Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
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 14 Introduction 15 ============ 16 17 The Virtual File System (also known as the Virtual Filesystem Switch) 18 is the software layer in the kernel that provides the filesystem 19 interface to userspace programs. It also provides an abstraction 20 within the kernel which allows different filesystem implementations to 21 coexist. 22 23 VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so 24 on are called from a process context. Filesystem locking is described 25 in the document Documentation/filesystems/Locking. 26 27 28 Directory Entry Cache (dcache) 29 ------------------------------ 30 31 The VFS implements the open(2), stat(2), chmod(2), and similar system 32 calls. The pathname argument that is passed to them is used by the VFS 33 to search through the directory entry cache (also known as the dentry 34 cache or dcache). This provides a very fast look-up mechanism to 35 translate a pathname (filename) into a specific dentry. Dentries live 36 in RAM and are never saved to disc: they exist only for performance. 37 38 The dentry cache is meant to be a view into your entire filespace. As 39 most computers cannot fit all dentries in the RAM at the same time, 40 some bits of the cache are missing. In order to resolve your pathname 41 into a dentry, the VFS may have to resort to creating dentries along 42 the way, and then loading the inode. This is done by looking up the 43 inode. 44 45 46 The Inode Object 47 ---------------- 48 49 An individual dentry usually has a pointer to an inode. Inodes are 50 filesystem objects such as regular files, directories, FIFOs and other 51 beasts. They live either on the disc (for block device filesystems) 52 or in the memory (for pseudo filesystems). Inodes that live on the 53 disc are copied into the memory when required and changes to the inode 54 are written back to disc. A single inode can be pointed to by multiple 55 dentries (hard links, for example, do this). 56 57 To look up an inode requires that the VFS calls the lookup() method of 58 the parent directory inode. This method is installed by the specific 59 filesystem implementation that the inode lives in. Once the VFS has 60 the required dentry (and hence the inode), we can do all those boring 61 things like open(2) the file, or stat(2) it to peek at the inode 62 data. The stat(2) operation is fairly simple: once the VFS has the 63 dentry, it peeks at the inode data and passes some of it back to 64 userspace. 65 66 67 The File Object 68 --------------- 69 70 Opening a file requires another operation: allocation of a file 71 structure (this is the kernel-side implementation of file 72 descriptors). The freshly allocated file structure is initialized with 73 a pointer to the dentry and a set of file operation member functions. 74 These are taken from the inode data. The open() file method is then 75 called so the specific filesystem implementation can do its work. You 76 can see that this is another switch performed by the VFS. The file 77 structure is placed into the file descriptor table for the process. 78 79 Reading, writing and closing files (and other assorted VFS operations) 80 is done by using the userspace file descriptor to grab the appropriate 81 file structure, and then calling the required file structure method to 82 do whatever is required. For as long as the file is open, it keeps the 83 dentry in use, which in turn means that the VFS inode is still in use. 84 85 86 Registering and Mounting a Filesystem 87 ===================================== 88 89 To register and unregister a filesystem, use the following API 90 functions: 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 97 The passed struct file_system_type describes your filesystem. When a 98 request is made to mount a filesystem onto a directory in your namespace, 99 the VFS will call the appropriate mount() method for the specific 100 filesystem. New vfsmount referring to the tree returned by ->mount() 101 will be attached to the mountpoint, so that when pathname resolution 102 reaches the mountpoint it will jump into the root of that vfsmount. 103 104 You can see all filesystems that are registered to the kernel in the 105 file /proc/filesystems. 106 107 108 struct file_system_type 109 ----------------------- 110 111 This describes the filesystem. As of kernel 2.6.39, the following 112 members are defined: 113 114 struct 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 144 145 The 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) 156 157 The mount() method must return the root dentry of the tree requested by 158 caller. An active reference to its superblock must be grabbed and the 159 superblock must be locked. On failure it should return ERR_PTR(error). 160 161 The arguments match those of mount(2) and their interpretation 162 depends on filesystem type. E.g. for block filesystems, dev_name is 163 interpreted as block device name, that device is opened and if it 164 contains a suitable filesystem image the method creates and initializes 165 struct super_block accordingly, returning its root dentry to caller. 166 167 ->mount() may choose to return a subtree of existing filesystem - it 168 doesn't have to create a new one. The main result from the caller's 169 point of view is a reference to dentry at the root of (sub)tree to 170 be attached; creation of new superblock is a common side effect. 