)(struct folio *); 813 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter); 814 int (*migrate_folio)(struct mapping *, struct folio *dst, 815 struct folio *src, enum migrate_mode); 816 int (*launder_folio) (struct folio *); 817 818 bool (*is_partially_uptodate) (struct folio *, size_t from, 819 size_t count); 820 void (*is_dirty_writeback)(struct folio *, bool *, bool *); 821 int (*error_remove_folio)(struct mapping *mapping, struct folio *); 822 int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span) 823 int (*swap_deactivate)(struct file *); 824 int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter); 825 }; 826 827``read_folio`` 828 Called by the page cache to read a folio from the backing store. 829 The 'file' argument supplies authentication information to network 830 filesystems, and is generally not used by block based filesystems. 831 It may be NULL if the caller does not have an open file (eg if 832 the kernel is performing a read for itself rather than on behalf 833 of a userspace process with an open file). 834 835 If the mapping does not support large folios, the folio will 836 contain a single page. The folio will be locked when read_folio 837 is called. If the read completes successfully, the folio should 838 be marked uptodate. The filesystem should unlock the folio 839 once the read has completed, whether it was successful or not. 840 The filesystem does not need to modify the refcount on the folio; 841 the page cache holds a reference count and that will not be 842 released until the folio is unlocked. 843 844 Filesystems may implement ->read_folio() synchronously. 845 In normal operation, folios are read through the ->readahead() 846 method. Only if this fails, or if the caller needs to wait for 847 the read to complete will the page cache call ->read_folio(). 848 Filesystems should not attempt to perform their own readahead 849 in the ->read_folio() operation. 850 851 If the filesystem cannot perform the read at this time, it can 852 unlock the folio, do whatever action it needs to ensure that the 853 read will succeed in the future and return AOP_TRUNCATED_PAGE. 854 In this case, the caller should look up the folio, lock it, 855 and call ->read_folio again. 856 857 Callers may invoke the ->read_folio() method directly, but using 858 read_mapping_folio() will take care of locking, waiting for the 859 read to complete and handle cases such as AOP_TRUNCATED_PAGE. 860 861``writepages`` 862 called by the VM to write out pages associated with the 863 address_space object. If wbc->sync_mode is WB_SYNC_ALL, then 864 the writeback_control will specify a range of pages that must be 865 written out. If it is WB_SYNC_NONE, then a nr_to_write is 866 given and that many pages should be written if possible. If no 867 ->writepages is given, then mpage_writepages is used instead. 868 This will choose pages from the address space that are tagged as 869 DIRTY and will write them back. 870 871``dirty_folio`` 872 called by the VM to mark a folio as dirty. This is particularly 873 needed if an address space attaches private data to a folio, and 874 that data needs to be updated when a folio is dirtied. This is 875 called, for example, when a memory mapped page gets modified. 876 If defined, it should set the folio dirty flag, and the 877 PAGECACHE_TAG_DIRTY search mark in i_pages. 878 879``readahead`` 880 Called by the VM to read pages associated with the address_space 881 object. The pages are consecutive in the page cache and are 882 locked. The implementation should decrement the page refcount 883 after starting I/O on each page. Usually the page will be 884 unlocked by the I/O completion handler. The set of pages are 885 divided into some sync pages followed by some async pages, 886 rac->ra->async_size gives the number of async pages. The 887 filesystem should attempt to read all sync pages but may decide 888 to stop once it reaches the async pages. If it does decide to 889 stop attempting I/O, it can simply return. The caller will 890 remove the remaining pages from the address space, unlock them 891 and decrement the page refcount. Set PageUptodate if the I/O 892 completes successfully. 893 894``write_begin`` 895 Called by the generic buffered write code to ask the filesystem 896 to prepare to write len bytes at the given offset in the file. 897 The address_space should check that the write will be able to 898 complete, by allocating space if necessary and doing any other 899 internal housekeeping. If the write will update parts of any 900 basic-blocks on storage, then those blocks should be pre-read 901 (if they haven't been read already) so that the updated blocks 902 can be written out properly. 