1.. SPDX-License-Identifier: GPL-2.0 2 3========================================= 4Overview of the Linux Virtual File System 5========================================= 6 7Original author: Richard Gooch <rgooch@atnf.csiro.au> 8 9- Copyright (C) 1999 Richard Gooch 10- Copyright (C) 2005 Pekka Enberg 11 12 13Introduction 14============ 15 16The Virtual File System (also known as the Virtual Filesystem Switch) is 17the software layer in the kernel that provides the filesystem interface 18to userspace programs. It also provides an abstraction within the 19kernel which allows different filesystem implementations to coexist. 20 21VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so on 22are called from a process context. Filesystem locking is described in 23the document Documentation/filesystems/locking.rst. 24 25 26Directory Entry Cache (dcache) 27------------------------------ 28 29The VFS implements the open(2), stat(2), chmod(2), and similar system 30calls. The pathname argument that is passed to them is used by the VFS 31to search through the directory entry cache (also known as the dentry 32cache or dcache). This provides a very fast look-up mechanism to 33translate a pathname (filename) into a specific dentry. Dentries live 34in RAM and are never saved to disc: they exist only for performance. 35 36The dentry cache is meant to be a view into your entire filespace. As 37most computers cannot fit all dentries in the RAM at the same time, some 38bits of the cache are missing. In order to resolve your pathname into a 39dentry, the VFS may have to resort to creating dentries along the way, 40and then loading the inode. This is done by looking up the inode. 41 42 43The Inode Object 44---------------- 45 46An individual dentry usually has a pointer to an inode. Inodes are 47filesystem objects such as regular files, directories, FIFOs and other 48beasts. They live either on the disc (for block device filesystems) or 49in the memory (for pseudo filesystems). Inodes that live on the disc 50are copied into the memory when required and changes to the inode are 51written back to disc. A single inode can be pointed to by multiple 52dentries (hard links, for example, do this). 53 54To look up an inode requires that the VFS calls the lookup() method of 55the parent directory inode. This method is installed by the specific 56filesystem implementation that the inode lives in. Once the VFS has the 57required dentry (and hence the inode), we can do all those boring things 58like open(2) the file, or stat(2) it to peek at the inode data. The 59stat(2) operation is fairly simple: once the VFS has the dentry, it 60peeks at the inode data and passes some of it back to userspace. 61 62 63The File Object 64--------------- 65 66Opening a file requires another operation: allocation of a file 67structure (this is the kernel-side implementation of file descriptors). 68The freshly allocated file structure is initialized with a pointer to 69the dentry and a set of file operation member functions. These are 70taken from the inode data. The open() file method is then called so the 71specific filesystem implementation can do its work. You can see that 72this is another switch performed by the VFS. The file structure is 73placed into the file descriptor table for the process. 74 75Reading, writing and closing files (and other assorted VFS operations) 76is done by using the userspace file descriptor to grab the appropriate 77file structure, and then calling the required file structure method to 78do whatever is required. For as long as the file is open, it keeps the 79dentry in use, which in turn means that the VFS inode is still in use. 80 81 82Registering and Mounting a Filesystem 83===================================== 84 85To register and unregister a filesystem, use the following API 86functions: 87 88.. code-block:: c 89 90 #include <linux/fs.h> 91 92 extern int register_filesystem(struct file_system_type *); 93 extern int unregister_filesystem(struct file_system_type *); 94 95The passed struct file_system_type describes your filesystem. When a 96request is made to mount a filesystem onto a directory in your 97namespace, the VFS will call the appropriate mount() method for the 98specific filesystem. New vfsmount referring to the tree returned by 99->mount() will be attached to the mountpoint, so that when pathname 100resolution reaches the mountpoint it will jump into the root of that 101vfsmount. 102 103You can see all filesystems that are registered to the kernel in the 104file /proc/filesystems. 105 106 107struct file_system_type 108----------------------- 109 110This describes the filesystem. As of kernel 2.6.39, the following 111members are defined: 112 113.. code-block:: c 114 115 struct file_system_type { 116 const char *name; 117 int fs_flags; 118 struct dentry *(*mount) (struct file_system_type *, int, 119 const char *, void *); 120 void (*kill_sb) (struct super_block *); 121 struct module *owner; 122 struct file_system_type * next; 123 struct list_head fs_supers; 124 struct lock_class_key s_lock_key; 125 struct lock_class_key s_umount_key; 126 }; 127 128``name`` 129 the name of the filesystem type, such as "ext2", "iso9660", 130 "msdos" and so on 131 132``fs_flags`` 133 various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.) 134 135``mount`` 136 the method to call when a new instance of this filesystem should 137 be mounted 138 139``kill_sb`` 140 the method to call when an instance of this filesystem should be 141 shut down 142 143 144``owner`` 145 for internal VFS use: you should initialize this to THIS_MODULE 146 in most cases. 147 148``next`` 149 for internal VFS use: you should initialize this to NULL 150 151 s_lock_key, s_umount_key: lockdep-specific 152 153The mount() method has the following arguments: 154 155``struct file_system_type *fs_type`` 156 describes the filesystem, partly initialized by the specific 157 filesystem code 158 159``int flags`` 160 mount flags 161 162``const char *dev_name`` 163 the device name we are mounting. 164 165``void *data`` 166 arbitrary mount options, usually comes as an ASCII string (see 167 "Mount Options" section) 168 169The mount() method must return the root dentry of the tree requested by 170caller. An active reference to its superblock must be grabbed and the 171superblock must be locked. On failure it should return ERR_PTR(error). 172 173The arguments match those of mount(2) and their interpretation depends 174on filesystem type. E.g. for block filesystems, dev_name is interpreted 175as block device name, that device is opened and if it contains a 176suitable filesystem image the method creates and initializes struct 177super_block accordingly, returning its root dentry to caller. 178 179->mount() may choose to return a subtree of existing filesystem - it 180doesn't have to create a new one. The main result from the caller's 181point of view is a reference to dentry at the root of (sub)tree to be 182attached; creation of new superblock is a common side effect. 183 184The most interesting member of the superblock structure that the mount() 185method fills in is the "s_op" field. This is a pointer to a "struct 186super_operations" which describes the next level of the filesystem 187implementation. 188 189Usually, a filesystem uses one of the generic mount() implementations 190and provides a fill_super() callback instead. The generic variants are: 191 192``mount_bdev`` 193 mount a filesystem residing on a block device 194 195``mount_nodev`` 196 mount a filesystem that is not backed by a device 197 198``mount_single`` 199 mount a filesystem which shares the instance between all mounts 200 201A fill_super() callback implementation has the following arguments: 202 203``struct super_block *sb`` 204 the superblock structure. The callback must initialize this 205 properly. 