171 172 The most interesting member of the superblock structure that the 173 mount() method fills in is the "s_op" field. This is a pointer to 174 a "struct super_operations" which describes the next level of the 175 filesystem implementation. 176 177 Usually, a filesystem uses one of the generic mount() implementations 178 and 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 186 187 A 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 196 197 198 The Superblock Object 199 ===================== 200 201 A superblock object represents a mounted filesystem. 202 203 204 struct super_operations 205 ----------------------- 206 207 This describes how the VFS can manipulate the superblock of your 208 filesystem. As of kernel 2.6.22, the following members are defined: 209 210 struct 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 }; 234 235 All methods are called without any locks being held, unless otherwise 236 noted. This means that most methods can block safely. All methods are 237 only called from a process context (i.e. not from an interrupt handler 238 or 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. 321 322 Whoever sets up the inode is responsible for filling in the "i_op" field. This 323 is a pointer to a "struct inode_operations" which describes the methods that 324 can be performed on individual inodes. 325 326 struct xattr_handlers 327 --------------------- 328 329 On filesystems that support extended attributes (xattrs), the s_xattr 330 superblock field points to a NULL-terminated array of xattr handlers. Extended 331 attributes 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. 350 351 When none of the xattr handlers of a filesystem match the specified attribute 352 name or when a filesystem doesn't support extended attributes, the various 353 *xattr(2) system calls return -EOPNOTSUPP. 354 355 356 The Inode Object 357 ================ 358 359 An inode object represents an object within the filesystem. 360 361 362 struct inode_operations 363 ----------------------- 364 365 This describes how the VFS can manipulate an inode in your 366 filesystem. As of kernel 2.6.22, the following members are defined: 367 368 struct 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, int *opened); 390 int (*tmpfile) (struct inode *, struct dentry *, umode_t); 391 }; 392 393 Again, all methods are called without any locks being held, unless 394 otherwise 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 cannot perform this (e.g. the file type 500 turned out to be wrong) it may signal this by returning 1 instead of 501 usual 0 or -ve . This method is only called if the last component is 502 negative or needs lookup. Cached positive dentries are still handled by 503 f_op->open(). If the file was created, the FILE_CREATED flag should be 504 set in "opened". In case of O_EXCL the method must only succeed if the 505 file didn't exist and hence FILE_CREATED shall always be set on success. 506 507 tmpfile: called in the end of O_TMPFILE open(). Optional, equivalent to 508 atomically creating, opening and unlinking a file in given directory. 509 510 The Address Space Object 511 ======================== 512 513 The address space object is used to group and manage pages in the page 514 cache. It can be used to keep track of the pages in a file (or 515 anything else) and also track the mapping of sections of the file into 516 process address spaces. 517 518 There are a number of distinct yet related services that an 519 address-space can provide. These include communicating memory 520 pressure, page lookup by address, and keeping track of pages tagged as 521 Dirty or Writeback. 522 523 The first can be used independently to the others. The VM can try to 524 either write dirty pages in order to clean them, or release clean 525 pages in order to reuse them. To do this it can call the ->writepage 526 method on dirty pages, and ->releasepage on clean pages with 527 PagePrivate set. Clean pages without PagePrivate and with no external 528 references will be released without notice being given to the 529 address_space. 530 531 To achieve this functionality, pages need to be placed on an LRU with 532 lru_cache_add and mark_page_active needs to be called whenever the 533 page is used. 534 535 Pages are normally kept in a radix tree index by ->index. This tree 536 maintains information about the PG_Dirty and PG_Writeback status of 537 each page, so that pages with either of these flags can be found 538 quickly. 539 540 The Dirty tag is primarily used by mpage_writepages - the default 541 ->writepages method. It uses the tag to find dirty pages to call 542 ->writepage on. If mpage_writepages is not used (i.e. the address 543 provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is 544 almost unused. write_inode_now and sync_inode do use it (through 545 __sync_single_inode) to check if ->writepages has been successful in 546 writing out the whole address_space. 547 548 The Writeback tag is used by filemap*wait* and sync_page* functions, 549 via filemap_fdatawait_range, to wait for all writeback to complete. 