1 2 Overview of the Linux Virtual File System 3 4 Original author: Richard Gooch <rgooch@atnf.csiro.au> 5 6 Last updated on June 24, 2007. 7 8 Copyright (C) 1999 Richard Gooch 9 Copyright (C) 2005 Pekka Enberg 10 11 This file is released under the GPLv2. 12 13 14Introduction 15============ 16 17The Virtual File System (also known as the Virtual Filesystem Switch) 18is the software layer in the kernel that provides the filesystem 19interface to userspace programs. It also provides an abstraction 20within the kernel which allows different filesystem implementations to 21coexist. 22 23VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so 24on are called from a process context. Filesystem locking is described 25in the document Documentation/filesystems/Locking. 26 27 28Directory Entry Cache (dcache) 29------------------------------ 30 31The VFS implements the open(2), stat(2), chmod(2), and similar system 32calls. The pathname argument that is passed to them is used by the VFS 33to search through the directory entry cache (also known as the dentry 34cache or dcache). This provides a very fast look-up mechanism to 35translate a pathname (filename) into a specific dentry. Dentries live 36in RAM and are never saved to disc: they exist only for performance. 37 38The dentry cache is meant to be a view into your entire filespace. As 39most computers cannot fit all dentries in the RAM at the same time, 40some bits of the cache are missing. In order to resolve your pathname 41into a dentry, the VFS may have to resort to creating dentries along 42the way, and then loading the inode. This is done by looking up the 43inode. 44 45 46The Inode Object 47---------------- 48 49An individual dentry usually has a pointer to an inode. Inodes are 50filesystem objects such as regular files, directories, FIFOs and other 51beasts. They live either on the disc (for block device filesystems) 52or in the memory (for pseudo filesystems). Inodes that live on the 53disc are copied into the memory when required and changes to the inode 54are written back to disc. A single inode can be pointed to by multiple 55dentries (hard links, for example, do this). 56 57To look up an inode requires that the VFS calls the lookup() method of 58the parent directory inode. This method is installed by the specific 59filesystem implementation that the inode lives in. Once the VFS has 60the required dentry (and hence the inode), we can do all those boring 61things like open(2) the file, or stat(2) it to peek at the inode 62data. The stat(2) operation is fairly simple: once the VFS has the 63dentry, it peeks at the inode data and passes some of it back to 64userspace. 65 66 67The File Object 68--------------- 69 70Opening a file requires another operation: allocation of a file 71structure (this is the kernel-side implementation of file 72descriptors). The freshly allocated file structure is initialized with 73a pointer to the dentry and a set of file operation member functions. 74These are taken from the inode data. The open() file method is then 75called so the specific filesystem implementation can do its work. You 76can see that this is another switch performed by the VFS. The file 77structure is placed into the file descriptor table for the process. 78 79Reading, writing and closing files (and other assorted VFS operations) 80is done by using the userspace file descriptor to grab the appropriate 81file structure, and then calling the required file structure method to 82do whatever is required. For as long as the file is open, it keeps the 83dentry in use, which in turn means that the VFS inode is still in use. 84 85 86Registering and Mounting a Filesystem 87===================================== 88 89To register and unregister a filesystem, use the following API 90functions: 91 92 #include <linux/fs.h> 93 94 extern int register_filesystem(struct file_system_type *); 95 extern int unregister_filesystem(struct file_system_type *); 96 97The passed struct file_system_type describes your filesystem. When a 98request is made to mount a filesystem onto a directory in your namespace, 99the VFS will call the appropriate mount() method for the specific 100filesystem. New vfsmount referring to the tree returned by ->mount() 101will be attached to the mountpoint, so that when pathname resolution 102reaches the mountpoint it will jump into the root of that vfsmount. 103 104You can see all filesystems that are registered to the kernel in the 105file /proc/filesystems. 106 107 108struct file_system_type 109----------------------- 110 111This describes the filesystem. As of kernel 2.6.39, the following 112members are defined: 113 114struct file_system_type { 115 const char *name; 116 int fs_flags; 117 struct dentry *(*mount) (struct file_system_type *, int, 118 const char *, void *); 119 void (*kill_sb) (struct super_block *); 120 struct module *owner; 121 struct file_system_type * next; 122 struct list_head fs_supers; 123 struct lock_class_key s_lock_key; 124 struct lock_class_key s_umount_key; 125}; 126 127 name: the name of the filesystem type, such as "ext2", "iso9660", 128 "msdos" and so on 129 130 fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.) 131 132 mount: the method to call when a new instance of this 133 filesystem should be mounted 134 135 kill_sb: the method to call when an instance of this filesystem 136 should be shut down 137 138 owner: for internal VFS use: you should initialize this to THIS_MODULE in 139 most cases. 140 141 next: for internal VFS use: you should initialize this to NULL 142 143 s_lock_key, s_umount_key: lockdep-specific 144 145The mount() method has the following arguments: 146 147 struct file_system_type *fs_type: describes the filesystem, partly initialized 148 by the specific filesystem code 149 150 int flags: mount flags 151 152 const char *dev_name: the device name we are mounting. 153 154 void *data: arbitrary mount options, usually comes as an ASCII 155 string (see "Mount Options" section) 156 157The mount() method must return the root dentry of the tree requested by 158caller. An active reference to its superblock must be grabbed and the 159superblock must be locked. On failure it should return ERR_PTR(error). 160 161The arguments match those of mount(2) and their interpretation 162depends on filesystem type. E.g. for block filesystems, dev_name is 163interpreted as block device name, that device is opened and if it 164contains a suitable filesystem image the method creates and initializes 165struct super_block accordingly, returning its root dentry to caller. 166 167->mount() may choose to return a subtree of existing filesystem - it 168doesn't have to create a new one. The main result from the caller's 169point of view is a reference to dentry at the root of (sub)tree to 170be attached; creation of new superblock is a common side effect. 171 172The most interesting member of the superblock structure that the 173mount() method fills in is the "s_op" field. This is a pointer to 174a "struct super_operations" which describes the next level of the 175filesystem implementation. 