903 904 The filesystem must return the locked pagecache folio for the 905 specified offset, in ``*foliop``, for the caller to write into. 906 907 It must be able to cope with short writes (where the length 908 passed to write_begin is greater than the number of bytes copied 909 into the folio). 910 911 A void * may be returned in fsdata, which then gets passed into 912 write_end. 913 914 Returns 0 on success; < 0 on failure (which is the error code), 915 in which case write_end is not called. 916 917``write_end`` 918 After a successful write_begin, and data copy, write_end must be 919 called. len is the original len passed to write_begin, and 920 copied is the amount that was able to be copied. 921 922 The filesystem must take care of unlocking the folio, 923 decrementing its refcount, and updating i_size. 924 925 Returns < 0 on failure, otherwise the number of bytes (<= 926 'copied') that were able to be copied into pagecache. 927 928``bmap`` 929 called by the VFS to map a logical block offset within object to 930 physical block number. This method is used by the FIBMAP ioctl 931 and for working with swap-files. To be able to swap to a file, 932 the file must have a stable mapping to a block device. The swap 933 system does not go through the filesystem but instead uses bmap 934 to find out where the blocks in the file are and uses those 935 addresses directly. 936 937``invalidate_folio`` 938 If a folio has private data, then invalidate_folio will be 939 called when part or all of the folio is to be removed from the 940 address space. This generally corresponds to either a 941 truncation, punch hole or a complete invalidation of the address 942 space (in the latter case 'offset' will always be 0 and 'length' 943 will be folio_size()). Any private data associated with the folio 944 should be updated to reflect this truncation. If offset is 0 945 and length is folio_size(), then the private data should be 946 released, because the folio must be able to be completely 947 discarded. This may be done by calling the ->release_folio 948 function, but in this case the release MUST succeed. 949 950``release_folio`` 951 release_folio is called on folios with private data to tell the 952 filesystem that the folio is about to be freed. ->release_folio 953 should remove any private data from the folio and clear the 954 private flag. If release_folio() fails, it should return false. 955 release_folio() is used in two distinct though related cases. 956 The first is when the VM wants to free a clean folio with no 957 active users. If ->release_folio succeeds, the folio will be 958 removed from the address_space and be freed. 959 960 The second case is when a request has been made to invalidate 961 some or all folios in an address_space. This can happen 962 through the fadvise(POSIX_FADV_DONTNEED) system call or by the 963 filesystem explicitly requesting it as nfs and 9p do (when they 964 believe the cache may be out of date with storage) by calling 965 invalidate_inode_pages2(). If the filesystem makes such a call, 966 and needs to be certain that all folios are invalidated, then 967 its release_folio will need to ensure this. Possibly it can 968 clear the uptodate flag if it cannot free private data yet. 969 970``free_folio`` 971 free_folio is called once the folio is no longer visible in the 972 page cache in order to allow the cleanup of any private data. 973 Since it may be called by the memory reclaimer, it should not 974 assume that the original address_space mapping still exists, and 975 it should not block. 976 977``direct_IO`` 978 called by the generic read/write routines to perform direct_IO - 979 that is IO requests which bypass the page cache and transfer 980 data directly between the storage and the application's address 981 space. 982 983``migrate_folio`` 984 This is used to compact the physical memory usage. If the VM 985 wants to relocate a folio (maybe from a memory device that is 986 signalling imminent failure) it will pass a new folio and an old 987 folio to this function. migrate_folio should transfer any private 988 data across and update any references that it has to the folio. 989 990``launder_folio`` 991 Called before freeing a folio - it writes back the dirty folio. 992 To prevent redirtying the folio, it is kept locked during the 993 whole operation. 