550 551 An address_space handler may attach extra information to a page, 552 typically using the 'private' field in the 'struct page'. If such 553 information is attached, the PG_Private flag should be set. This will 554 cause various VM routines to make extra calls into the address_space 555 handler to deal with that data. 556 557 An address space acts as an intermediate between storage and 558 application. Data is read into the address space a whole page at a 559 time, and provided to the application either by copying of the page, 560 or by memory-mapping the page. 561 Data is written into the address space by the application, and then 562 written-back to storage typically in whole pages, however the 563 address_space has finer control of write sizes. 564 565 The read process essentially only requires 'readpage'. The write 566 process is more complicated and uses write_begin/write_end or 567 set_page_dirty to write data into the address_space, and writepage 568 and writepages to writeback data to storage. 569 570 Adding and removing pages to/from an address_space is protected by the 571 inode's i_mutex. 572 573 When data is written to a page, the PG_Dirty flag should be set. It 574 typically remains set until writepage asks for it to be written. This 575 should clear PG_Dirty and set PG_Writeback. It can be actually 576 written at any point after PG_Dirty is clear. Once it is known to be 577 safe, PG_Writeback is cleared. 578 579 Writeback makes use of a writeback_control structure to direct the 580 operations. This gives the the writepage and writepages operations some 581 information about the nature of and reason for the writeback request, 582 and the constraints under which it is being done. It is also used to 583 return information back to the caller about the result of a writepage or 584 writepages request. 585 586 Handling errors during writeback 587 -------------------------------- 588 Most applications that do buffered I/O will periodically call a file 589 synchronization call (fsync, fdatasync, msync or sync_file_range) to 590 ensure that data written has made it to the backing store. When there 591 is an error during writeback, they expect that error to be reported when 592 a file sync request is made. After an error has been reported on one 593 request, subsequent requests on the same file descriptor should return 594 0, unless further writeback errors have occurred since the previous file 595 syncronization. 596 597 Ideally, the kernel would report errors only on file descriptions on 598 which writes were done that subsequently failed to be written back. The 599 generic pagecache infrastructure does not track the file descriptions 600 that have dirtied each individual page however, so determining which 601 file descriptors should get back an error is not possible. 602 603 Instead, the generic writeback error tracking infrastructure in the 604 kernel settles for reporting errors to fsync on all file descriptions 605 that were open at the time that the error occurred. In a situation with 606 multiple writers, all of them will get back an error on a subsequent fsync, 607 even if all of the writes done through that particular file descriptor 608 succeeded (or even if there were no writes on that file descriptor at all). 609 610 Filesystems that wish to use this infrastructure should call 611 mapping_set_error to record the error in the address_space when it 612 occurs. Then, after writing back data from the pagecache in their 613 file->fsync operation, they should call file_check_and_advance_wb_err to 614 ensure that the struct file's error cursor has advanced to the correct 615 point in the stream of errors emitted by the backing device(s). 616 617 struct address_space_operations 618 ------------------------------- 619 620 This describes how the VFS can manipulate mapping of a file to page cache in 621 your filesystem. The following members are defined: 622 623 struct address_space_operations { 624 int (*writepage)(struct page *page, struct writeback_control *wbc); 625 int (*readpage)(struct file *, struct page *); 626 int (*writepages)(struct address_space *, struct writeback_control *); 627 int (*set_page_dirty)(struct page *page); 628 int (*readpages)(struct file *filp, struct address_space *mapping, 629 struct list_head *pages, unsigned nr_pages); 630 int (*write_begin)(struct file *, struct address_space *mapping, 631 loff_t pos, unsigned len, unsigned flags, 632 struct page **pagep, void **fsdata); 633 int (*write_end)(struct file *, struct address_space *mapping, 634 loff_t pos, unsigned len, unsigned copied, 635 struct page *page, void *fsdata); 636 sector_t (*bmap)(struct address_space *, sector_t); 637 void (*invalidatepage) (struct page *, unsigned int, unsigned int); 638 int (*releasepage) (struct page *, int); 639 void (*freepage)(struct page *); 640 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter); 641 /* isolate a page for migration */ 642 bool (*isolate_page) (struct page *, isolate_mode_t); 643 /* migrate the contents of a page to the specified target */ 644 int (*migratepage) (struct page *, struct page *); 645 /* put migration-failed page back to right list */ 646 void (*putback_page) (struct page *); 647 int (*launder_page) (struct page *); 648 649 int (*is_partially_uptodate) (struct page *, unsigned long, 650 unsigned long); 651 void (*is_dirty_writeback) (struct page *, bool *, bool *); 652 int (*error_remove_page) (struct mapping *mapping, struct page *page); 653 int (*swap_activate)(struct file *); 654 int (*swap_deactivate)(struct file *); 655 }; 656 657 writepage: called by the VM to write a dirty page to backing store. 