176 177Usually, a filesystem uses one of the generic mount() implementations 178and provides a fill_super() callback instead. The generic variants are: 179 180 mount_bdev: mount a filesystem residing on a block device 181 182 mount_nodev: mount a filesystem that is not backed by a device 183 184 mount_single: mount a filesystem which shares the instance between 185 all mounts 186 187A fill_super() callback implementation has the following arguments: 188 189 struct super_block *sb: the superblock structure. The callback 190 must initialize this properly. 191 192 void *data: arbitrary mount options, usually comes as an ASCII 193 string (see "Mount Options" section) 194 195 int silent: whether or not to be silent on error 196 197 198The Superblock Object 199===================== 200 201A superblock object represents a mounted filesystem. 202 203 204struct super_operations 205----------------------- 206 207This describes how the VFS can manipulate the superblock of your 208filesystem. As of kernel 2.6.22, the following members are defined: 209 210struct super_operations { 211 struct inode *(*alloc_inode)(struct super_block *sb); 212 void (*destroy_inode)(struct inode *); 213 214 void (*dirty_inode) (struct inode *, int flags); 215 int (*write_inode) (struct inode *, int); 216 void (*drop_inode) (struct inode *); 217 void (*delete_inode) (struct inode *); 218 void (*put_super) (struct super_block *); 219 int (*sync_fs)(struct super_block *sb, int wait); 220 int (*freeze_fs) (struct super_block *); 221 int (*unfreeze_fs) (struct super_block *); 222 int (*statfs) (struct dentry *, struct kstatfs *); 223 int (*remount_fs) (struct super_block *, int *, char *); 224 void (*clear_inode) (struct inode *); 225 void (*umount_begin) (struct super_block *); 226 227 int (*show_options)(struct seq_file *, struct dentry *); 228 229 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t); 230 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t); 231 int (*nr_cached_objects)(struct super_block *); 232 void (*free_cached_objects)(struct super_block *, int); 233}; 234 235All methods are called without any locks being held, unless otherwise 236noted. This means that most methods can block safely. All methods are 237only called from a process context (i.e. not from an interrupt handler 238or bottom half). 239 240 alloc_inode: this method is called by alloc_inode() to allocate memory 241 for struct inode and initialize it. If this function is not 242 defined, a simple 'struct inode' is allocated. Normally 243 alloc_inode will be used to allocate a larger structure which 244 contains a 'struct inode' embedded within it. 245 246 destroy_inode: this method is called by destroy_inode() to release 247 resources allocated for struct inode. It is only required if 248 ->alloc_inode was defined and simply undoes anything done by 249 ->alloc_inode. 250 251 dirty_inode: this method is called by the VFS to mark an inode dirty. 252 253 write_inode: this method is called when the VFS needs to write an 254 inode to disc. The second parameter indicates whether the write 255 should be synchronous or not, not all filesystems check this flag. 256 257 drop_inode: called when the last access to the inode is dropped, 258 with the inode->i_lock spinlock held. 259 260 This method should be either NULL (normal UNIX filesystem 261 semantics) or "generic_delete_inode" (for filesystems that do not 262 want to cache inodes - causing "delete_inode" to always be 263 called regardless of the value of i_nlink) 264 265 The "generic_delete_inode()" behavior is equivalent to the 266 old practice of using "force_delete" in the put_inode() case, 267 but does not have the races that the "force_delete()" approach 268 had. 269 270 delete_inode: called when the VFS wants to delete an inode 271 272 put_super: called when the VFS wishes to free the superblock 273 (i.e. unmount). This is called with the superblock lock held 274 275 sync_fs: called when VFS is writing out all dirty data associated with 276 a superblock. The second parameter indicates whether the method 277 should wait until the write out has been completed. Optional. 278 279 freeze_fs: called when VFS is locking a filesystem and 280 forcing it into a consistent state. This method is currently 281 used by the Logical Volume Manager (LVM). 282 283 unfreeze_fs: called when VFS is unlocking a filesystem and making it writable 284 again. 285 286 statfs: called when the VFS needs to get filesystem statistics. 287 288 remount_fs: called when the filesystem is remounted. This is called 289 with the kernel lock held 290 291 clear_inode: called then the VFS clears the inode. Optional 292 293 umount_begin: called when the VFS is unmounting a filesystem. 294 295 show_options: called by the VFS to show mount options for 296 /proc/<pid>/mounts. (see "Mount Options" section) 297 298 quota_read: called by the VFS to read from filesystem quota file. 299 300 quota_write: called by the VFS to write to filesystem quota file. 301 302 nr_cached_objects: called by the sb cache shrinking function for the 303 filesystem to return the number of freeable cached objects it contains. 304 Optional. 305 306 free_cache_objects: called by the sb cache shrinking function for the 307 filesystem to scan the number of objects indicated to try to free them. 308 Optional, but any filesystem implementing this method needs to also 309 implement ->nr_cached_objects for it to be called correctly. 310 311 We can't do anything with any errors that the filesystem might 312 encountered, hence the void return type. This will never be called if 313 the VM is trying to reclaim under GFP_NOFS conditions, hence this 314 method does not need to handle that situation itself. 315 316 Implementations must include conditional reschedule calls inside any 317 scanning loop that is done. This allows the VFS to determine 318 appropriate scan batch sizes without having to worry about whether 319 implementations will cause holdoff problems due to large scan batch 320 sizes. 321 322Whoever sets up the inode is responsible for filling in the "i_op" field. This 323is a pointer to a "struct inode_operations" which describes the methods that 324can be performed on individual inodes. 325 326struct xattr_handlers 327--------------------- 328 329On filesystems that support extended attributes (xattrs), the s_xattr 330superblock field points to a NULL-terminated array of xattr handlers. Extended 331attributes are name:value pairs. 332 333 name: Indicates that the handler matches attributes with the specified name 334 (such as "system.posix_acl_access"); the prefix field must be NULL. 335 336 prefix: Indicates that the handler matches all attributes with the specified 337 name prefix (such as "user."); the name field must be NULL. 338 339 list: Determine if attributes matching this xattr handler should be listed 340 for a particular dentry. Used by some listxattr implementations like 341 generic_listxattr. 342 343 get: Called by the VFS to get the value of a particular extended attribute. 344 This method is called by the getxattr(2) system call. 345 346 set: Called by the VFS to set the value of a particular extended attribute. 