994 995``is_partially_uptodate`` 996 Called by the VM when reading a file through the pagecache when 997 the underlying blocksize is smaller than the size of the folio. 998 If the required block is up to date then the read can complete 999 without needing I/O to bring the whole page up to date. 1000 1001``is_dirty_writeback`` 1002 Called by the VM when attempting to reclaim a folio. The VM uses 1003 dirty and writeback information to determine if it needs to 1004 stall to allow flushers a chance to complete some IO. 1005 Ordinarily it can use folio_test_dirty and folio_test_writeback but 1006 some filesystems have more complex state (unstable folios in NFS 1007 prevent reclaim) or do not set those flags due to locking 1008 problems. This callback allows a filesystem to indicate to the 1009 VM if a folio should be treated as dirty or writeback for the 1010 purposes of stalling. 1011 1012``error_remove_folio`` 1013 normally set to generic_error_remove_folio if truncation is ok 1014 for this address space. Used for memory failure handling. 1015 Setting this implies you deal with pages going away under you, 1016 unless you have them locked or reference counts increased. 1017 1018``swap_activate`` 1019 1020 Called to prepare the given file for swap. It should perform 1021 any validation and preparation necessary to ensure that writes 1022 can be performed with minimal memory allocation. It should call 1023 add_swap_extent(), or the helper iomap_swapfile_activate(), and 1024 return the number of extents added. If IO should be submitted 1025 through ->swap_rw(), it should set SWP_FS_OPS, otherwise IO will 1026 be submitted directly to the block device ``sis->bdev``. 1027 1028``swap_deactivate`` 1029 Called during swapoff on files where swap_activate was 1030 successful. 1031 1032``swap_rw`` 1033 Called to read or write swap pages when SWP_FS_OPS is set. 1034 1035The File Object 1036=============== 1037 1038A file object represents a file opened by a process. This is also known 1039as an "open file description" in POSIX parlance. 1040 1041 1042struct file_operations 1043---------------------- 1044 1045This describes how the VFS can manipulate an open file. As of kernel 10464.18, the following members are defined: 1047 1048.. code-block:: c 1049 1050 struct file_operations { 1051 struct module *owner; 1052 fop_flags_t fop_flags; 1053 loff_t (*llseek) (struct file *, loff_t, int); 1054 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); 1055 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); 1056 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *); 1057 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *); 1058 int (*iopoll)(struct kiocb *kiocb, struct io_comp_batch *, 1059 unsigned int flags); 1060 int (*iterate_shared) (struct file *, struct dir_context *); 1061 __poll_t (*poll) (struct file *, struct poll_table_struct *); 1062 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); 1063 long (*compat_ioctl) (struct file *, unsigned int, unsigned long); 1064 int (*mmap) (struct file *, struct vm_area_struct *); 1065 int (*open) (struct inode *, struct file *); 1066 int (*flush) (struct file *, fl_owner_t id); 1067 int (*release) (struct inode *, struct file *); 1068 int (*fsync) (struct file *, loff_t, loff_t, int datasync); 1069 int (*fasync) (int, struct file *, int); 1070 int (*lock) (struct file *, int, struct file_lock *); 1071 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long); 1072 int (*check_flags)(int); 1073 int (*flock) (struct file *, int, struct file_lock *); 1074 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int); 1075 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int); 1076 void (*splice_eof)(struct file *file); 1077 int (*setlease)(struct file *, int, struct file_lease **, void **); 1078 long (*fallocate)(struct file *file, int mode, loff_t offset, 1079 loff_t len); 1080 void (*show_fdinfo)(struct seq_file *m, struct file *f); 1081 #ifndef CONFIG_MMU 1082 unsigned (*mmap_capabilities)(struct file *); 1083 #endif 1084 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, 1085 loff_t, size_t, unsigned int); 1086 loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in, 1087 struct file *file_out, loff_t pos_out, 1088 loff_t len, unsigned int remap_flags); 1089 int (*fadvise)(struct file *, loff_t, loff_t, int); 1090 int (*uring_cmd)(struct io_uring_cmd *ioucmd, unsigned int issue_flags); 1091 int (*uring_cmd_iopoll)(struct io_uring_cmd *, struct io_comp_batch *, 1092 unsigned int poll_flags); 1093 int (*mmap_prepare)(struct vm_area_desc *); 1094 }; 1095 1096Again, all methods are called without any locks being held, unless 1097otherwise noted. 