658 This may happen for data integrity reasons (i.e. 'sync'), or 659 to free up memory (flush). The difference can be seen in 660 wbc->sync_mode. 661 The PG_Dirty flag has been cleared and PageLocked is true. 662 writepage should start writeout, should set PG_Writeback, 663 and should make sure the page is unlocked, either synchronously 664 or asynchronously when the write operation completes. 665 666 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to 667 try too hard if there are problems, and may choose to write out 668 other pages from the mapping if that is easier (e.g. due to 669 internal dependencies). If it chooses not to start writeout, it 670 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep 671 calling ->writepage on that page. 672 673 See the file "Locking" for more details. 674 675 readpage: called by the VM to read a page from backing store. 676 The page will be Locked when readpage is called, and should be 677 unlocked and marked uptodate once the read completes. 678 If ->readpage discovers that it needs to unlock the page for 679 some reason, it can do so, and then return AOP_TRUNCATED_PAGE. 680 In this case, the page will be relocated, relocked and if 681 that all succeeds, ->readpage will be called again. 682 683 writepages: called by the VM to write out pages associated with the 684 address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then 685 the writeback_control will specify a range of pages that must be 686 written out. If it is WBC_SYNC_NONE, then a nr_to_write is given 687 and that many pages should be written if possible. 688 If no ->writepages is given, then mpage_writepages is used 689 instead. This will choose pages from the address space that are 690 tagged as DIRTY and will pass them to ->writepage. 691 692 set_page_dirty: called by the VM to set a page dirty. 693 This is particularly needed if an address space attaches 694 private data to a page, and that data needs to be updated when 695 a page is dirtied. This is called, for example, when a memory 696 mapped page gets modified. 697 If defined, it should set the PageDirty flag, and the 698 PAGECACHE_TAG_DIRTY tag in the radix tree. 699 700 readpages: called by the VM to read pages associated with the address_space 701 object. This is essentially just a vector version of 702 readpage. Instead of just one page, several pages are 703 requested. 704 readpages is only used for read-ahead, so read errors are 705 ignored. If anything goes wrong, feel free to give up. 706 707 write_begin: 708 Called by the generic buffered write code to ask the filesystem to 709 prepare to write len bytes at the given offset in the file. The 710 address_space should check that the write will be able to complete, 711 by allocating space if necessary and doing any other internal 712 housekeeping. If the write will update parts of any basic-blocks on 713 storage, then those blocks should be pre-read (if they haven't been 714 read already) so that the updated blocks can be written out properly. 715 716 The filesystem must return the locked pagecache page for the specified 717 offset, in *pagep, for the caller to write into. 718 719 It must be able to cope with short writes (where the length passed to 720 write_begin is greater than the number of bytes copied into the page). 721 722 flags is a field for AOP_FLAG_xxx flags, described in 723 include/linux/fs.h. 724 725 A void * may be returned in fsdata, which then gets passed into 726 write_end. 727 728 Returns 0 on success; < 0 on failure (which is the error code), in 729 which case write_end is not called. 730 731 write_end: After a successful write_begin, and data copy, write_end must 732 be called. len is the original len passed to write_begin, and copied 733 is the amount that was able to be copied. 734 735 The filesystem must take care of unlocking the page and releasing it 736 refcount, and updating i_size. 737 738 Returns < 0 on failure, otherwise the number of bytes (<= 'copied') 739 that were able to be copied into pagecache. 740 741 bmap: called by the VFS to map a logical block offset within object to 742 physical block number. This method is used by the FIBMAP 743 ioctl and for working with swap-files. To be able to swap to 744 a file, the file must have a stable mapping to a block 745 device. The swap system does not go through the filesystem 746 but instead uses bmap to find out where the blocks in the file 747 are and uses those addresses directly. 748 749 invalidatepage: If a page has PagePrivate set, then invalidatepage 750 will be called when part or all of the page is to be removed 751 from the address space. This generally corresponds to either a 752 truncation, punch hole or a complete invalidation of the address 753 space (in the latter case 'offset' will always be 0 and 'length' 754 will be PAGE_SIZE). Any private data associated with the page 755 should be updated to reflect this truncation. If offset is 0 and 756 length is PAGE_SIZE, then the private data should be released, 757 because the page must be able to be completely discarded. This may 758 be done by calling the ->releasepage function, but in this case the 759 release MUST succeed. 760 761 releasepage: releasepage is called on PagePrivate pages to indicate 762 that the page should be freed if possible. ->releasepage 763 should remove any private data from the page and clear the 764 PagePrivate flag. If releasepage() fails for some reason, it must 765 indicate failure with a 0 return value. 