347 When the new value is NULL, called to remove a particular extended 348 attribute. This method is called by the the setxattr(2) and 349 removexattr(2) system calls. 350 351When none of the xattr handlers of a filesystem match the specified attribute 352name or when a filesystem doesn't support extended attributes, the various 353*xattr(2) system calls return -EOPNOTSUPP. 354 355 356The Inode Object 357================ 358 359An inode object represents an object within the filesystem. 360 361 362struct inode_operations 363----------------------- 364 365This describes how the VFS can manipulate an inode in your 366filesystem. As of kernel 2.6.22, the following members are defined: 367 368struct inode_operations { 369 int (*create) (struct inode *,struct dentry *, umode_t, bool); 370 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int); 371 int (*link) (struct dentry *,struct inode *,struct dentry *); 372 int (*unlink) (struct inode *,struct dentry *); 373 int (*symlink) (struct inode *,struct dentry *,const char *); 374 int (*mkdir) (struct inode *,struct dentry *,umode_t); 375 int (*rmdir) (struct inode *,struct dentry *); 376 int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t); 377 int (*rename) (struct inode *, struct dentry *, 378 struct inode *, struct dentry *, unsigned int); 379 int (*readlink) (struct dentry *, char __user *,int); 380 const char *(*get_link) (struct dentry *, struct inode *, 381 struct delayed_call *); 382 int (*permission) (struct inode *, int); 383 int (*get_acl)(struct inode *, int); 384 int (*setattr) (struct dentry *, struct iattr *); 385 int (*getattr) (const struct path *, struct kstat *, u32, unsigned int); 386 ssize_t (*listxattr) (struct dentry *, char *, size_t); 387 void (*update_time)(struct inode *, struct timespec *, int); 388 int (*atomic_open)(struct inode *, struct dentry *, struct file *, 389 unsigned open_flag, umode_t create_mode); 390 int (*tmpfile) (struct inode *, struct dentry *, umode_t); 391}; 392 393Again, all methods are called without any locks being held, unless 394otherwise noted. 395 396 create: called by the open(2) and creat(2) system calls. Only 397 required if you want to support regular files. The dentry you 398 get should not have an inode (i.e. it should be a negative 399 dentry). Here you will probably call d_instantiate() with the 400 dentry and the newly created inode 401 402 lookup: called when the VFS needs to look up an inode in a parent 403 directory. The name to look for is found in the dentry. This 404 method must call d_add() to insert the found inode into the 405 dentry. The "i_count" field in the inode structure should be 406 incremented. If the named inode does not exist a NULL inode 407 should be inserted into the dentry (this is called a negative 408 dentry). Returning an error code from this routine must only 409 be done on a real error, otherwise creating inodes with system 410 calls like create(2), mknod(2), mkdir(2) and so on will fail. 411 If you wish to overload the dentry methods then you should 412 initialise the "d_dop" field in the dentry; this is a pointer 413 to a struct "dentry_operations". 414 This method is called with the directory inode semaphore held 415 416 link: called by the link(2) system call. Only required if you want 417 to support hard links. You will probably need to call 418 d_instantiate() just as you would in the create() method 419 420 unlink: called by the unlink(2) system call. Only required if you 421 want to support deleting inodes 422 423 symlink: called by the symlink(2) system call. Only required if you 424 want to support symlinks. You will probably need to call 425 d_instantiate() just as you would in the create() method 426 427 mkdir: called by the mkdir(2) system call. Only required if you want 428 to support creating subdirectories. You will probably need to 429 call d_instantiate() just as you would in the create() method 430 431 rmdir: called by the rmdir(2) system call. Only required if you want 432 to support deleting subdirectories 433 434 mknod: called by the mknod(2) system call to create a device (char, 435 block) inode or a named pipe (FIFO) or socket. Only required 436 if you want to support creating these types of inodes. You 437 will probably need to call d_instantiate() just as you would 438 in the create() method 439 440 rename: called by the rename(2) system call to rename the object to 441 have the parent and name given by the second inode and dentry. 442 443 The filesystem must return -EINVAL for any unsupported or 444 unknown flags. Currently the following flags are implemented: 445 (1) RENAME_NOREPLACE: this flag indicates that if the target 446 of the rename exists the rename should fail with -EEXIST 447 instead of replacing the target. The VFS already checks for 448 existence, so for local filesystems the RENAME_NOREPLACE 449 implementation is equivalent to plain rename. 450 (2) RENAME_EXCHANGE: exchange source and target. Both must 451 exist; this is checked by the VFS. Unlike plain rename, 452 source and target may be of different type. 453 454 get_link: called by the VFS to follow a symbolic link to the 455 inode it points to. Only required if you want to support 456 symbolic links. This method returns the symlink body 457 to traverse (and possibly resets the current position with 458 nd_jump_link()). If the body won't go away until the inode 459 is gone, nothing else is needed; if it needs to be otherwise 460 pinned, arrange for its release by having get_link(..., ..., done) 461 do set_delayed_call(done, destructor, argument). 462 In that case destructor(argument) will be called once VFS is 463 done with the body you've returned. 464 May be called in RCU mode; that is indicated by NULL dentry 465 argument. If request can't be handled without leaving RCU mode, 466 have it return ERR_PTR(-ECHILD). 467 468 readlink: this is now just an override for use by readlink(2) for the 469 cases when ->get_link uses nd_jump_link() or object is not in 470 fact a symlink. Normally filesystems should only implement 471 ->get_link for symlinks and readlink(2) will automatically use 472 that. 473 474 permission: called by the VFS to check for access rights on a POSIX-like 475 filesystem. 476 477 May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk 478 mode, the filesystem must check the permission without blocking or 479 storing to the inode. 480 481 If a situation is encountered that rcu-walk cannot handle, return 482 -ECHILD and it will be called again in ref-walk mode. 483 484 setattr: called by the VFS to set attributes for a file. This method 485 is called by chmod(2) and related system calls. 486 487 getattr: called by the VFS to get attributes of a file. This method 488 is called by stat(2) and related system calls. 