206 207``void *data`` 208 arbitrary mount options, usually comes as an ASCII string (see 209 "Mount Options" section) 210 211``int silent`` 212 whether or not to be silent on error 213 214 215The Superblock Object 216===================== 217 218A superblock object represents a mounted filesystem. 219 220 221struct super_operations 222----------------------- 223 224This describes how the VFS can manipulate the superblock of your 225filesystem. As of kernel 2.6.22, the following members are defined: 226 227.. code-block:: c 228 229 struct super_operations { 230 struct inode *(*alloc_inode)(struct super_block *sb); 231 void (*destroy_inode)(struct inode *); 232 233 void (*dirty_inode) (struct inode *, int flags); 234 int (*write_inode) (struct inode *, int); 235 void (*drop_inode) (struct inode *); 236 void (*delete_inode) (struct inode *); 237 void (*put_super) (struct super_block *); 238 int (*sync_fs)(struct super_block *sb, int wait); 239 int (*freeze_fs) (struct super_block *); 240 int (*unfreeze_fs) (struct super_block *); 241 int (*statfs) (struct dentry *, struct kstatfs *); 242 int (*remount_fs) (struct super_block *, int *, char *); 243 void (*clear_inode) (struct inode *); 244 void (*umount_begin) (struct super_block *); 245 246 int (*show_options)(struct seq_file *, struct dentry *); 247 248 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t); 249 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t); 250 int (*nr_cached_objects)(struct super_block *); 251 void (*free_cached_objects)(struct super_block *, int); 252 }; 253 254All methods are called without any locks being held, unless otherwise 255noted. This means that most methods can block safely. All methods are 256only called from a process context (i.e. not from an interrupt handler 257or bottom half). 258 259``alloc_inode`` 260 this method is called by alloc_inode() to allocate memory for 261 struct inode and initialize it. If this function is not 262 defined, a simple 'struct inode' is allocated. Normally 263 alloc_inode will be used to allocate a larger structure which 264 contains a 'struct inode' embedded within it. 265 266``destroy_inode`` 267 this method is called by destroy_inode() to release resources 268 allocated for struct inode. It is only required if 269 ->alloc_inode was defined and simply undoes anything done by 270 ->alloc_inode. 271 272``dirty_inode`` 273 this method is called by the VFS when an inode is marked dirty. 274 This is specifically for the inode itself being marked dirty, 275 not its data. If the update needs to be persisted by fdatasync(), 276 then I_DIRTY_DATASYNC will be set in the flags argument. 277 278``write_inode`` 279 this method is called when the VFS needs to write an inode to 280 disc. The second parameter indicates whether the write should 281 be synchronous or not, not all filesystems check this flag. 282 283``drop_inode`` 284 called when the last access to the inode is dropped, with the 285 inode->i_lock spinlock held. 286 287 This method should be either NULL (normal UNIX filesystem 288 semantics) or "generic_delete_inode" (for filesystems that do 289 not want to cache inodes - causing "delete_inode" to always be 290 called regardless of the value of i_nlink) 291 292 The "generic_delete_inode()" behavior is equivalent to the old 293 practice of using "force_delete" in the put_inode() case, but 294 does not have the races that the "force_delete()" approach had. 295 296``delete_inode`` 297 called when the VFS wants to delete an inode 298 299``put_super`` 300 called when the VFS wishes to free the superblock 301 (i.e. unmount). This is called with the superblock lock held 302 303``sync_fs`` 304 called when VFS is writing out all dirty data associated with a 305 superblock. The second parameter indicates whether the method 306 should wait until the write out has been completed. Optional. 307 308``freeze_fs`` 309 called when VFS is locking a filesystem and forcing it into a 310 consistent state. This method is currently used by the Logical 311 Volume Manager (LVM). 312 313``unfreeze_fs`` 314 called when VFS is unlocking a filesystem and making it writable 315 again. 316 317``statfs`` 318 called when the VFS needs to get filesystem statistics. 319 320``remount_fs`` 321 called when the filesystem is remounted. This is called with 322 the kernel lock held 323 324``clear_inode`` 325 called then the VFS clears the inode. Optional 326 327``umount_begin`` 328 called when the VFS is unmounting a filesystem. 329 330``show_options`` 331 called by the VFS to show mount options for /proc/<pid>/mounts. 332 (see "Mount Options" section) 333 334``quota_read`` 335 called by the VFS to read from filesystem quota file. 336 337``quota_write`` 338 called by the VFS to write to filesystem quota file. 339 340``nr_cached_objects`` 341 called by the sb cache shrinking function for the filesystem to 342 return the number of freeable cached objects it contains. 343 Optional. 344 345``free_cache_objects`` 346 called by the sb cache shrinking function for the filesystem to 347 scan the number of objects indicated to try to free them. 348 Optional, but any filesystem implementing this method needs to 349 also implement ->nr_cached_objects for it to be called 350 correctly. 351 352 We can't do anything with any errors that the filesystem might 353 encountered, hence the void return type. This will never be 354 called if the VM is trying to reclaim under GFP_NOFS conditions, 355 hence this method does not need to handle that situation itself. 356 357 Implementations must include conditional reschedule calls inside 358 any scanning loop that is done. This allows the VFS to 359 determine appropriate scan batch sizes without having to worry 360 about whether implementations will cause holdoff problems due to 361 large scan batch sizes. 362 363Whoever sets up the inode is responsible for filling in the "i_op" 364field. This is a pointer to a "struct inode_operations" which describes 365the methods that can be performed on individual inodes. 366 367 368struct xattr_handlers 369--------------------- 370 371On filesystems that support extended attributes (xattrs), the s_xattr 372superblock field points to a NULL-terminated array of xattr handlers. 373Extended attributes are name:value pairs. 374 375``name`` 376 Indicates that the handler matches attributes with the specified 377 name (such as "system.posix_acl_access"); the prefix field must 378 be NULL. 379 380``prefix`` 381 Indicates that the handler matches all attributes with the 382 specified name prefix (such as "user."); the name field must be 383 NULL. 384 385``list`` 386 Determine if attributes matching this xattr handler should be 387 listed for a particular dentry. Used by some listxattr 388 implementations like generic_listxattr. 389 390``get`` 391 Called by the VFS to get the value of a particular extended 392 attribute. This method is called by the getxattr(2) system 393 call. 394 395``set`` 396 Called by the VFS to set the value of a particular extended 397 attribute. When the new value is NULL, called to remove a 398 particular extended attribute. This method is called by the 399 setxattr(2) and removexattr(2) system calls. 400 401When none of the xattr handlers of a filesystem match the specified 402attribute name or when a filesystem doesn't support extended attributes, 403the various ``*xattr(2)`` system calls return -EOPNOTSUPP. 404 405 406The Inode Object 407================ 408 409An inode object represents an object within the filesystem. 410 411 412struct inode_operations 413----------------------- 414 415This describes how the VFS can manipulate an inode in your filesystem. 