linux/Documentation/filesystems/porting.rst
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   1====================
   2Changes since 2.5.0:
   3====================
   4
   5---
   6
   7**recommended**
   8
   9New helpers: sb_bread(), sb_getblk(), sb_find_get_block(), set_bh(),
  10sb_set_blocksize() and sb_min_blocksize().
  11
  12Use them.
  13
  14(sb_find_get_block() replaces 2.4's get_hash_table())
  15
  16---
  17
  18**recommended**
  19
  20New methods: ->alloc_inode() and ->destroy_inode().
  21
  22Remove inode->u.foo_inode_i
  23
  24Declare::
  25
  26        struct foo_inode_info {
  27                /* fs-private stuff */
  28                struct inode vfs_inode;
  29        };
  30        static inline struct foo_inode_info *FOO_I(struct inode *inode)
  31        {
  32                return list_entry(inode, struct foo_inode_info, vfs_inode);
  33        }
  34
  35Use FOO_I(inode) instead of &inode->u.foo_inode_i;
  36
  37Add foo_alloc_inode() and foo_destroy_inode() - the former should allocate
  38foo_inode_info and return the address of ->vfs_inode, the latter should free
  39FOO_I(inode) (see in-tree filesystems for examples).
  40
  41Make them ->alloc_inode and ->destroy_inode in your super_operations.
  42
  43Keep in mind that now you need explicit initialization of private data
  44typically between calling iget_locked() and unlocking the inode.
  45
  46At some point that will become mandatory.
  47
  48---
  49
  50**mandatory**
  51
  52Change of file_system_type method (->read_super to ->get_sb)
  53
  54->read_super() is no more.  Ditto for DECLARE_FSTYPE and DECLARE_FSTYPE_DEV.
  55
  56Turn your foo_read_super() into a function that would return 0 in case of
  57success and negative number in case of error (-EINVAL unless you have more
  58informative error value to report).  Call it foo_fill_super().  Now declare::
  59
  60  int foo_get_sb(struct file_system_type *fs_type,
  61        int flags, const char *dev_name, void *data, struct vfsmount *mnt)
  62  {
  63        return get_sb_bdev(fs_type, flags, dev_name, data, foo_fill_super,
  64                           mnt);
  65  }
  66
  67(or similar with s/bdev/nodev/ or s/bdev/single/, depending on the kind of
  68filesystem).
  69
  70Replace DECLARE_FSTYPE... with explicit initializer and have ->get_sb set as
  71foo_get_sb.
  72
  73---
  74
  75**mandatory**
  76
  77Locking change: ->s_vfs_rename_sem is taken only by cross-directory renames.
  78Most likely there is no need to change anything, but if you relied on
  79global exclusion between renames for some internal purpose - you need to
  80change your internal locking.  Otherwise exclusion warranties remain the
  81same (i.e. parents and victim are locked, etc.).
  82
  83---
  84
  85**informational**
  86
  87Now we have the exclusion between ->lookup() and directory removal (by
  88->rmdir() and ->rename()).  If you used to need that exclusion and do
  89it by internal locking (most of filesystems couldn't care less) - you
  90can relax your locking.
  91
  92---
  93
  94**mandatory**
  95
  96->lookup(), ->truncate(), ->create(), ->unlink(), ->mknod(), ->mkdir(),
  97->rmdir(), ->link(), ->lseek(), ->symlink(), ->rename()
  98and ->readdir() are called without BKL now.  Grab it on entry, drop upon return
  99- that will guarantee the same locking you used to have.  If your method or its
 100parts do not need BKL - better yet, now you can shift lock_kernel() and
 101unlock_kernel() so that they would protect exactly what needs to be
 102protected.
 103
 104---
 105
 106**mandatory**
 107
 108BKL is also moved from around sb operations. BKL should have been shifted into
 109individual fs sb_op functions.  If you don't need it, remove it.