489 490 listxattr: called by the VFS to list all extended attributes for a 491 given file. This method is called by the listxattr(2) system call. 492 493 update_time: called by the VFS to update a specific time or the i_version of 494 an inode. If this is not defined the VFS will update the inode itself 495 and call mark_inode_dirty_sync. 496 497 atomic_open: called on the last component of an open. Using this optional 498 method the filesystem can look up, possibly create and open the file in 499 one atomic operation. If it wants to leave actual opening to the 500 caller (e.g. if the file turned out to be a symlink, device, or just 501 something filesystem won't do atomic open for), it may signal this by 502 returning finish_no_open(file, dentry). This method is only called if 503 the last component is negative or needs lookup. Cached positive dentries 504 are still handled by f_op->open(). If the file was created, 505 FMODE_CREATED flag should be set in file->f_mode. In case of O_EXCL 506 the method must only succeed if the file didn't exist and hence FMODE_CREATED 507 shall always be set on success. 508 509 tmpfile: called in the end of O_TMPFILE open(). Optional, equivalent to 510 atomically creating, opening and unlinking a file in given directory. 511 512The Address Space Object 513======================== 514 515The address space object is used to group and manage pages in the page 516cache. It can be used to keep track of the pages in a file (or 517anything else) and also track the mapping of sections of the file into 518process address spaces. 519 520There are a number of distinct yet related services that an 521address-space can provide. These include communicating memory 522pressure, page lookup by address, and keeping track of pages tagged as 523Dirty or Writeback. 524 525The first can be used independently to the others. The VM can try to 526either write dirty pages in order to clean them, or release clean 527pages in order to reuse them. To do this it can call the ->writepage 528method on dirty pages, and ->releasepage on clean pages with 529PagePrivate set. Clean pages without PagePrivate and with no external 530references will be released without notice being given to the 531address_space. 532 533To achieve this functionality, pages need to be placed on an LRU with 534lru_cache_add and mark_page_active needs to be called whenever the 535page is used. 536 537Pages are normally kept in a radix tree index by ->index. This tree 538maintains information about the PG_Dirty and PG_Writeback status of 539each page, so that pages with either of these flags can be found 540quickly. 541 542The Dirty tag is primarily used by mpage_writepages - the default 543->writepages method. It uses the tag to find dirty pages to call 544->writepage on. If mpage_writepages is not used (i.e. the address 545provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is 546almost unused. write_inode_now and sync_inode do use it (through 547__sync_single_inode) to check if ->writepages has been successful in 548writing out the whole address_space. 549 550The Writeback tag is used by filemap*wait* and sync_page* functions, 551via filemap_fdatawait_range, to wait for all writeback to complete. 552 553An address_space handler may attach extra information to a page, 554typically using the 'private' field in the 'struct page'. If such 555information is attached, the PG_Private flag should be set. This will 556cause various VM routines to make extra calls into the address_space 557handler to deal with that data. 558 559An address space acts as an intermediate between storage and 560application. Data is read into the address space a whole page at a 561time, and provided to the application either by copying of the page, 562or by memory-mapping the page. 563Data is written into the address space by the application, and then 564written-back to storage typically in whole pages, however the 565address_space has finer control of write sizes. 566 567The read process essentially only requires 'readpage'. The write 568process is more complicated and uses write_begin/write_end or 569set_page_dirty to write data into the address_space, and writepage 570and writepages to writeback data to storage. 571 572Adding and removing pages to/from an address_space is protected by the 573inode's i_mutex. 574 575When data is written to a page, the PG_Dirty flag should be set. It 576typically remains set until writepage asks for it to be written. This 577should clear PG_Dirty and set PG_Writeback. It can be actually 578written at any point after PG_Dirty is clear. Once it is known to be 579safe, PG_Writeback is cleared. 580 581Writeback makes use of a writeback_control structure to direct the 582operations. This gives the the writepage and writepages operations some 583information about the nature of and reason for the writeback request, 584and the constraints under which it is being done. It is also used to 585return information back to the caller about the result of a writepage or 586writepages request. 587 588Handling errors during writeback 589-------------------------------- 590Most applications that do buffered I/O will periodically call a file 591synchronization call (fsync, fdatasync, msync or sync_file_range) to 592ensure that data written has made it to the backing store. When there 593is an error during writeback, they expect that error to be reported when 594a file sync request is made. After an error has been reported on one 595request, subsequent requests on the same file descriptor should return 5960, unless further writeback errors have occurred since the previous file 597syncronization. 598 599Ideally, the kernel would report errors only on file descriptions on 600which writes were done that subsequently failed to be written back. The 601generic pagecache infrastructure does not track the file descriptions 602that have dirtied each individual page however, so determining which 603file descriptors should get back an error is not possible. 604 605Instead, the generic writeback error tracking infrastructure in the 606kernel settles for reporting errors to fsync on all file descriptions 607that were open at the time that the error occurred. In a situation with 608multiple writers, all of them will get back an error on a subsequent fsync, 609even if all of the writes done through that particular file descriptor 610succeeded (or even if there were no writes on that file descriptor at all). 611 612Filesystems that wish to use this infrastructure should call 613mapping_set_error to record the error in the address_space when it 614occurs. Then, after writing back data from the pagecache in their 615file->fsync operation, they should call file_check_and_advance_wb_err to 616ensure that the struct file's error cursor has advanced to the correct 617point in the stream of errors emitted by the backing device(s). 