416As of kernel 2.6.22, the following members are defined: 417 418.. code-block:: c 419 420 struct inode_operations { 421 int (*create) (struct user_namespace *, struct inode *,struct dentry *, umode_t, bool); 422 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int); 423 int (*link) (struct dentry *,struct inode *,struct dentry *); 424 int (*unlink) (struct inode *,struct dentry *); 425 int (*symlink) (struct user_namespace *, struct inode *,struct dentry *,const char *); 426 int (*mkdir) (struct user_namespace *, struct inode *,struct dentry *,umode_t); 427 int (*rmdir) (struct inode *,struct dentry *); 428 int (*mknod) (struct user_namespace *, struct inode *,struct dentry *,umode_t,dev_t); 429 int (*rename) (struct user_namespace *, struct inode *, struct dentry *, 430 struct inode *, struct dentry *, unsigned int); 431 int (*readlink) (struct dentry *, char __user *,int); 432 const char *(*get_link) (struct dentry *, struct inode *, 433 struct delayed_call *); 434 int (*permission) (struct user_namespace *, struct inode *, int); 435 struct posix_acl * (*get_acl)(struct inode *, int, bool); 436 int (*setattr) (struct user_namespace *, struct dentry *, struct iattr *); 437 int (*getattr) (struct user_namespace *, const struct path *, struct kstat *, u32, unsigned int); 438 ssize_t (*listxattr) (struct dentry *, char *, size_t); 439 void (*update_time)(struct inode *, struct timespec *, int); 440 int (*atomic_open)(struct inode *, struct dentry *, struct file *, 441 unsigned open_flag, umode_t create_mode); 442 int (*tmpfile) (struct user_namespace *, struct inode *, struct dentry *, umode_t); 443 int (*set_acl)(struct user_namespace *, struct inode *, struct posix_acl *, int); 444 int (*fileattr_set)(struct user_namespace *mnt_userns, 445 struct dentry *dentry, struct fileattr *fa); 446 int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa); 447 }; 448 449Again, all methods are called without any locks being held, unless 450otherwise noted. 451 452``create`` 453 called by the open(2) and creat(2) system calls. Only required 454 if you want to support regular files. The dentry you get should 455 not have an inode (i.e. it should be a negative dentry). Here 456 you will probably call d_instantiate() with the dentry and the 457 newly created inode 458 459``lookup`` 460 called when the VFS needs to look up an inode in a parent 461 directory. The name to look for is found in the dentry. This 462 method must call d_add() to insert the found inode into the 463 dentry. The "i_count" field in the inode structure should be 464 incremented. If the named inode does not exist a NULL inode 465 should be inserted into the dentry (this is called a negative 466 dentry). Returning an error code from this routine must only be 467 done on a real error, otherwise creating inodes with system 468 calls like create(2), mknod(2), mkdir(2) and so on will fail. 469 If you wish to overload the dentry methods then you should 470 initialise the "d_dop" field in the dentry; this is a pointer to 471 a struct "dentry_operations". This method is called with the 472 directory inode semaphore held 473 474``link`` 475 called by the link(2) system call. Only required if you want to 476 support hard links. You will probably need to call 477 d_instantiate() just as you would in the create() method 478 479``unlink`` 480 called by the unlink(2) system call. Only required if you want 481 to support deleting inodes 482 483``symlink`` 484 called by the symlink(2) system call. Only required if you want 485 to support symlinks. You will probably need to call 486 d_instantiate() just as you would in the create() method 487 488``mkdir`` 489 called by the mkdir(2) system call. Only required if you want 490 to support creating subdirectories. You will probably need to 491 call d_instantiate() just as you would in the create() method 492 493``rmdir`` 494 called by the rmdir(2) system call. Only required if you want 495 to support deleting subdirectories 496 497``mknod`` 498 called by the mknod(2) system call to create a device (char, 499 block) inode or a named pipe (FIFO) or socket. Only required if 500 you want to support creating these types of inodes. You will 501 probably need to call d_instantiate() just as you would in the 502 create() method 503 504``rename`` 505 called by the rename(2) system call to rename the object to have 506 the parent and name given by the second inode and dentry. 507 508 The filesystem must return -EINVAL for any unsupported or 509 unknown flags. Currently the following flags are implemented: 510 (1) RENAME_NOREPLACE: this flag indicates that if the target of 511 the rename exists the rename should fail with -EEXIST instead of 512 replacing the target. The VFS already checks for existence, so 513 for local filesystems the RENAME_NOREPLACE implementation is 514 equivalent to plain rename. 515 (2) RENAME_EXCHANGE: exchange source and target. Both must 516 exist; this is checked by the VFS. Unlike plain rename, source 517 and target may be of different type. 518 519``get_link`` 520 called by the VFS to follow a symbolic link to the inode it 521 points to. Only required if you want to support symbolic links. 522 This method returns the symlink body to traverse (and possibly 523 resets the current position with nd_jump_link()). If the body 524 won't go away until the inode is gone, nothing else is needed; 525 if it needs to be otherwise pinned, arrange for its release by 526 having get_link(..., ..., done) do set_delayed_call(done, 527 destructor, argument). In that case destructor(argument) will 528 be called once VFS is done with the body you've returned. May 529 be called in RCU mode; that is indicated by NULL dentry 530 argument. If request can't be handled without leaving RCU mode, 531 have it return ERR_PTR(-ECHILD). 532 533 If the filesystem stores the symlink target in ->i_link, the 534 VFS may use it directly without calling ->get_link(); however, 535 ->get_link() must still be provided. ->i_link must not be 536 freed until after an RCU grace period. Writing to ->i_link 537 post-iget() time requires a 'release' memory barrier. 538 539``readlink`` 540 this is now just an override for use by readlink(2) for the 541 cases when ->get_link uses nd_jump_link() or object is not in 542 fact a symlink. Normally filesystems should only implement 543 ->get_link for symlinks and readlink(2) will automatically use 544 that. 545 546``permission`` 547 called by the VFS to check for access rights on a POSIX-like 548 filesystem. 549 550 May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in 551 rcu-walk mode, the filesystem must check the permission without 552 blocking or storing to the inode. 553 554 If a situation is encountered that rcu-walk cannot handle, 555 return 556 -ECHILD and it will be called again in ref-walk mode. 557 558``setattr`` 559 called by the VFS to set attributes for a file. This method is 560 called by chmod(2) and related system calls. 561 562``getattr`` 563 called by the VFS to get attributes of a file. This method is 564 called by stat(2) and related system calls. 565 566``listxattr`` 567 called by the VFS to list all extended attributes for a given 568 file. This method is called by the listxattr(2) system call. 569 570``update_time`` 571 called by the VFS to update a specific time or the i_version of 572 an inode. If this is not defined the VFS will update the inode 573 itself and call mark_inode_dirty_sync. 574 575``atomic_open`` 576 called on the last component of an open. Using this optional 577 method the filesystem can look up, possibly create and open the 578 file in one atomic operation. If it wants to leave actual 579 opening to the caller (e.g. if the file turned out to be a 580 symlink, device, or just something filesystem won't do atomic 581 open for), it may signal this by returning finish_no_open(file, 582 dentry). This method is only called if the last component is 583 negative or needs lookup. Cached positive dentries are still 584 handled by f_op->open(). If the file was created, FMODE_CREATED 585 flag should be set in file->f_mode. In case of O_EXCL the 586 method must only succeed if the file didn't exist and hence 587 FMODE_CREATED shall always be set on success. 