766 releasepage() is used in two distinct though related cases. The 767 first is when the VM finds a clean page with no active users and 768 wants to make it a free page. If ->releasepage succeeds, the 769 page will be removed from the address_space and become free. 770 771 The second case is when a request has been made to invalidate 772 some or all pages in an address_space. This can happen 773 through the fadvise(POSIX_FADV_DONTNEED) system call or by the 774 filesystem explicitly requesting it as nfs and 9fs do (when 775 they believe the cache may be out of date with storage) by 776 calling invalidate_inode_pages2(). 777 If the filesystem makes such a call, and needs to be certain 778 that all pages are invalidated, then its releasepage will 779 need to ensure this. Possibly it can clear the PageUptodate 780 bit if it cannot free private data yet. 781 782 freepage: freepage is called once the page is no longer visible in 783 the page cache in order to allow the cleanup of any private 784 data. Since it may be called by the memory reclaimer, it 785 should not assume that the original address_space mapping still 786 exists, and it should not block. 787 788 direct_IO: called by the generic read/write routines to perform 789 direct_IO - that is IO requests which bypass the page cache 790 and transfer data directly between the storage and the 791 application's address space. 792 793 isolate_page: Called by the VM when isolating a movable non-lru page. 794 If page is successfully isolated, VM marks the page as PG_isolated 795 via __SetPageIsolated. 796 797 migrate_page: This is used to compact the physical memory usage. 798 If the VM wants to relocate a page (maybe off a memory card 799 that is signalling imminent failure) it will pass a new page 800 and an old page to this function. migrate_page should 801 transfer any private data across and update any references 802 that it has to the page. 803 804 putback_page: Called by the VM when isolated page's migration fails. 805 806 launder_page: Called before freeing a page - it writes back the dirty page. To 807 prevent redirtying the page, it is kept locked during the whole 808 operation. 809 810 is_partially_uptodate: Called by the VM when reading a file through the 811 pagecache when the underlying blocksize != pagesize. If the required 812 block is up to date then the read can complete without needing the IO 813 to bring the whole page up to date. 814 815 is_dirty_writeback: Called by the VM when attempting to reclaim a page. 816 The VM uses dirty and writeback information to determine if it needs 817 to stall to allow flushers a chance to complete some IO. Ordinarily 818 it can use PageDirty and PageWriteback but some filesystems have 819 more complex state (unstable pages in NFS prevent reclaim) or 820 do not set those flags due to locking problems. This callback 821 allows a filesystem to indicate to the VM if a page should be 822 treated as dirty or writeback for the purposes of stalling. 823 824 error_remove_page: normally set to generic_error_remove_page if truncation 825 is ok for this address space. Used for memory failure handling. 826 Setting this implies you deal with pages going away under you, 827 unless you have them locked or reference counts increased. 828 829 swap_activate: Called when swapon is used on a file to allocate 830 space if necessary and pin the block lookup information in 831 memory. A return value of zero indicates success, 832 in which case this file can be used to back swapspace. 833 834 swap_deactivate: Called during swapoff on files where swap_activate 835 was successful. 836 837 838 The File Object 839 =============== 840 841 A file object represents a file opened by a process. This is also known 842 as an "open file description" in POSIX parlance. 843 844 845 struct file_operations 846 ---------------------- 847 848 This describes how the VFS can manipulate an open file. As of kernel 849 4.1, the following members are defined: 850 851 struct file_operations { 852 struct module *owner; 853 loff_t (*llseek) (struct file *, loff_t, int); 854 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); 855 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); 856 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *); 857 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *); 858 int (*iterate) (struct file *, struct dir_context *); 859 unsigned int (*poll) (struct file *, struct poll_table_struct *); 860 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); 861 long (*compat_ioctl) (struct file *, unsigned int, unsigned long); 862 int (*mmap) (struct file *, struct vm_area_struct *); 863 int (*mremap)(struct file *, struct vm_area_struct *); 864 int (*open) (struct inode *, struct file *); 865 int (*flush) (struct file *, fl_owner_t id); 866 int (*release) (struct inode *, struct file *); 867 int (*fsync) (struct file *, loff_t, loff_t, int datasync); 868 int (*fasync) (int, struct file *, int); 869 int (*lock) (struct file *, int, struct file_lock *); 870 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int); 871 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long); 872 int (*check_flags)(int); 873 int (*flock) (struct file *, int, struct file_lock *); 874 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int); 875 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int); 876 int (*setlease)(struct file *, long, struct file_lock **, void **); 877 long (*fallocate)(struct file *file, int mode, loff_t offset, 878 loff_t len); 879 void (*show_fdinfo)(struct seq_file *m, struct file *f); 880 #ifndef CONFIG_MMU 881 unsigned (*mmap_capabilities)(struct file *); 882 #endif 883 }; 884 885 Again, all methods are called without any locks being held, unless 886 otherwise noted. 