618 619struct address_space_operations 620------------------------------- 621 622This describes how the VFS can manipulate mapping of a file to page cache in 623your filesystem. The following members are defined: 624 625struct address_space_operations { 626 int (*writepage)(struct page *page, struct writeback_control *wbc); 627 int (*readpage)(struct file *, struct page *); 628 int (*writepages)(struct address_space *, struct writeback_control *); 629 int (*set_page_dirty)(struct page *page); 630 int (*readpages)(struct file *filp, struct address_space *mapping, 631 struct list_head *pages, unsigned nr_pages); 632 int (*write_begin)(struct file *, struct address_space *mapping, 633 loff_t pos, unsigned len, unsigned flags, 634 struct page **pagep, void **fsdata); 635 int (*write_end)(struct file *, struct address_space *mapping, 636 loff_t pos, unsigned len, unsigned copied, 637 struct page *page, void *fsdata); 638 sector_t (*bmap)(struct address_space *, sector_t); 639 void (*invalidatepage) (struct page *, unsigned int, unsigned int); 640 int (*releasepage) (struct page *, int); 641 void (*freepage)(struct page *); 642 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter); 643 /* isolate a page for migration */ 644 bool (*isolate_page) (struct page *, isolate_mode_t); 645 /* migrate the contents of a page to the specified target */ 646 int (*migratepage) (struct page *, struct page *); 647 /* put migration-failed page back to right list */ 648 void (*putback_page) (struct page *); 649 int (*launder_page) (struct page *); 650 651 int (*is_partially_uptodate) (struct page *, unsigned long, 652 unsigned long); 653 void (*is_dirty_writeback) (struct page *, bool *, bool *); 654 int (*error_remove_page) (struct mapping *mapping, struct page *page); 655 int (*swap_activate)(struct file *); 656 int (*swap_deactivate)(struct file *); 657}; 658 659 writepage: called by the VM to write a dirty page to backing store. 660 This may happen for data integrity reasons (i.e. 'sync'), or 661 to free up memory (flush). The difference can be seen in 662 wbc->sync_mode. 663 The PG_Dirty flag has been cleared and PageLocked is true. 664 writepage should start writeout, should set PG_Writeback, 665 and should make sure the page is unlocked, either synchronously 666 or asynchronously when the write operation completes. 667 668 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to 669 try too hard if there are problems, and may choose to write out 670 other pages from the mapping if that is easier (e.g. due to 671 internal dependencies). If it chooses not to start writeout, it 672 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep 673 calling ->writepage on that page. 674 675 See the file "Locking" for more details. 676 677 readpage: called by the VM to read a page from backing store. 678 The page will be Locked when readpage is called, and should be 679 unlocked and marked uptodate once the read completes. 680 If ->readpage discovers that it needs to unlock the page for 681 some reason, it can do so, and then return AOP_TRUNCATED_PAGE. 682 In this case, the page will be relocated, relocked and if 683 that all succeeds, ->readpage will be called again. 684 685 writepages: called by the VM to write out pages associated with the 686 address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then 687 the writeback_control will specify a range of pages that must be 688 written out. If it is WBC_SYNC_NONE, then a nr_to_write is given 689 and that many pages should be written if possible. 690 If no ->writepages is given, then mpage_writepages is used 691 instead. This will choose pages from the address space that are 692 tagged as DIRTY and will pass them to ->writepage. 693 694 set_page_dirty: called by the VM to set a page dirty. 695 This is particularly needed if an address space attaches 696 private data to a page, and that data needs to be updated when 697 a page is dirtied. This is called, for example, when a memory 698 mapped page gets modified. 699 If defined, it should set the PageDirty flag, and the 700 PAGECACHE_TAG_DIRTY tag in the radix tree. 701 702 readpages: called by the VM to read pages associated with the address_space 703 object. This is essentially just a vector version of 704 readpage. Instead of just one page, several pages are 705 requested. 706 readpages is only used for read-ahead, so read errors are 707 ignored. If anything goes wrong, feel free to give up. 708 709 write_begin: 710 Called by the generic buffered write code to ask the filesystem to 711 prepare to write len bytes at the given offset in the file. The 712 address_space should check that the write will be able to complete, 713 by allocating space if necessary and doing any other internal 714 housekeeping. If the write will update parts of any basic-blocks on 715 storage, then those blocks should be pre-read (if they haven't been 716 read already) so that the updated blocks can be written out properly. 717 718 The filesystem must return the locked pagecache page for the specified 719 offset, in *pagep, for the caller to write into. 720 721 It must be able to cope with short writes (where the length passed to 722 write_begin is greater than the number of bytes copied into the page). 723 724 flags is a field for AOP_FLAG_xxx flags, described in 725 include/linux/fs.h. 726 727 A void * may be returned in fsdata, which then gets passed into 728 write_end. 729 730 Returns 0 on success; < 0 on failure (which is the error code), in 731 which case write_end is not called. 732 733 write_end: After a successful write_begin, and data copy, write_end must 734 be called. len is the original len passed to write_begin, and copied 735 is the amount that was able to be copied. 736 737 The filesystem must take care of unlocking the page and releasing it 738 refcount, and updating i_size. 739 740 Returns < 0 on failure, otherwise the number of bytes (<= 'copied') 741 that were able to be copied into pagecache. 742 743 bmap: called by the VFS to map a logical block offset within object to 744 physical block number. This method is used by the FIBMAP 745 ioctl and for working with swap-files. To be able to swap to 746 a file, the file must have a stable mapping to a block 747 device. The swap system does not go through the filesystem 748 but instead uses bmap to find out where the blocks in the file 749 are and uses those addresses directly. 750 751 invalidatepage: If a page has PagePrivate set, then invalidatepage 752 will be called when part or all of the page is to be removed 753 from the address space. This generally corresponds to either a 754 truncation, punch hole or a complete invalidation of the address 755 space (in the latter case 'offset' will always be 0 and 'length' 756 will be PAGE_SIZE). Any private data associated with the page 757 should be updated to reflect this truncation. If offset is 0 and 758 length is PAGE_SIZE, then the private data should be released, 759 because the page must be able to be completely discarded. This may 760 be done by calling the ->releasepage function, but in this case the 761 release MUST succeed. 762 763 releasepage: releasepage is called on PagePrivate pages to indicate 764 that the page should be freed if possible. ->releasepage 765 should remove any private data from the page and clear the 766 PagePrivate flag. If releasepage() fails for some reason, it must 767 indicate failure with a 0 return value. 768 releasepage() is used in two distinct though related cases. The 769 first is when the VM finds a clean page with no active users and 770 wants to make it a free page. If ->releasepage succeeds, the 771 page will be removed from the address_space and become free. 772 773 The second case is when a request has been made to invalidate 774 some or all pages in an address_space. This can happen 775 through the fadvise(POSIX_FADV_DONTNEED) system call or by the 776 filesystem explicitly requesting it as nfs and 9fs do (when 777 they believe the cache may be out of date with storage) by 778 calling invalidate_inode_pages2(). 