588 589``tmpfile`` 590 called in the end of O_TMPFILE open(). Optional, equivalent to 591 atomically creating, opening and unlinking a file in given 592 directory. 593 594``fileattr_get`` 595 called on ioctl(FS_IOC_GETFLAGS) and ioctl(FS_IOC_FSGETXATTR) to 596 retrieve miscellaneous file flags and attributes. Also called 597 before the relevant SET operation to check what is being changed 598 (in this case with i_rwsem locked exclusive). If unset, then 599 fall back to f_op->ioctl(). 600 601``fileattr_set`` 602 called on ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_FSSETXATTR) to 603 change miscellaneous file flags and attributes. Callers hold 604 i_rwsem exclusive. If unset, then fall back to f_op->ioctl(). 605 606 607The Address Space Object 608======================== 609 610The address space object is used to group and manage pages in the page 611cache. It can be used to keep track of the pages in a file (or anything 612else) and also track the mapping of sections of the file into process 613address spaces. 614 615There are a number of distinct yet related services that an 616address-space can provide. These include communicating memory pressure, 617page lookup by address, and keeping track of pages tagged as Dirty or 618Writeback. 619 620The first can be used independently to the others. The VM can try to 621either write dirty pages in order to clean them, or release clean pages 622in order to reuse them. To do this it can call the ->writepage method 623on dirty pages, and ->releasepage on clean pages with PagePrivate set. 624Clean pages without PagePrivate and with no external references will be 625released without notice being given to the address_space. 626 627To achieve this functionality, pages need to be placed on an LRU with 628lru_cache_add and mark_page_active needs to be called whenever the page 629is used. 630 631Pages are normally kept in a radix tree index by ->index. This tree 632maintains information about the PG_Dirty and PG_Writeback status of each 633page, so that pages with either of these flags can be found quickly. 634 635The Dirty tag is primarily used by mpage_writepages - the default 636->writepages method. It uses the tag to find dirty pages to call 637->writepage on. If mpage_writepages is not used (i.e. the address 638provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is almost 639unused. write_inode_now and sync_inode do use it (through 640__sync_single_inode) to check if ->writepages has been successful in 641writing out the whole address_space. 642 643The Writeback tag is used by filemap*wait* and sync_page* functions, via 644filemap_fdatawait_range, to wait for all writeback to complete. 645 646An address_space handler may attach extra information to a page, 647typically using the 'private' field in the 'struct page'. If such 648information is attached, the PG_Private flag should be set. This will 649cause various VM routines to make extra calls into the address_space 650handler to deal with that data. 651 652An address space acts as an intermediate between storage and 653application. Data is read into the address space a whole page at a 654time, and provided to the application either by copying of the page, or 655by memory-mapping the page. Data is written into the address space by 656the application, and then written-back to storage typically in whole 657pages, however the address_space has finer control of write sizes. 658 659The read process essentially only requires 'readpage'. The write 660process is more complicated and uses write_begin/write_end or 661set_page_dirty to write data into the address_space, and writepage and 662writepages to writeback data to storage. 663 664Adding and removing pages to/from an address_space is protected by the 665inode's i_mutex. 666 667When data is written to a page, the PG_Dirty flag should be set. It 668typically remains set until writepage asks for it to be written. This 669should clear PG_Dirty and set PG_Writeback. It can be actually written 670at any point after PG_Dirty is clear. Once it is known to be safe, 671PG_Writeback is cleared. 672 673Writeback makes use of a writeback_control structure to direct the 674operations. This gives the writepage and writepages operations some 675information about the nature of and reason for the writeback request, 676and the constraints under which it is being done. It is also used to 677return information back to the caller about the result of a writepage or 678writepages request. 679 680 681Handling errors during writeback 682-------------------------------- 683 684Most applications that do buffered I/O will periodically call a file 685synchronization call (fsync, fdatasync, msync or sync_file_range) to 686ensure that data written has made it to the backing store. When there 687is an error during writeback, they expect that error to be reported when 688a file sync request is made. After an error has been reported on one 689request, subsequent requests on the same file descriptor should return 6900, unless further writeback errors have occurred since the previous file 691syncronization. 692 693Ideally, the kernel would report errors only on file descriptions on 694which writes were done that subsequently failed to be written back. The 695generic pagecache infrastructure does not track the file descriptions 696that have dirtied each individual page however, so determining which 697file descriptors should get back an error is not possible. 698 699Instead, the generic writeback error tracking infrastructure in the 700kernel settles for reporting errors to fsync on all file descriptions 701that were open at the time that the error occurred. In a situation with 702multiple writers, all of them will get back an error on a subsequent 703fsync, even if all of the writes done through that particular file 704descriptor succeeded (or even if there were no writes on that file 705descriptor at all). 706 707Filesystems that wish to use this infrastructure should call 708mapping_set_error to record the error in the address_space when it 709occurs. Then, after writing back data from the pagecache in their 710file->fsync operation, they should call file_check_and_advance_wb_err to 711ensure that the struct file's error cursor has advanced to the correct 712point in the stream of errors emitted by the backing device(s). 