1098 1099``llseek`` 1100 called when the VFS needs to move the file position index 1101 1102``read`` 1103 called by read(2) and related system calls 1104 1105``read_iter`` 1106 possibly asynchronous read with iov_iter as destination 1107 1108``write`` 1109 called by write(2) and related system calls 1110 1111``write_iter`` 1112 possibly asynchronous write with iov_iter as source 1113 1114``iopoll`` 1115 called when aio wants to poll for completions on HIPRI iocbs 1116 1117``iterate_shared`` 1118 called when the VFS needs to read the directory contents 1119 1120``poll`` 1121 called by the VFS when a process wants to check if there is 1122 activity on this file and (optionally) go to sleep until there 1123 is activity. Called by the select(2) and poll(2) system calls 1124 1125``unlocked_ioctl`` 1126 called by the ioctl(2) system call. 1127 1128``compat_ioctl`` 1129 called by the ioctl(2) system call when 32 bit system calls are 1130 used on 64 bit kernels. 1131 1132``mmap`` 1133 called by the mmap(2) system call. Deprecated in favour of 1134 ``mmap_prepare``. 1135 1136``open`` 1137 called by the VFS when an inode should be opened. When the VFS 1138 opens a file, it creates a new "struct file". It then calls the 1139 open method for the newly allocated file structure. You might 1140 think that the open method really belongs in "struct 1141 inode_operations", and you may be right. I think it's done the 1142 way it is because it makes filesystems simpler to implement. 1143 The open() method is a good place to initialize the 1144 "private_data" member in the file structure if you want to point 1145 to a device structure 1146 1147``flush`` 1148 called by the close(2) system call to flush a file 1149 1150``release`` 1151 called when the last reference to an open file is closed 1152 1153``fsync`` 1154 called by the fsync(2) system call. Also see the section above 1155 entitled "Handling errors during writeback". 1156 1157``fasync`` 1158 called by the fcntl(2) system call when asynchronous 1159 (non-blocking) mode is enabled for a file 1160 1161``lock`` 1162 called by the fcntl(2) system call for F_GETLK, F_SETLK, and 1163 F_SETLKW commands 1164 1165``get_unmapped_area`` 1166 called by the mmap(2) system call 1167 1168``check_flags`` 1169 called by the fcntl(2) system call for F_SETFL command 1170 1171``flock`` 1172 called by the flock(2) system call 1173 1174``splice_write`` 1175 called by the VFS to splice data from a pipe to a file. This 1176 method is used by the splice(2) system call 1177 1178``splice_read`` 1179 called by the VFS to splice data from file to a pipe. This 1180 method is used by the splice(2) system call 1181 1182``setlease`` 1183 called by the VFS to set or release a file lock lease. setlease 1184 implementations should call generic_setlease to record or remove 1185 the lease in the inode after setting it. 1186 1187``fallocate`` 1188 called by the VFS to preallocate blocks or punch a hole. 1189 1190``copy_file_range`` 1191 called by the copy_file_range(2) system call. 1192 1193``remap_file_range`` 1194 called by the ioctl(2) system call for FICLONERANGE and FICLONE 1195 and FIDEDUPERANGE commands to remap file ranges. An 1196 implementation should remap len bytes at pos_in of the source 1197 file into the dest file at pos_out. Implementations must handle 1198 callers passing in len == 0; this means "remap to the end of the 1199 source file". The return value should the number of bytes 1200 remapped, or the usual negative error code if errors occurred 1201 before any bytes were remapped. The remap_flags parameter 1202 accepts REMAP_FILE_* flags. If REMAP_FILE_DEDUP is set then the 1203 implementation must only remap if the requested file ranges have 1204 identical contents. If REMAP_FILE_CAN_SHORTEN is set, the caller is 1205 ok with the implementation shortening the request length to 1206 satisfy alignment or EOF requirements (or any other reason). 1207 1208``fadvise`` 1209 possibly called by the fadvise64() system call. 1210 1211``mmap_prepare`` 1212 Called by the mmap(2) system call. Allows a VFS to set up a 1213 file-backed memory mapping, most notably establishing relevant 1214 private state and VMA callbacks. 1215 1216Note that the file operations are implemented by the specific 1217filesystem in which the inode resides. When opening a device node 1218(character or block special) most filesystems will call special 1219support routines in the VFS which will locate the required device 1220driver information. These support routines replace the filesystem file 1221operations with those for the device driver, and then proceed to call 1222the new open() method for the file. This is how opening a device file 1223in the filesystem eventually ends up calling the device driver open() 1224method. 