779 If the filesystem makes such a call, and needs to be certain 780 that all pages are invalidated, then its releasepage will 781 need to ensure this. Possibly it can clear the PageUptodate 782 bit if it cannot free private data yet. 783 784 freepage: freepage is called once the page is no longer visible in 785 the page cache in order to allow the cleanup of any private 786 data. Since it may be called by the memory reclaimer, it 787 should not assume that the original address_space mapping still 788 exists, and it should not block. 789 790 direct_IO: called by the generic read/write routines to perform 791 direct_IO - that is IO requests which bypass the page cache 792 and transfer data directly between the storage and the 793 application's address space. 794 795 isolate_page: Called by the VM when isolating a movable non-lru page. 796 If page is successfully isolated, VM marks the page as PG_isolated 797 via __SetPageIsolated. 798 799 migrate_page: This is used to compact the physical memory usage. 800 If the VM wants to relocate a page (maybe off a memory card 801 that is signalling imminent failure) it will pass a new page 802 and an old page to this function. migrate_page should 803 transfer any private data across and update any references 804 that it has to the page. 805 806 putback_page: Called by the VM when isolated page's migration fails. 807 808 launder_page: Called before freeing a page - it writes back the dirty page. To 809 prevent redirtying the page, it is kept locked during the whole 810 operation. 811 812 is_partially_uptodate: Called by the VM when reading a file through the 813 pagecache when the underlying blocksize != pagesize. If the required 814 block is up to date then the read can complete without needing the IO 815 to bring the whole page up to date. 816 817 is_dirty_writeback: Called by the VM when attempting to reclaim a page. 818 The VM uses dirty and writeback information to determine if it needs 819 to stall to allow flushers a chance to complete some IO. Ordinarily 820 it can use PageDirty and PageWriteback but some filesystems have 821 more complex state (unstable pages in NFS prevent reclaim) or 822 do not set those flags due to locking problems. This callback 823 allows a filesystem to indicate to the VM if a page should be 824 treated as dirty or writeback for the purposes of stalling. 825 826 error_remove_page: normally set to generic_error_remove_page if truncation 827 is ok for this address space. Used for memory failure handling. 828 Setting this implies you deal with pages going away under you, 829 unless you have them locked or reference counts increased. 830 831 swap_activate: Called when swapon is used on a file to allocate 832 space if necessary and pin the block lookup information in 833 memory. A return value of zero indicates success, 834 in which case this file can be used to back swapspace. 835 836 swap_deactivate: Called during swapoff on files where swap_activate 837 was successful. 838 839 840The File Object 841=============== 842 843A file object represents a file opened by a process. This is also known 844as an "open file description" in POSIX parlance. 845 846 847struct file_operations 848---------------------- 849 850This describes how the VFS can manipulate an open file. As of kernel 8514.18, the following members are defined: 852 853struct file_operations { 854 struct module *owner; 855 loff_t (*llseek) (struct file *, loff_t, int); 856 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); 857 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); 858 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *); 859 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *); 860 int (*iterate) (struct file *, struct dir_context *); 861 int (*iterate_shared) (struct file *, struct dir_context *); 862 __poll_t (*poll) (struct file *, struct poll_table_struct *); 863 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); 864 long (*compat_ioctl) (struct file *, unsigned int, unsigned long); 865 int (*mmap) (struct file *, struct vm_area_struct *); 866 int (*open) (struct inode *, struct file *); 867 int (*flush) (struct file *, fl_owner_t id); 868 int (*release) (struct inode *, struct file *); 869 int (*fsync) (struct file *, loff_t, loff_t, int datasync); 870 int (*fasync) (int, struct file *, int); 871 int (*lock) (struct file *, int, struct file_lock *); 872 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int); 873 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long); 874 int (*check_flags)(int); 875 int (*flock) (struct file *, int, struct file_lock *); 876 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int); 877 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int); 878 int (*setlease)(struct file *, long, struct file_lock **, void **); 879 long (*fallocate)(struct file *file, int mode, loff_t offset, 880 loff_t len); 881 void (*show_fdinfo)(struct seq_file *m, struct file *f); 882#ifndef CONFIG_MMU 883 unsigned (*mmap_capabilities)(struct file *); 884#endif 885 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int); 886 int (*clone_file_range)(struct file *, loff_t, struct file *, loff_t, u64); 887 int (*dedupe_file_range)(struct file *, loff_t, struct file *, loff_t, u64); 888 int (*fadvise)(struct file *, loff_t, loff_t, int); 889}; 890 891Again, all methods are called without any locks being held, unless 892otherwise noted. 893 894 llseek: called when the VFS needs to move the file position index 895 896 read: called by read(2) and related system calls 897 898 read_iter: possibly asynchronous read with iov_iter as destination 899 900 write: called by write(2) and related system calls 901 902 write_iter: possibly asynchronous write with iov_iter as source 903 904 iterate: called when the VFS needs to read the directory contents 905 906 iterate_shared: called when the VFS needs to read the directory contents 907 when filesystem supports concurrent dir iterators 908 909 poll: called by the VFS when a process wants to check if there is 910 activity on this file and (optionally) go to sleep until there 911 is activity. Called by the select(2) and poll(2) system calls 912 913 unlocked_ioctl: called by the ioctl(2) system call. 914 915 compat_ioctl: called by the ioctl(2) system call when 32 bit system calls 916 are used on 64 bit kernels. 917 918 mmap: called by the mmap(2) system call 919 920 open: called by the VFS when an inode should be opened. When the VFS 921 opens a file, it creates a new "struct file". It then calls the 922 open method for the newly allocated file structure. You might 923 think that the open method really belongs in 924 "struct inode_operations", and you may be right. I think it's 925 done the way it is because it makes filesystems simpler to 926 implement. The open() method is a good place to initialize the 927 "private_data" member in the file structure if you want to point 928 to a device structure 929 930 flush: called by the close(2) system call to flush a file 931 932 release: called when the last reference to an open file is closed 933 934 fsync: called by the fsync(2) system call. Also see the section above 935 entitled "Handling errors during writeback". 