713 714 715struct address_space_operations 716------------------------------- 717 718This describes how the VFS can manipulate mapping of a file to page 719cache in your filesystem. The following members are defined: 720 721.. code-block:: c 722 723 struct address_space_operations { 724 int (*writepage)(struct page *page, struct writeback_control *wbc); 725 int (*readpage)(struct file *, struct page *); 726 int (*writepages)(struct address_space *, struct writeback_control *); 727 int (*set_page_dirty)(struct page *page); 728 void (*readahead)(struct readahead_control *); 729 int (*readpages)(struct file *filp, struct address_space *mapping, 730 struct list_head *pages, unsigned nr_pages); 731 int (*write_begin)(struct file *, struct address_space *mapping, 732 loff_t pos, unsigned len, unsigned flags, 733 struct page **pagep, void **fsdata); 734 int (*write_end)(struct file *, struct address_space *mapping, 735 loff_t pos, unsigned len, unsigned copied, 736 struct page *page, void *fsdata); 737 sector_t (*bmap)(struct address_space *, sector_t); 738 void (*invalidatepage) (struct page *, unsigned int, unsigned int); 739 int (*releasepage) (struct page *, int); 740 void (*freepage)(struct page *); 741 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter); 742 /* isolate a page for migration */ 743 bool (*isolate_page) (struct page *, isolate_mode_t); 744 /* migrate the contents of a page to the specified target */ 745 int (*migratepage) (struct page *, struct page *); 746 /* put migration-failed page back to right list */ 747 void (*putback_page) (struct page *); 748 int (*launder_page) (struct page *); 749 750 int (*is_partially_uptodate) (struct page *, unsigned long, 751 unsigned long); 752 void (*is_dirty_writeback) (struct page *, bool *, bool *); 753 int (*error_remove_page) (struct mapping *mapping, struct page *page); 754 int (*swap_activate)(struct file *); 755 int (*swap_deactivate)(struct file *); 756 }; 757 758``writepage`` 759 called by the VM to write a dirty page to backing store. This 760 may happen for data integrity reasons (i.e. 'sync'), or to free 761 up memory (flush). The difference can be seen in 762 wbc->sync_mode. The PG_Dirty flag has been cleared and 763 PageLocked is true. writepage should start writeout, should set 764 PG_Writeback, and should make sure the page is unlocked, either 765 synchronously or asynchronously when the write operation 766 completes. 767 768 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to 769 try too hard if there are problems, and may choose to write out 770 other pages from the mapping if that is easier (e.g. due to 771 internal dependencies). If it chooses not to start writeout, it 772 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not 773 keep calling ->writepage on that page. 774 775 See the file "Locking" for more details. 776 777``readpage`` 778 called by the VM to read a page from backing store. The page 779 will be Locked when readpage is called, and should be unlocked 780 and marked uptodate once the read completes. If ->readpage 781 discovers that it needs to unlock the page for some reason, it 782 can do so, and then return AOP_TRUNCATED_PAGE. In this case, 783 the page will be relocated, relocked and if that all succeeds, 784 ->readpage will be called again. 785 786``writepages`` 787 called by the VM to write out pages associated with the 788 address_space object. If wbc->sync_mode is WB_SYNC_ALL, then 789 the writeback_control will specify a range of pages that must be 790 written out. If it is WB_SYNC_NONE, then a nr_to_write is 791 given and that many pages should be written if possible. If no 792 ->writepages is given, then mpage_writepages is used instead. 793 This will choose pages from the address space that are tagged as 794 DIRTY and will pass them to ->writepage. 795 796``set_page_dirty`` 797 called by the VM to set a page dirty. This is particularly 798 needed if an address space attaches private data to a page, and 799 that data needs to be updated when a page is dirtied. This is 800 called, for example, when a memory mapped page gets modified. 801 If defined, it should set the PageDirty flag, and the 802 PAGECACHE_TAG_DIRTY tag in the radix tree. 803 804``readahead`` 805 Called by the VM to read pages associated with the address_space 806 object. The pages are consecutive in the page cache and are 807 locked. The implementation should decrement the page refcount 808 after starting I/O on each page. Usually the page will be 809 unlocked by the I/O completion handler. If the filesystem decides 810 to stop attempting I/O before reaching the end of the readahead 811 window, it can simply return. The caller will decrement the page 812 refcount and unlock the remaining pages for you. Set PageUptodate 813 if the I/O completes successfully. Setting PageError on any page 814 will be ignored; simply unlock the page if an I/O error occurs. 815 816``readpages`` 817 called by the VM to read pages associated with the address_space 818 object. This is essentially just a vector version of readpage. 819 Instead of just one page, several pages are requested. 820 readpages is only used for read-ahead, so read errors are 821 ignored. If anything goes wrong, feel free to give up. 822 This interface is deprecated and will be removed by the end of 823 2020; implement readahead instead. 824 825``write_begin`` 826 Called by the generic buffered write code to ask the filesystem 827 to prepare to write len bytes at the given offset in the file. 828 The address_space should check that the write will be able to 829 complete, by allocating space if necessary and doing any other 830 internal housekeeping. If the write will update parts of any 831 basic-blocks on storage, then those blocks should be pre-read 832 (if they haven't been read already) so that the updated blocks 833 can be written out properly. 834 835 The filesystem must return the locked pagecache page for the 836 specified offset, in ``*pagep``, for the caller to write into. 837 838 It must be able to cope with short writes (where the length 839 passed to write_begin is greater than the number of bytes copied 840 into the page). 841 842 flags is a field for AOP_FLAG_xxx flags, described in 843 include/linux/fs.h. 844 845 A void * may be returned in fsdata, which then gets passed into 846 write_end. 847 848 Returns 0 on success; < 0 on failure (which is the error code), 849 in which case write_end is not called. 850 851``write_end`` 852 After a successful write_begin, and data copy, write_end must be 853 called. len is the original len passed to write_begin, and 854 copied is the amount that was able to be copied. 855 856 The filesystem must take care of unlocking the page and 857 releasing it refcount, and updating i_size. 858 859 Returns < 0 on failure, otherwise the number of bytes (<= 860 'copied') that were able to be copied into pagecache. 861 862``bmap`` 863 called by the VFS to map a logical block offset within object to 864 physical block number. This method is used by the FIBMAP ioctl 865 and for working with swap-files. To be able to swap to a file, 866 the file must have a stable mapping to a block device. The swap 867 system does not go through the filesystem but instead uses bmap 868 to find out where the blocks in the file are and uses those 869 addresses directly. 870 871``invalidatepage`` 872 If a page has PagePrivate set, then invalidatepage will be 873 called when part or all of the page is to be removed from the 874 address space. This generally corresponds to either a 875 truncation, punch hole or a complete invalidation of the address 876 space (in the latter case 'offset' will always be 0 and 'length' 877 will be PAGE_SIZE). Any private data associated with the page 878 should be updated to reflect this truncation. If offset is 0 879 and length is PAGE_SIZE, then the private data should be 880 released, because the page must be able to be completely 881 discarded. This may be done by calling the ->releasepage 882 function, but in this case the release MUST succeed. 