1225 1226 1227Directory Entry Cache (dcache) 1228============================== 1229 1230 1231struct dentry_operations 1232------------------------ 1233 1234This describes how a filesystem can overload the standard dentry 1235operations. Dentries and the dcache are the domain of the VFS and the 1236individual filesystem implementations. Device drivers have no business 1237here. These methods may be set to NULL, as they are either optional or 1238the VFS uses a default. As of kernel 2.6.22, the following members are 1239defined: 1240 1241.. code-block:: c 1242 1243 struct dentry_operations { 1244 int (*d_revalidate)(struct inode *, const struct qstr *, 1245 struct dentry *, unsigned int); 1246 int (*d_weak_revalidate)(struct dentry *, unsigned int); 1247 int (*d_hash)(const struct dentry *, struct qstr *); 1248 int (*d_compare)(const struct dentry *, 1249 unsigned int, const char *, const struct qstr *); 1250 int (*d_delete)(const struct dentry *); 1251 int (*d_init)(struct dentry *); 1252 void (*d_release)(struct dentry *); 1253 void (*d_iput)(struct dentry *, struct inode *); 1254 char *(*d_dname)(struct dentry *, char *, int); 1255 struct vfsmount *(*d_automount)(struct path *); 1256 int (*d_manage)(const struct path *, bool); 1257 struct dentry *(*d_real)(struct dentry *, enum d_real_type type); 1258 bool (*d_unalias_trylock)(const struct dentry *); 1259 void (*d_unalias_unlock)(const struct dentry *); 1260 }; 1261 1262``d_revalidate`` 1263 called when the VFS needs to revalidate a dentry. This is 1264 called whenever a name look-up finds a dentry in the dcache. 1265 Most local filesystems leave this as NULL, because all their 1266 dentries in the dcache are valid. Network filesystems are 1267 different since things can change on the server without the 1268 client necessarily being aware of it. 1269 1270 This function should return a positive value if the dentry is 1271 still valid, and zero or a negative error code if it isn't. 1272 1273 d_revalidate may be called in rcu-walk mode (flags & 1274 LOOKUP_RCU). If in rcu-walk mode, the filesystem must 1275 revalidate the dentry without blocking or storing to the dentry, 1276 d_parent and d_inode should not be used without care (because 1277 they can change and, in d_inode case, even become NULL under 1278 us). 1279 1280 If a situation is encountered that rcu-walk cannot handle, 1281 return 1282 -ECHILD and it will be called again in ref-walk mode. 1283 1284``d_weak_revalidate`` 1285 called when the VFS needs to revalidate a "jumped" dentry. This 1286 is called when a path-walk ends at dentry that was not acquired 1287 by doing a lookup in the parent directory. This includes "/", 1288 "." and "..", as well as procfs-style symlinks and mountpoint 1289 traversal. 1290 1291 In this case, we are less concerned with whether the dentry is 1292 still fully correct, but rather that the inode is still valid. 1293 As with d_revalidate, most local filesystems will set this to 1294 NULL since their dcache entries are always valid. 1295 1296 This function has the same return code semantics as 1297 d_revalidate. 1298 1299 d_weak_revalidate is only called after leaving rcu-walk mode. 1300 1301``d_hash`` 1302 called when the VFS adds a dentry to the hash table. The first 1303 dentry passed to d_hash is the parent directory that the name is 1304 to be hashed into. 1305 1306 Same locking and synchronisation rules as d_compare regarding 1307 what is safe to dereference etc. 1308 1309``d_compare`` 1310 called to compare a dentry name with a given name. The first 1311 dentry is the parent of the dentry to be compared, the second is 1312 the child dentry. len and name string are properties of the 1313 dentry to be compared. qstr is the name to compare it with. 1314 1315 Must be constant and idempotent, and should not take locks if 1316 possible, and should not or store into the dentry. Should not 1317 dereference pointers outside the dentry without lots of care 1318 (eg. d_parent, d_inode, d_name should not be used). 1319 1320 However, our vfsmount is pinned, and RCU held, so the dentries 1321 and inodes won't disappear, neither will our sb or filesystem 1322 module. ->d_sb may be used. 1323 1324 It is a tricky calling convention because it needs to be called 1325 under "rcu-walk", ie. without any locks or references on things. 