936 937 fasync: called by the fcntl(2) system call when asynchronous 938 (non-blocking) mode is enabled for a file 939 940 lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW 941 commands 942 943 get_unmapped_area: called by the mmap(2) system call 944 945 check_flags: called by the fcntl(2) system call for F_SETFL command 946 947 flock: called by the flock(2) system call 948 949 splice_write: called by the VFS to splice data from a pipe to a file. This 950 method is used by the splice(2) system call 951 952 splice_read: called by the VFS to splice data from file to a pipe. This 953 method is used by the splice(2) system call 954 955 setlease: called by the VFS to set or release a file lock lease. setlease 956 implementations should call generic_setlease to record or remove 957 the lease in the inode after setting it. 958 959 fallocate: called by the VFS to preallocate blocks or punch a hole. 960 961 copy_file_range: called by the copy_file_range(2) system call. 962 963 clone_file_range: called by the ioctl(2) system call for FICLONERANGE and 964 FICLONE commands. 965 966 dedupe_file_range: called by the ioctl(2) system call for FIDEDUPERANGE 967 command. 968 969 fadvise: possibly called by the fadvise64() system call. 970 971Note that the file operations are implemented by the specific 972filesystem in which the inode resides. When opening a device node 973(character or block special) most filesystems will call special 974support routines in the VFS which will locate the required device 975driver information. These support routines replace the filesystem file 976operations with those for the device driver, and then proceed to call 977the new open() method for the file. This is how opening a device file 978in the filesystem eventually ends up calling the device driver open() 979method. 980 981 982Directory Entry Cache (dcache) 983============================== 984 985 986struct dentry_operations 987------------------------ 988 989This describes how a filesystem can overload the standard dentry 990operations. Dentries and the dcache are the domain of the VFS and the 991individual filesystem implementations. Device drivers have no business 992here. These methods may be set to NULL, as they are either optional or 993the VFS uses a default. As of kernel 2.6.22, the following members are 994defined: 995 996struct dentry_operations { 997 int (*d_revalidate)(struct dentry *, unsigned int); 998 int (*d_weak_revalidate)(struct dentry *, unsigned int); 999 int (*d_hash)(const struct dentry *, struct qstr *); 1000 int (*d_compare)(const struct dentry *, 1001 unsigned int, const char *, const struct qstr *); 1002 int (*d_delete)(const struct dentry *); 1003 int (*d_init)(struct dentry *); 1004 void (*d_release)(struct dentry *); 1005 void (*d_iput)(struct dentry *, struct inode *); 1006 char *(*d_dname)(struct dentry *, char *, int); 1007 struct vfsmount *(*d_automount)(struct path *); 1008 int (*d_manage)(const struct path *, bool); 1009 struct dentry *(*d_real)(struct dentry *, const struct inode *); 1010}; 1011 1012 d_revalidate: called when the VFS needs to revalidate a dentry. This 1013 is called whenever a name look-up finds a dentry in the 1014 dcache. Most local filesystems leave this as NULL, because all their 1015 dentries in the dcache are valid. Network filesystems are different 1016 since things can change on the server without the client necessarily 1017 being aware of it. 1018 1019 This function should return a positive value if the dentry is still 1020 valid, and zero or a negative error code if it isn't. 1021 1022 d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU). 1023 If in rcu-walk mode, the filesystem must revalidate the dentry without 1024 blocking or storing to the dentry, d_parent and d_inode should not be 1025 used without care (because they can change and, in d_inode case, even 1026 become NULL under us). 1027 1028 If a situation is encountered that rcu-walk cannot handle, return 1029 -ECHILD and it will be called again in ref-walk mode. 1030 1031 d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry. 1032 This is called when a path-walk ends at dentry that was not acquired by 1033 doing a lookup in the parent directory. This includes "/", "." and "..", 1034 as well as procfs-style symlinks and mountpoint traversal. 1035 1036 In this case, we are less concerned with whether the dentry is still 1037 fully correct, but rather that the inode is still valid. As with 1038 d_revalidate, most local filesystems will set this to NULL since their 1039 dcache entries are always valid. 1040 1041 This function has the same return code semantics as d_revalidate. 1042 1043 d_weak_revalidate is only called after leaving rcu-walk mode. 1044 1045 d_hash: called when the VFS adds a dentry to the hash table. The first 1046 dentry passed to d_hash is the parent directory that the name is 1047 to be hashed into. 1048 1049 Same locking and synchronisation rules as d_compare regarding 1050 what is safe to dereference etc. 1051 1052 d_compare: called to compare a dentry name with a given name. The first 1053 dentry is the parent of the dentry to be compared, the second is 1054 the child dentry. len and name string are properties of the dentry 1055 to be compared. qstr is the name to compare it with. 1056 1057 Must be constant and idempotent, and should not take locks if 1058 possible, and should not or store into the dentry. 1059 Should not dereference pointers outside the dentry without 1060 lots of care (eg. d_parent, d_inode, d_name should not be used). 1061 1062 However, our vfsmount is pinned, and RCU held, so the dentries and 1063 inodes won't disappear, neither will our sb or filesystem module. 1064 ->d_sb may be used. 1065 1066 It is a tricky calling convention because it needs to be called under 1067 "rcu-walk", ie. without any locks or references on things. 1068 1069 d_delete: called when the last reference to a dentry is dropped and the 1070 dcache is deciding whether or not to cache it. Return 1 to delete 1071 immediately, or 0 to cache the dentry. Default is NULL which means to 1072 always cache a reachable dentry. d_delete must be constant and 1073 idempotent. 1074 1075 d_init: called when a dentry is allocated 1076 1077 d_release: called when a dentry is really deallocated 1078 1079 d_iput: called when a dentry loses its inode (just prior to its 1080 being deallocated). The default when this is NULL is that the 1081 VFS calls iput(). If you define this method, you must call 1082 iput() yourself 1083 1084 d_dname: called when the pathname of a dentry should be generated. 