887 888 llseek: called when the VFS needs to move the file position index 889 890 read: called by read(2) and related system calls 891 892 read_iter: possibly asynchronous read with iov_iter as destination 893 894 write: called by write(2) and related system calls 895 896 write_iter: possibly asynchronous write with iov_iter as source 897 898 iterate: called when the VFS needs to read the directory contents 899 900 poll: called by the VFS when a process wants to check if there is 901 activity on this file and (optionally) go to sleep until there 902 is activity. Called by the select(2) and poll(2) system calls 903 904 unlocked_ioctl: called by the ioctl(2) system call. 905 906 compat_ioctl: called by the ioctl(2) system call when 32 bit system calls 907 are used on 64 bit kernels. 908 909 mmap: called by the mmap(2) system call 910 911 open: called by the VFS when an inode should be opened. When the VFS 912 opens a file, it creates a new "struct file". It then calls the 913 open method for the newly allocated file structure. You might 914 think that the open method really belongs in 915 "struct inode_operations", and you may be right. I think it's 916 done the way it is because it makes filesystems simpler to 917 implement. The open() method is a good place to initialize the 918 "private_data" member in the file structure if you want to point 919 to a device structure 920 921 flush: called by the close(2) system call to flush a file 922 923 release: called when the last reference to an open file is closed 924 925 fsync: called by the fsync(2) system call. Also see the section above 926 entitled "Handling errors during writeback". 927 928 fasync: called by the fcntl(2) system call when asynchronous 929 (non-blocking) mode is enabled for a file 930 931 lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW 932 commands 933 934 get_unmapped_area: called by the mmap(2) system call 935 936 check_flags: called by the fcntl(2) system call for F_SETFL command 937 938 flock: called by the flock(2) system call 939 940 splice_write: called by the VFS to splice data from a pipe to a file. This 941 method is used by the splice(2) system call 942 943 splice_read: called by the VFS to splice data from file to a pipe. This 944 method is used by the splice(2) system call 945 946 setlease: called by the VFS to set or release a file lock lease. setlease 947 implementations should call generic_setlease to record or remove 948 the lease in the inode after setting it. 949 950 fallocate: called by the VFS to preallocate blocks or punch a hole. 951 952 Note that the file operations are implemented by the specific 953 filesystem in which the inode resides. When opening a device node 954 (character or block special) most filesystems will call special 955 support routines in the VFS which will locate the required device 956 driver information. These support routines replace the filesystem file 957 operations with those for the device driver, and then proceed to call 958 the new open() method for the file. This is how opening a device file 959 in the filesystem eventually ends up calling the device driver open() 960 method. 961 962 963 Directory Entry Cache (dcache) 964 ============================== 965 966 967 struct dentry_operations 968 ------------------------ 969 970 This describes how a filesystem can overload the standard dentry 971 operations. Dentries and the dcache are the domain of the VFS and the 972 individual filesystem implementations. Device drivers have no business 973 here. These methods may be set to NULL, as they are either optional or 974 the VFS uses a default. As of kernel 2.6.22, the following members are 975 defined: 976 977 struct dentry_operations { 978 int (*d_revalidate)(struct dentry *, unsigned int); 979 int (*d_weak_revalidate)(struct dentry *, unsigned int); 980 int (*d_hash)(const struct dentry *, struct qstr *); 981 int (*d_compare)(const struct dentry *, 982 unsigned int, const char *, const struct qstr *); 983 int (*d_delete)(const struct dentry *); 984 int (*d_init)(struct dentry *); 985 void (*d_release)(struct dentry *); 986 void (*d_iput)(struct dentry *, struct inode *); 987 char *(*d_dname)(struct dentry *, char *, int); 988 struct vfsmount *(*d_automount)(struct path *); 989 int (*d_manage)(const struct path *, bool); 990 struct dentry *(*d_real)(struct dentry *, const struct inode *, 991 unsigned int, unsigned int); 992 }; 993 994 d_revalidate: called when the VFS needs to revalidate a dentry. This 995 is called whenever a name look-up finds a dentry in the 996 dcache. Most local filesystems leave this as NULL, because all their 997 dentries in the dcache are valid. Network filesystems are different 998 since things can change on the server without the client necessarily 999 being aware of it. 1000 1001 This function should return a positive value if the dentry is still 1002 valid, and zero or a negative error code if it isn't. 1003 1004 d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU). 