883 884``releasepage`` 885 releasepage is called on PagePrivate pages to indicate that the 886 page should be freed if possible. ->releasepage should remove 887 any private data from the page and clear the PagePrivate flag. 888 If releasepage() fails for some reason, it must indicate failure 889 with a 0 return value. releasepage() is used in two distinct 890 though related cases. The first is when the VM finds a clean 891 page with no active users and wants to make it a free page. If 892 ->releasepage succeeds, the page will be removed from the 893 address_space and become free. 894 895 The second case is when a request has been made to invalidate 896 some or all pages in an address_space. This can happen through 897 the fadvise(POSIX_FADV_DONTNEED) system call or by the 898 filesystem explicitly requesting it as nfs and 9fs do (when they 899 believe the cache may be out of date with storage) by calling 900 invalidate_inode_pages2(). If the filesystem makes such a call, 901 and needs to be certain that all pages are invalidated, then its 902 releasepage will need to ensure this. Possibly it can clear the 903 PageUptodate bit if it cannot free private data yet. 904 905``freepage`` 906 freepage is called once the page is no longer visible in the 907 page cache in order to allow the cleanup of any private data. 908 Since it may be called by the memory reclaimer, it should not 909 assume that the original address_space mapping still exists, and 910 it should not block. 911 912``direct_IO`` 913 called by the generic read/write routines to perform direct_IO - 914 that is IO requests which bypass the page cache and transfer 915 data directly between the storage and the application's address 916 space. 917 918``isolate_page`` 919 Called by the VM when isolating a movable non-lru page. If page 920 is successfully isolated, VM marks the page as PG_isolated via 921 __SetPageIsolated. 922 923``migrate_page`` 924 This is used to compact the physical memory usage. If the VM 925 wants to relocate a page (maybe off a memory card that is 926 signalling imminent failure) it will pass a new page and an old 927 page to this function. migrate_page should transfer any private 928 data across and update any references that it has to the page. 929 930``putback_page`` 931 Called by the VM when isolated page's migration fails. 932 933``launder_page`` 934 Called before freeing a page - it writes back the dirty page. 935 To prevent redirtying the page, it is kept locked during the 936 whole operation. 937 938``is_partially_uptodate`` 939 Called by the VM when reading a file through the pagecache when 940 the underlying blocksize != pagesize. If the required block is 941 up to date then the read can complete without needing the IO to 942 bring the whole page up to date. 943 944``is_dirty_writeback`` 945 Called by the VM when attempting to reclaim a page. The VM uses 946 dirty and writeback information to determine if it needs to 947 stall to allow flushers a chance to complete some IO. 948 Ordinarily it can use PageDirty and PageWriteback but some 949 filesystems have more complex state (unstable pages in NFS 950 prevent reclaim) or do not set those flags due to locking 951 problems. This callback allows a filesystem to indicate to the 952 VM if a page should be treated as dirty or writeback for the 953 purposes of stalling. 954 955``error_remove_page`` 956 normally set to generic_error_remove_page if truncation is ok 957 for this address space. Used for memory failure handling. 958 Setting this implies you deal with pages going away under you, 959 unless you have them locked or reference counts increased. 960 961``swap_activate`` 962 Called when swapon is used on a file to allocate space if 963 necessary and pin the block lookup information in memory. A 964 return value of zero indicates success, in which case this file 965 can be used to back swapspace. 966 967``swap_deactivate`` 968 Called during swapoff on files where swap_activate was 969 successful. 970 971 972The File Object 973=============== 974 975A file object represents a file opened by a process. This is also known 976as an "open file description" in POSIX parlance. 977 978 979struct file_operations 980---------------------- 981 982This describes how the VFS can manipulate an open file. As of kernel 9834.18, the following members are defined: 984 985.. code-block:: c 986 987 struct file_operations { 988 struct module *owner; 989 loff_t (*llseek) (struct file *, loff_t, int); 990 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); 991 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); 992 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *); 993 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *); 994 int (*iopoll)(struct kiocb *kiocb, bool spin); 995 int (*iterate) (struct file *, struct dir_context *); 996 int (*iterate_shared) (struct file *, struct dir_context *); 997 __poll_t (*poll) (struct file *, struct poll_table_struct *); 998 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); 999 long (*compat_ioctl) (struct file *, unsigned int, unsigned long); 1000 int (*mmap) (struct file *, struct vm_area_struct *); 1001 int (*open) (struct inode *, struct file *); 1002 int (*flush) (struct file *, fl_owner_t id); 1003 int (*release) (struct inode *, struct file *); 1004 int (*fsync) (struct file *, loff_t, loff_t, int datasync); 1005 int (*fasync) (int, struct file *, int); 1006 int (*lock) (struct file *, int, struct file_lock *); 1007 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int); 1008 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long); 1009 int (*check_flags)(int); 1010 int (*flock) (struct file *, int, struct file_lock *); 1011 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int); 1012 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int); 1013 int (*setlease)(struct file *, long, struct file_lock **, void **); 1014 long (*fallocate)(struct file *file, int mode, loff_t offset, 1015 loff_t len); 1016 void (*show_fdinfo)(struct seq_file *m, struct file *f); 1017 #ifndef CONFIG_MMU 1018 unsigned (*mmap_capabilities)(struct file *); 1019 #endif 1020 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int); 1021 loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in, 1022 struct file *file_out, loff_t pos_out, 1023 loff_t len, unsigned int remap_flags); 1024 int (*fadvise)(struct file *, loff_t, loff_t, int); 1025 }; 1026 1027Again, all methods are called without any locks being held, unless 1028otherwise noted. 1029 1030``llseek`` 1031 called when the VFS needs to move the file position index 1032 1033``read`` 1034 called by read(2) and related system calls 1035 1036``read_iter`` 1037 possibly asynchronous read with iov_iter as destination 1038 1039``write`` 1040 called by write(2) and related system calls 1041 1042``write_iter`` 1043 possibly asynchronous write with iov_iter as source 1044 1045``iopoll`` 1046 called when aio wants to poll for completions on HIPRI iocbs 1047 1048``iterate`` 1049 called when the VFS needs to read the directory contents 1050 1051``iterate_shared`` 1052 called when the VFS needs to read the directory contents when 1053 filesystem supports concurrent dir iterators 1054 1055``poll`` 1056 called by the VFS when a process wants to check if there is 1057 activity on this file and (optionally) go to sleep until there 1058 is activity. Called by the select(2) and poll(2) system calls 1059 1060``unlocked_ioctl`` 1061 called by the ioctl(2) system call. 1062 1063``compat_ioctl`` 1064 called by the ioctl(2) system call when 32 bit system calls are 1065 used on 64 bit kernels. 1066 1067``mmap`` 1068 called by the mmap(2) system call 1069 1070``open`` 1071 called by the VFS when an inode should be opened. When the VFS 1072 opens a file, it creates a new "struct file". It then calls the 1073 open method for the newly allocated file structure. You might 1074 think that the open method really belongs in "struct 1075 inode_operations", and you may be right. I think it's done the 1076 way it is because it makes filesystems simpler to implement. 