1085 Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay 1086 pathname generation. (Instead of doing it when dentry is created, 1087 it's done only when the path is needed.). Real filesystems probably 1088 dont want to use it, because their dentries are present in global 1089 dcache hash, so their hash should be an invariant. As no lock is 1090 held, d_dname() should not try to modify the dentry itself, unless 1091 appropriate SMP safety is used. CAUTION : d_path() logic is quite 1092 tricky. The correct way to return for example "Hello" is to put it 1093 at the end of the buffer, and returns a pointer to the first char. 1094 dynamic_dname() helper function is provided to take care of this. 1095 1096 Example : 1097 1098 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen) 1099 { 1100 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]", 1101 dentry->d_inode->i_ino); 1102 } 1103 1104 d_automount: called when an automount dentry is to be traversed (optional). 1105 This should create a new VFS mount record and return the record to the 1106 caller. The caller is supplied with a path parameter giving the 1107 automount directory to describe the automount target and the parent 1108 VFS mount record to provide inheritable mount parameters. NULL should 1109 be returned if someone else managed to make the automount first. If 1110 the vfsmount creation failed, then an error code should be returned. 1111 If -EISDIR is returned, then the directory will be treated as an 1112 ordinary directory and returned to pathwalk to continue walking. 1113 1114 If a vfsmount is returned, the caller will attempt to mount it on the 1115 mountpoint and will remove the vfsmount from its expiration list in 1116 the case of failure. The vfsmount should be returned with 2 refs on 1117 it to prevent automatic expiration - the caller will clean up the 1118 additional ref. 1119 1120 This function is only used if DCACHE_NEED_AUTOMOUNT is set on the 1121 dentry. This is set by __d_instantiate() if S_AUTOMOUNT is set on the 1122 inode being added. 1123 1124 d_manage: called to allow the filesystem to manage the transition from a 1125 dentry (optional). This allows autofs, for example, to hold up clients 1126 waiting to explore behind a 'mountpoint' whilst letting the daemon go 1127 past and construct the subtree there. 0 should be returned to let the 1128 calling process continue. -EISDIR can be returned to tell pathwalk to 1129 use this directory as an ordinary directory and to ignore anything 1130 mounted on it and not to check the automount flag. Any other error 1131 code will abort pathwalk completely. 1132 1133 If the 'rcu_walk' parameter is true, then the caller is doing a 1134 pathwalk in RCU-walk mode. Sleeping is not permitted in this mode, 1135 and the caller can be asked to leave it and call again by returning 1136 -ECHILD. -EISDIR may also be returned to tell pathwalk to 1137 ignore d_automount or any mounts. 1138 1139 This function is only used if DCACHE_MANAGE_TRANSIT is set on the 1140 dentry being transited from. 1141 1142 d_real: overlay/union type filesystems implement this method to return one of 1143 the underlying dentries hidden by the overlay. It is used in two 1144 different modes: 1145 1146 Called from file_dentry() it returns the real dentry matching the inode 1147 argument. The real dentry may be from a lower layer already copied up, 1148 but still referenced from the file. This mode is selected with a 1149 non-NULL inode argument. 1150 1151 With NULL inode the topmost real underlying dentry is returned. 1152 1153Each dentry has a pointer to its parent dentry, as well as a hash list 1154of child dentries. Child dentries are basically like files in a 1155directory. 1156 1157 1158Directory Entry Cache API 1159-------------------------- 1160 1161There are a number of functions defined which permit a filesystem to 1162manipulate dentries: 1163 1164 dget: open a new handle for an existing dentry (this just increments 1165 the usage count) 1166 1167 dput: close a handle for a dentry (decrements the usage count). If 1168 the usage count drops to 0, and the dentry is still in its 1169 parent's hash, the "d_delete" method is called to check whether 1170 it should be cached. If it should not be cached, or if the dentry 1171 is not hashed, it is deleted. Otherwise cached dentries are put 1172 into an LRU list to be reclaimed on memory shortage. 1173 1174 d_drop: this unhashes a dentry from its parents hash list. A 1175 subsequent call to dput() will deallocate the dentry if its 1176 usage count drops to 0 1177 1178 d_delete: delete a dentry. If there are no other open references to 1179 the dentry then the dentry is turned into a negative dentry 1180 (the d_iput() method is called). If there are other 1181 references, then d_drop() is called instead 1182 1183 d_add: add a dentry to its parents hash list and then calls 1184 d_instantiate() 1185 1186 d_instantiate: add a dentry to the alias hash list for the inode and 1187 updates the "d_inode" member. The "i_count" member in the 1188 inode structure should be set/incremented. If the inode 1189 pointer is NULL, the dentry is called a "negative 1190 dentry". This function is commonly called when an inode is 1191 created for an existing negative dentry 1192 1193 d_lookup: look up a dentry given its parent and path name component 1194 It looks up the child of that given name from the dcache 1195 hash table. If it is found, the reference count is incremented 1196 and the dentry is returned. The caller must use dput() 1197 to free the dentry when it finishes using it. 1198 1199Mount Options 1200============= 1201 1202Parsing options 1203--------------- 1204 1205On mount and remount the filesystem is passed a string containing a 1206comma separated list of mount options. The options can have either of 1207these forms: 1208 1209 option 1210 option=value 1211 1212The <linux/parser.h> header defines an API that helps parse these 1213options. There are plenty of examples on how to use it in existing 1214filesystems. 1215 1216Showing options 1217--------------- 1218 1219If a filesystem accepts mount options, it must define show_options() 1220to show all the currently active options. The rules are: 1221 1222 - options MUST be shown which are not default or their values differ 1223 from the default 1224 1225 - options MAY be shown which are enabled by default or have their 1226 default value 1227 1228Options used only internally between a mount helper and the kernel 1229(such as file descriptors), or which only have an effect during the 1230mounting (such as ones controlling the creation of a journal) are exempt 1231from the above rules. 1232 1233The underlying reason for the above rules is to make sure, that a 1234mount can be accurately replicated (e.g. umounting and mounting again) 1235based on the information found in /proc/mounts. 1236 1237Resources 1238========= 1239 1240(Note some of these resources are not up-to-date with the latest kernel 1241 version.) 1242 1243Creating Linux virtual filesystems. 2002 1244 <http://lwn.net/Articles/13325/> 1245 1246The Linux Virtual File-system Layer by Neil Brown. 1999 1247 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html> 1248 1249A tour of the Linux VFS by Michael K. Johnson. 1996 1250 <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html> 1251 1252A small trail through the Linux kernel by Andries Brouwer. 2001 1253 <http://www.win.tue.nl/~aeb/linux/vfs/trail.html> 1254