1005 If in rcu-walk mode, the filesystem must revalidate the dentry without 1006 blocking or storing to the dentry, d_parent and d_inode should not be 1007 used without care (because they can change and, in d_inode case, even 1008 become NULL under us). 1009 1010 If a situation is encountered that rcu-walk cannot handle, return 1011 -ECHILD and it will be called again in ref-walk mode. 1012 1013 d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry. 1014 This is called when a path-walk ends at dentry that was not acquired by 1015 doing a lookup in the parent directory. This includes "/", "." and "..", 1016 as well as procfs-style symlinks and mountpoint traversal. 1017 1018 In this case, we are less concerned with whether the dentry is still 1019 fully correct, but rather that the inode is still valid. As with 1020 d_revalidate, most local filesystems will set this to NULL since their 1021 dcache entries are always valid. 1022 1023 This function has the same return code semantics as d_revalidate. 1024 1025 d_weak_revalidate is only called after leaving rcu-walk mode. 1026 1027 d_hash: called when the VFS adds a dentry to the hash table. The first 1028 dentry passed to d_hash is the parent directory that the name is 1029 to be hashed into. 1030 1031 Same locking and synchronisation rules as d_compare regarding 1032 what is safe to dereference etc. 1033 1034 d_compare: called to compare a dentry name with a given name. The first 1035 dentry is the parent of the dentry to be compared, the second is 1036 the child dentry. len and name string are properties of the dentry 1037 to be compared. qstr is the name to compare it with. 1038 1039 Must be constant and idempotent, and should not take locks if 1040 possible, and should not or store into the dentry. 1041 Should not dereference pointers outside the dentry without 1042 lots of care (eg. d_parent, d_inode, d_name should not be used). 1043 1044 However, our vfsmount is pinned, and RCU held, so the dentries and 1045 inodes won't disappear, neither will our sb or filesystem module. 1046 ->d_sb may be used. 1047 1048 It is a tricky calling convention because it needs to be called under 1049 "rcu-walk", ie. without any locks or references on things. 1050 1051 d_delete: called when the last reference to a dentry is dropped and the 1052 dcache is deciding whether or not to cache it. Return 1 to delete 1053 immediately, or 0 to cache the dentry. Default is NULL which means to 1054 always cache a reachable dentry. d_delete must be constant and 1055 idempotent. 1056 1057 d_init: called when a dentry is allocated 1058 1059 d_release: called when a dentry is really deallocated 1060 1061 d_iput: called when a dentry loses its inode (just prior to its 1062 being deallocated). The default when this is NULL is that the 1063 VFS calls iput(). If you define this method, you must call 1064 iput() yourself 1065 1066 d_dname: called when the pathname of a dentry should be generated. 1067 Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay 1068 pathname generation. (Instead of doing it when dentry is created, 1069 it's done only when the path is needed.). Real filesystems probably 1070 dont want to use it, because their dentries are present in global 1071 dcache hash, so their hash should be an invariant. As no lock is 1072 held, d_dname() should not try to modify the dentry itself, unless 1073 appropriate SMP safety is used. CAUTION : d_path() logic is quite 1074 tricky. The correct way to return for example "Hello" is to put it 1075 at the end of the buffer, and returns a pointer to the first char. 1076 dynamic_dname() helper function is provided to take care of this. 1077 1078 Example : 1079 1080 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen) 1081 { 1082 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]", 1083 dentry->d_inode->i_ino); 1084 } 1085 1086 d_automount: called when an automount dentry is to be traversed (optional). 1087 This should create a new VFS mount record and return the record to the 1088 caller. The caller is supplied with a path parameter giving the 1089 automount directory to describe the automount target and the parent 1090 VFS mount record to provide inheritable mount parameters. NULL should 1091 be returned if someone else managed to make the automount first. If 1092 the vfsmount creation failed, then an error code should be returned. 1093 If -EISDIR is returned, then the directory will be treated as an 1094 ordinary directory and returned to pathwalk to continue walking. 1095 1096 If a vfsmount is returned, the caller will attempt to mount it on the 1097 mountpoint and will remove the vfsmount from its expiration list in 1098 the case of failure. The vfsmount should be returned with 2 refs on 1099 it to prevent automatic expiration - the caller will clean up the 1100 additional ref. 1101 1102 This function is only used if DCACHE_NEED_AUTOMOUNT is set on the 1103 dentry. This is set by __d_instantiate() if S_AUTOMOUNT is set on the 1104 inode being added. 1105 1106 d_manage: called to allow the filesystem to manage the transition from a 1107 dentry (optional). This allows autofs, for example, to hold up clients 1108 waiting to explore behind a 'mountpoint' whilst letting the daemon go 1109 past and construct the subtree there. 