1077 The open() method is a good place to initialize the 1078 "private_data" member in the file structure if you want to point 1079 to a device structure 1080 1081``flush`` 1082 called by the close(2) system call to flush a file 1083 1084``release`` 1085 called when the last reference to an open file is closed 1086 1087``fsync`` 1088 called by the fsync(2) system call. Also see the section above 1089 entitled "Handling errors during writeback". 1090 1091``fasync`` 1092 called by the fcntl(2) system call when asynchronous 1093 (non-blocking) mode is enabled for a file 1094 1095``lock`` 1096 called by the fcntl(2) system call for F_GETLK, F_SETLK, and 1097 F_SETLKW commands 1098 1099``get_unmapped_area`` 1100 called by the mmap(2) system call 1101 1102``check_flags`` 1103 called by the fcntl(2) system call for F_SETFL command 1104 1105``flock`` 1106 called by the flock(2) system call 1107 1108``splice_write`` 1109 called by the VFS to splice data from a pipe to a file. This 1110 method is used by the splice(2) system call 1111 1112``splice_read`` 1113 called by the VFS to splice data from file to a pipe. This 1114 method is used by the splice(2) system call 1115 1116``setlease`` 1117 called by the VFS to set or release a file lock lease. setlease 1118 implementations should call generic_setlease to record or remove 1119 the lease in the inode after setting it. 1120 1121``fallocate`` 1122 called by the VFS to preallocate blocks or punch a hole. 1123 1124``copy_file_range`` 1125 called by the copy_file_range(2) system call. 1126 1127``remap_file_range`` 1128 called by the ioctl(2) system call for FICLONERANGE and FICLONE 1129 and FIDEDUPERANGE commands to remap file ranges. An 1130 implementation should remap len bytes at pos_in of the source 1131 file into the dest file at pos_out. Implementations must handle 1132 callers passing in len == 0; this means "remap to the end of the 1133 source file". The return value should the number of bytes 1134 remapped, or the usual negative error code if errors occurred 1135 before any bytes were remapped. The remap_flags parameter 1136 accepts REMAP_FILE_* flags. If REMAP_FILE_DEDUP is set then the 1137 implementation must only remap if the requested file ranges have 1138 identical contents. If REMAP_FILE_CAN_SHORTEN is set, the caller is 1139 ok with the implementation shortening the request length to 1140 satisfy alignment or EOF requirements (or any other reason). 1141 1142``fadvise`` 1143 possibly called by the fadvise64() system call. 1144 1145Note that the file operations are implemented by the specific 1146filesystem in which the inode resides. When opening a device node 1147(character or block special) most filesystems will call special 1148support routines in the VFS which will locate the required device 1149driver information. These support routines replace the filesystem file 1150operations with those for the device driver, and then proceed to call 1151the new open() method for the file. This is how opening a device file 1152in the filesystem eventually ends up calling the device driver open() 1153method. 1154 1155 1156Directory Entry Cache (dcache) 1157============================== 1158 1159 1160struct dentry_operations 1161------------------------ 1162 1163This describes how a filesystem can overload the standard dentry 1164operations. Dentries and the dcache are the domain of the VFS and the 1165individual filesystem implementations. Device drivers have no business 1166here. These methods may be set to NULL, as they are either optional or 1167the VFS uses a default. As of kernel 2.6.22, the following members are 1168defined: 1169 1170.. code-block:: c 1171 1172 struct dentry_operations { 1173 int (*d_revalidate)(struct dentry *, unsigned int); 1174 int (*d_weak_revalidate)(struct dentry *, unsigned int); 1175 int (*d_hash)(const struct dentry *, struct qstr *); 1176 int (*d_compare)(const struct dentry *, 1177 unsigned int, const char *, const struct qstr *); 1178 int (*d_delete)(const struct dentry *); 1179 int (*d_init)(struct dentry *); 1180 void (*d_release)(struct dentry *); 1181 void (*d_iput)(struct dentry *, struct inode *); 1182 char *(*d_dname)(struct dentry *, char *, int); 1183 struct vfsmount *(*d_automount)(struct path *); 1184 int (*d_manage)(const struct path *, bool); 1185 struct dentry *(*d_real)(struct dentry *, const struct inode *); 1186 }; 1187 1188``d_revalidate`` 1189 called when the VFS needs to revalidate a dentry. This is 1190 called whenever a name look-up finds a dentry in the dcache. 1191 Most local filesystems leave this as NULL, because all their 1192 dentries in the dcache are valid. Network filesystems are 1193 different since things can change on the server without the 1194 client necessarily being aware of it. 1195 1196 This function should return a positive value if the dentry is 1197 still valid, and zero or a negative error code if it isn't. 1198 1199 d_revalidate may be called in rcu-walk mode (flags & 1200 LOOKUP_RCU). If in rcu-walk mode, the filesystem must 1201 revalidate the dentry without blocking or storing to the dentry, 1202 d_parent and d_inode should not be used without care (because 1203 they can change and, in d_inode case, even become NULL under 1204 us). 1205 1206 If a situation is encountered that rcu-walk cannot handle, 1207 return 1208 -ECHILD and it will be called again in ref-walk mode. 1209 1210``_weak_revalidate`` 1211 called when the VFS needs to revalidate a "jumped" dentry. This 1212 is called when a path-walk ends at dentry that was not acquired 1213 by doing a lookup in the parent directory. This includes "/", 1214 "." and "..", as well as procfs-style symlinks and mountpoint 1215 traversal. 1216 1217 In this case, we are less concerned with whether the dentry is 1218 still fully correct, but rather that the inode is still valid. 1219 As with d_revalidate, most local filesystems will set this to 1220 NULL since their dcache entries are always valid. 1221 1222 This function has the same return code semantics as 1223 d_revalidate. 1224 1225 d_weak_revalidate is only called after leaving rcu-walk mode. 1226 1227``d_hash`` 1228 called when the VFS adds a dentry to the hash table. The first 1229 dentry passed to d_hash is the parent directory that the name is 1230 to be hashed into. 1231 1232 Same locking and synchronisation rules as d_compare regarding 1233 what is safe to dereference etc. 1234 1235``d_compare`` 1236 called to compare a dentry name with a given name. The first 1237 dentry is the parent of the dentry to be compared, the second is 1238 the child dentry. len and name string are properties of the 1239 dentry to be compared. qstr is the name to compare it with. 1240 1241 Must be constant and idempotent, and should not take locks if 1242 possible, and should not or store into the dentry. Should not 1243 dereference pointers outside the dentry without lots of care 1244 (eg. d_parent, d_inode, d_name should not be used). 1245 1246 However, our vfsmount is pinned, and RCU held, so the dentries 1247 and inodes won't disappear, neither will our sb or filesystem 1248 module. ->d_sb may be used. 1249 1250 It is a tricky calling convention because it needs to be called 1251 under "rcu-walk", ie. without any locks or references on things. 1252 1253``d_delete`` 1254 called when the last reference to a dentry is dropped and the 1255 dcache is deciding whether or not to cache it. Return 1 to 1256 delete immediately, or 0 to cache the dentry. Default is NULL 1257 which means to always cache a reachable dentry. d_delete must 1258 be constant and idempotent. 1259 1260``d_init`` 1261 called when a dentry is allocated 1262 1263``d_release`` 1264 called when a dentry is really deallocated 1265 1266``d_iput`` 1267 called when a dentry loses its inode (just prior to its being 1268 deallocated). The default when this is NULL is that the VFS 1269 calls iput(). If you define this method, you must call iput() 1270 yourself 1271 1272``d_dname`` 1273 called when the pathname of a dentry should be generated. 