1326 1327``d_delete`` 1328 called when the last reference to a dentry is dropped and the 1329 dcache is deciding whether or not to cache it. Return 1 to 1330 delete immediately, or 0 to cache the dentry. Default is NULL 1331 which means to always cache a reachable dentry. d_delete must 1332 be constant and idempotent. 1333 1334``d_init`` 1335 called when a dentry is allocated 1336 1337``d_release`` 1338 called when a dentry is really deallocated 1339 1340``d_iput`` 1341 called when a dentry loses its inode (just prior to its being 1342 deallocated). The default when this is NULL is that the VFS 1343 calls iput(). If you define this method, you must call iput() 1344 yourself 1345 1346``d_dname`` 1347 called when the pathname of a dentry should be generated. 1348 Useful for some pseudo filesystems (sockfs, pipefs, ...) to 1349 delay pathname generation. (Instead of doing it when dentry is 1350 created, it's done only when the path is needed.). Real 1351 filesystems probably dont want to use it, because their dentries 1352 are present in global dcache hash, so their hash should be an 1353 invariant. As no lock is held, d_dname() should not try to 1354 modify the dentry itself, unless appropriate SMP safety is used. 1355 CAUTION : d_path() logic is quite tricky. The correct way to 1356 return for example "Hello" is to put it at the end of the 1357 buffer, and returns a pointer to the first char. 1358 dynamic_dname() helper function is provided to take care of 1359 this. 1360 1361 Example : 1362 1363.. code-block:: c 1364 1365 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen) 1366 { 1367 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]", 1368 dentry->d_inode->i_ino); 1369 } 1370 1371``d_automount`` 1372 called when an automount dentry is to be traversed (optional). 1373 This should create a new VFS mount record and return the record 1374 to the caller. The caller is supplied with a path parameter 1375 giving the automount directory to describe the automount target 1376 and the parent VFS mount record to provide inheritable mount 1377 parameters. NULL should be returned if someone else managed to 1378 make the automount first. If the vfsmount creation failed, then 1379 an error code should be returned. If -EISDIR is returned, then 1380 the directory will be treated as an ordinary directory and 1381 returned to pathwalk to continue walking. 1382 1383 If a vfsmount is returned, the caller will attempt to mount it 1384 on the mountpoint and will remove the vfsmount from its 1385 expiration list in the case of failure. 1386 1387 This function is only used if DCACHE_NEED_AUTOMOUNT is set on 1388 the dentry. This is set by __d_instantiate() if S_AUTOMOUNT is 1389 set on the inode being added. 1390 1391``d_manage`` 1392 called to allow the filesystem to manage the transition from a 1393 dentry (optional). This allows autofs, for example, to hold up 1394 clients waiting to explore behind a 'mountpoint' while letting 1395 the daemon go past and construct the subtree there. 0 should be 1396 returned to let the calling process continue. -EISDIR can be 1397 returned to tell pathwalk to use this directory as an ordinary 1398 directory and to ignore anything mounted on it and not to check 1399 the automount flag. Any other error code will abort pathwalk 1400 completely. 1401 1402 If the 'rcu_walk' parameter is true, then the caller is doing a 1403 pathwalk in RCU-walk mode. Sleeping is not permitted in this 1404 mode, and the caller can be asked to leave it and call again by 1405 returning -ECHILD. -EISDIR may also be returned to tell 1406 pathwalk to ignore d_automount or any mounts. 1407 1408 This function is only used if DCACHE_MANAGE_TRANSIT is set on 1409 the dentry being transited from. 1410 1411``d_real`` 1412 overlay/union type filesystems implement this method to return one 1413 of the underlying dentries of a regular file hidden by the overlay. 1414 1415 The 'type' argument takes the values D_REAL_DATA or D_REAL_METADATA 1416 for returning the real underlying dentry that refers to the inode 1417 hosting the file's data or metadata respectively. 1418 1419 For non-regular files, the 'dentry' argument is returned. 1420 1421``d_unalias_trylock`` 1422 if present, will be called by d_splice_alias() before moving a 1423 preexisting attached alias. Returning false prevents __d_move(), 1424 making d_splice_alias() fail with -ESTALE. 