0 should be returned to let the 1110 calling process continue. -EISDIR can be returned to tell pathwalk to 1111 use this directory as an ordinary directory and to ignore anything 1112 mounted on it and not to check the automount flag. Any other error 1113 code will abort pathwalk completely. 1114 1115 If the 'rcu_walk' parameter is true, then the caller is doing a 1116 pathwalk in RCU-walk mode. Sleeping is not permitted in this mode, 1117 and the caller can be asked to leave it and call again by returning 1118 -ECHILD. -EISDIR may also be returned to tell pathwalk to 1119 ignore d_automount or any mounts. 1120 1121 This function is only used if DCACHE_MANAGE_TRANSIT is set on the 1122 dentry being transited from. 1123 1124 d_real: overlay/union type filesystems implement this method to return one of 1125 the underlying dentries hidden by the overlay. It is used in three 1126 different modes: 1127 1128 Called from open it may need to copy-up the file depending on the 1129 supplied open flags. This mode is selected with a non-zero flags 1130 argument. In this mode the d_real method can return an error. 1131 1132 Called from file_dentry() it returns the real dentry matching the inode 1133 argument. The real dentry may be from a lower layer already copied up, 1134 but still referenced from the file. This mode is selected with a 1135 non-NULL inode argument. This will always succeed. 1136 1137 With NULL inode and zero flags the topmost real underlying dentry is 1138 returned. This will always succeed. 1139 1140 This method is never called with both non-NULL inode and non-zero flags. 1141 1142 Each dentry has a pointer to its parent dentry, as well as a hash list 1143 of child dentries. Child dentries are basically like files in a 1144 directory. 1145 1146 1147 Directory Entry Cache API 1148 -------------------------- 1149 1150 There are a number of functions defined which permit a filesystem to 1151 manipulate dentries: 1152 1153 dget: open a new handle for an existing dentry (this just increments 1154 the usage count) 1155 1156 dput: close a handle for a dentry (decrements the usage count). If 1157 the usage count drops to 0, and the dentry is still in its 1158 parent's hash, the "d_delete" method is called to check whether 1159 it should be cached. If it should not be cached, or if the dentry 1160 is not hashed, it is deleted. Otherwise cached dentries are put 1161 into an LRU list to be reclaimed on memory shortage. 1162 1163 d_drop: this unhashes a dentry from its parents hash list. A 1164 subsequent call to dput() will deallocate the dentry if its 1165 usage count drops to 0 1166 1167 d_delete: delete a dentry. If there are no other open references to 1168 the dentry then the dentry is turned into a negative dentry 1169 (the d_iput() method is called). If there are other 1170 references, then d_drop() is called instead 1171 1172 d_add: add a dentry to its parents hash list and then calls 1173 d_instantiate() 1174 1175 d_instantiate: add a dentry to the alias hash list for the inode and 1176 updates the "d_inode" member. The "i_count" member in the 1177 inode structure should be set/incremented. If the inode 1178 pointer is NULL, the dentry is called a "negative 1179 dentry". This function is commonly called when an inode is 1180 created for an existing negative dentry 1181 1182 d_lookup: look up a dentry given its parent and path name component 1183 It looks up the child of that given name from the dcache 1184 hash table. If it is found, the reference count is incremented 1185 and the dentry is returned. The caller must use dput() 1186 to free the dentry when it finishes using it. 1187 1188 Mount Options 1189 ============= 1190 1191 Parsing options 1192 --------------- 1193 1194 On mount and remount the filesystem is passed a string containing a 1195 comma separated list of mount options. The options can have either of 1196 these forms: 1197 1198 option 1199 option=value 1200 1201 The <linux/parser.h> header defines an API that helps parse these 1202 options. There are plenty of examples on how to use it in existing 1203 filesystems. 1204 1205 Showing options 1206 --------------- 1207 1208 If a filesystem accepts mount options, it must define show_options() 1209 to show all the currently active options. The rules are: 1210 1211 - options MUST be shown which are not default or their values differ 1212 from the default 1213 1214 - options MAY be shown which are enabled by default or have their 1215 default value 1216 1217 Options used only internally between a mount helper and the kernel 1218 (such as file descriptors), or which only have an effect during the 1219 mounting (such as ones controlling the creation of a journal) are exempt 1220 from the above rules. 1221 1222 The underlying reason for the above rules is to make sure, that a 1223 mount can be accurately replicated (e.g. umounting and mounting again) 1224 based on the information found in /proc/mounts. 1225 1226 Resources 1227 ========= 1228 1229 (Note some of these resources are not up-to-date with the latest kernel 1230 version.) 1231 1232 Creating Linux virtual filesystems. 2002 1233 <http://lwn.net/Articles/13325/> 1234 1235 The Linux Virtual File-system Layer by Neil Brown. 1999 1236 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html> 1237 1238 A tour of the Linux VFS by Michael K. Johnson. 1996 1239 <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html> 1240 1241 A small trail through the Linux kernel by Andries Brouwer. 2001 1242 <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>