1274 Useful for some pseudo filesystems (sockfs, pipefs, ...) to 1275 delay pathname generation. (Instead of doing it when dentry is 1276 created, it's done only when the path is needed.). Real 1277 filesystems probably dont want to use it, because their dentries 1278 are present in global dcache hash, so their hash should be an 1279 invariant. As no lock is held, d_dname() should not try to 1280 modify the dentry itself, unless appropriate SMP safety is used. 1281 CAUTION : d_path() logic is quite tricky. The correct way to 1282 return for example "Hello" is to put it at the end of the 1283 buffer, and returns a pointer to the first char. 1284 dynamic_dname() helper function is provided to take care of 1285 this. 1286 1287 Example : 1288 1289.. code-block:: c 1290 1291 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen) 1292 { 1293 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]", 1294 dentry->d_inode->i_ino); 1295 } 1296 1297``d_automount`` 1298 called when an automount dentry is to be traversed (optional). 1299 This should create a new VFS mount record and return the record 1300 to the caller. The caller is supplied with a path parameter 1301 giving the automount directory to describe the automount target 1302 and the parent VFS mount record to provide inheritable mount 1303 parameters. NULL should be returned if someone else managed to 1304 make the automount first. If the vfsmount creation failed, then 1305 an error code should be returned. If -EISDIR is returned, then 1306 the directory will be treated as an ordinary directory and 1307 returned to pathwalk to continue walking. 1308 1309 If a vfsmount is returned, the caller will attempt to mount it 1310 on the mountpoint and will remove the vfsmount from its 1311 expiration list in the case of failure. The vfsmount should be 1312 returned with 2 refs on it to prevent automatic expiration - the 1313 caller will clean up the additional ref. 1314 1315 This function is only used if DCACHE_NEED_AUTOMOUNT is set on 1316 the dentry. This is set by __d_instantiate() if S_AUTOMOUNT is 1317 set on the inode being added. 1318 1319``d_manage`` 1320 called to allow the filesystem to manage the transition from a 1321 dentry (optional). This allows autofs, for example, to hold up 1322 clients waiting to explore behind a 'mountpoint' while letting 1323 the daemon go past and construct the subtree there. 0 should be 1324 returned to let the calling process continue. -EISDIR can be 1325 returned to tell pathwalk to use this directory as an ordinary 1326 directory and to ignore anything mounted on it and not to check 1327 the automount flag. Any other error code will abort pathwalk 1328 completely. 1329 1330 If the 'rcu_walk' parameter is true, then the caller is doing a 1331 pathwalk in RCU-walk mode. Sleeping is not permitted in this 1332 mode, and the caller can be asked to leave it and call again by 1333 returning -ECHILD. -EISDIR may also be returned to tell 1334 pathwalk to ignore d_automount or any mounts. 1335 1336 This function is only used if DCACHE_MANAGE_TRANSIT is set on 1337 the dentry being transited from. 1338 1339``d_real`` 1340 overlay/union type filesystems implement this method to return 1341 one of the underlying dentries hidden by the overlay. It is 1342 used in two different modes: 1343 1344 Called from file_dentry() it returns the real dentry matching 1345 the inode argument. The real dentry may be from a lower layer 1346 already copied up, but still referenced from the file. This 1347 mode is selected with a non-NULL inode argument. 1348 1349 With NULL inode the topmost real underlying dentry is returned. 1350 1351Each dentry has a pointer to its parent dentry, as well as a hash list 1352of child dentries. Child dentries are basically like files in a 1353directory. 1354 1355 1356Directory Entry Cache API 1357-------------------------- 1358 1359There are a number of functions defined which permit a filesystem to 1360manipulate dentries: 1361 1362``dget`` 1363 open a new handle for an existing dentry (this just increments 1364 the usage count) 1365 1366``dput`` 1367 close a handle for a dentry (decrements the usage count). If 1368 the usage count drops to 0, and the dentry is still in its 1369 parent's hash, the "d_delete" method is called to check whether 1370 it should be cached. If it should not be cached, or if the 1371 dentry is not hashed, it is deleted. Otherwise cached dentries 1372 are put into an LRU list to be reclaimed on memory shortage. 1373 1374``d_drop`` 1375 this unhashes a dentry from its parents hash list. A subsequent 1376 call to dput() will deallocate the dentry if its usage count 1377 drops to 0 1378 1379``d_delete`` 1380 delete a dentry. If there are no other open references to the 1381 dentry then the dentry is turned into a negative dentry (the 1382 d_iput() method is called). If there are other references, then 1383 d_drop() is called instead 1384 1385``d_add`` 1386 add a dentry to its parents hash list and then calls 1387 d_instantiate() 1388 1389``d_instantiate`` 1390 add a dentry to the alias hash list for the inode and updates 1391 the "d_inode" member. The "i_count" member in the inode 1392 structure should be set/incremented. If the inode pointer is 1393 NULL, the dentry is called a "negative dentry". This function 1394 is commonly called when an inode is created for an existing 1395 negative dentry 1396 1397``d_lookup`` 1398 look up a dentry given its parent and path name component It 1399 looks up the child of that given name from the dcache hash 1400 table. If it is found, the reference count is incremented and 1401 the dentry is returned. The caller must use dput() to free the 1402 dentry when it finishes using it. 1403 1404 1405Mount Options 1406============= 1407 1408 1409Parsing options 1410--------------- 1411 1412On mount and remount the filesystem is passed a string containing a 1413comma separated list of mount options. The options can have either of 1414these forms: 1415 1416 option 1417 option=value 1418 1419The <linux/parser.h> header defines an API that helps parse these 1420options. There are plenty of examples on how to use it in existing 1421filesystems. 1422 1423 1424Showing options 1425--------------- 1426 1427If a filesystem accepts mount options, it must define show_options() to 1428show all the currently active options. The rules are: 1429 1430 - options MUST be shown which are not default or their values differ 1431 from the default 1432 1433 - options MAY be shown which are enabled by default or have their 1434 default value 1435 1436Options used only internally between a mount helper and the kernel (such 1437as file descriptors), or which only have an effect during the mounting 1438(such as ones controlling the creation of a journal) are exempt from the 1439above rules. 1440 1441The underlying reason for the above rules is to make sure, that a mount 1442can be accurately replicated (e.g. umounting and mounting again) based 1443on the information found in /proc/mounts. 1444 1445 1446Resources 1447========= 1448 1449(Note some of these resources are not up-to-date with the latest kernel 1450 version.) 1451 1452Creating Linux virtual filesystems. 2002 1453 <https://lwn.net/Articles/13325/> 1454 1455The Linux Virtual File-system Layer by Neil Brown. 1999 1456 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html> 1457 1458A tour of the Linux VFS by Michael K. Johnson. 1996 1459 <https://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html> 1460 1461A small trail through the Linux kernel by Andries Brouwer. 2001 1462 <https://www.win.tue.nl/~aeb/linux/vfs/trail.html> 1463