1425 1426 Rationale: setting FS_RENAME_DOES_D_MOVE will prevent d_move() 1427 and d_exchange() calls from the outside of filesystem methods; 1428 however, it does not guarantee that attached dentries won't 1429 be renamed or moved by d_splice_alias() finding a preexisting 1430 alias for a directory inode. Normally we would not care; 1431 however, something that wants to stabilize the entire path to 1432 root over a blocking operation might need that. See 9p for one 1433 (and hopefully only) example. 1434 1435``d_unalias_unlock`` 1436 should be paired with ``d_unalias_trylock``; that one is called after 1437 __d_move() call in __d_unalias(). 1438 1439 1440Each dentry has a pointer to its parent dentry, as well as a hash list 1441of child dentries. Child dentries are basically like files in a 1442directory. 1443 1444 1445Directory Entry Cache API 1446-------------------------- 1447 1448There are a number of functions defined which permit a filesystem to 1449manipulate dentries: 1450 1451``dget`` 1452 open a new handle for an existing dentry (this just increments 1453 the usage count) 1454 1455``dput`` 1456 close a handle for a dentry (decrements the usage count). If 1457 the usage count drops to 0, and the dentry is still in its 1458 parent's hash, the "d_delete" method is called to check whether 1459 it should be cached. If it should not be cached, or if the 1460 dentry is not hashed, it is deleted. Otherwise cached dentries 1461 are put into an LRU list to be reclaimed on memory shortage. 1462 1463``d_drop`` 1464 this unhashes a dentry from its parents hash list. A subsequent 1465 call to dput() will deallocate the dentry if its usage count 1466 drops to 0 1467 1468``d_delete`` 1469 delete a dentry. If there are no other open references to the 1470 dentry then the dentry is turned into a negative dentry (the 1471 d_iput() method is called). If there are other references, then 1472 d_drop() is called instead 1473 1474``d_add`` 1475 add a dentry to its parents hash list and then calls 1476 d_instantiate() 1477 1478``d_instantiate`` 1479 add a dentry to the alias hash list for the inode and updates 1480 the "d_inode" member. The "i_count" member in the inode 1481 structure should be set/incremented. If the inode pointer is 1482 NULL, the dentry is called a "negative dentry". This function 1483 is commonly called when an inode is created for an existing 1484 negative dentry 1485 1486``d_lookup`` 1487 look up a dentry given its parent and path name component It 1488 looks up the child of that given name from the dcache hash 1489 table. If it is found, the reference count is incremented and 1490 the dentry is returned. The caller must use dput() to free the 1491 dentry when it finishes using it. 1492 1493 1494Mount Options 1495============= 1496 1497 1498Parsing options 1499--------------- 1500 1501On mount and remount the filesystem is passed a string containing a 1502comma separated list of mount options. The options can have either of 1503these forms: 1504 1505 option 1506 option=value 1507 1508The <linux/parser.h> header defines an API that helps parse these 1509options. There are plenty of examples on how to use it in existing 1510filesystems. 1511 1512 1513Showing options 1514--------------- 1515 1516If a filesystem accepts mount options, it must define show_options() to 1517show all the currently active options. The rules are: 1518 1519 - options MUST be shown which are not default or their values differ 1520 from the default 1521 1522 - options MAY be shown which are enabled by default or have their 1523 default value 1524 1525Options used only internally between a mount helper and the kernel (such 1526as file descriptors), or which only have an effect during the mounting 1527(such as ones controlling the creation of a journal) are exempt from the 1528above rules. 1529 1530The underlying reason for the above rules is to make sure, that a mount 1531can be accurately replicated (e.g. umounting and mounting again) based 1532on the information found in /proc/mounts. 1533 1534 1535Resources 1536========= 1537 1538(Note some of these resources are not up-to-date with the latest kernel 1539 version.) 1540 1541Creating Linux virtual filesystems. 2002 1542 <https://lwn.net/Articles/13325/> 1543 1544The Linux Virtual File-system Layer by Neil Brown. 1999 1545 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html> 1546 1547A tour of the Linux VFS by Michael K. Johnson. 1996 1548 <https://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html> 1549 1550A small trail through the Linux kernel by Andries Brouwer. 2001 1551 <https://www.win.tue.nl/~aeb/linux/vfs/trail.html> 1552