 110
 111---
 112
 113**informational**
 114
 115check for ->link() target not being a directory is done by callers.  Feel
 116free to drop it...
 117
 118---
 119
 120**informational**
 121
 122->link() callers hold ->i_mutex on the object we are linking to.  Some of your
 123problems might be over...
 124
 125---
 126
 127**mandatory**
 128
 129new file_system_type method - kill_sb(superblock).  If you are converting
 130an existing filesystem, set it according to ->fs_flags::
 131
 132        FS_REQUIRES_DEV         -       kill_block_super
 133        FS_LITTER               -       kill_litter_super
 134        neither                 -       kill_anon_super
 135
 136FS_LITTER is gone - just remove it from fs_flags.
 137
 138---
 139
 140**mandatory**
 141
 142FS_SINGLE is gone (actually, that had happened back when ->get_sb()
 143went in - and hadn't been documented ;-/).  Just remove it from fs_flags
 144(and see ->get_sb() entry for other actions).
 145
 146---
 147
 148**mandatory**
 149
 150->setattr() is called without BKL now.  Caller _always_ holds ->i_mutex, so
 151watch for ->i_mutex-grabbing code that might be used by your ->setattr().
 152Callers of notify_change() need ->i_mutex now.
 153
 154---
 155
 156**recommended**
 157
 158New super_block field ``struct export_operations *s_export_op`` for
 159explicit support for exporting, e.g. via NFS.  The structure is fully
 160documented at its declaration in include/linux/fs.h, and in
 161Documentation/filesystems/nfs/exporting.rst.
 162
 163Briefly it allows for the definition of decode_fh and encode_fh operations
 164to encode and decode filehandles, and allows the filesystem to use
 165a standard helper function for decode_fh, and provide file-system specific
 166support for this helper, particularly get_parent.
 167
 168It is planned that this will be required for exporting once the code
 169settles down a bit.
 170
 171**mandatory**
 172
 173s_export_op is now required for exporting a filesystem.
 174isofs, ext2, ext3, resierfs, fat
 175can be used as examples of very different filesystems.
 176
 177---
 178
 179**mandatory**
 180
 181iget4() and the read_inode2 callback have been superseded by iget5_locked()
 182which has the following prototype::
 183
 184    struct inode *iget5_locked(struct super_block *sb, unsigned long ino,
 185                                int (*test)(struct inode *, void *),
 186                                int (*set)(struct inode *, void *),
 187                                void *data);
 188
 189'test' is an additional function that can be used when the inode
 190number is not sufficient to identify the actual file object. 'set'
 191should be a non-blocking function that initializes those parts of a
 192newly created inode to allow the test function to succeed. 'data' is
 193passed as an opaque value to both test and set functions.
 194
 195When the inode has been created by iget5_locked(), it will be returned with the
 196I_NEW flag set and will still be locked.  The filesystem then needs to finalize
 197the initialization. Once the inode is initialized it must be unlocked by
 198calling unlock_new_inode().
 199
 200The filesystem is responsible for setting (and possibly testing) i_ino
 201when appropriate. There is also a simpler iget_locked function that
 202just takes the superblock and inode number as arguments and does the
 203test and set for you.
 204
 205e.g.::
 206
 207        inode = iget_locked(sb, ino);
 208        if (inode->i_state & I_NEW) {
 209                err = read_inode_from_disk(inode);
 210                if (err < 0) {
 211                        iget_failed(inode);
 212                        return err;
 213                }
 214                unlock_new_inode(inode);
 215        }
 216
 217Note that if the process of setting up a new inode fails, then iget_failed()
 218should be called on the inode to render it dead, and an appropriate error
 219should be passed back to the caller.
 220
 221---
 222
 223**recommended**
 224
 225->getattr() finally getting used.  See instances in nfs, minix, etc.
 226
 227---
 228
 229**mandatory**
 230
 231->revalidate() is gone.  If your filesystem had it - provide ->getattr()
 232and let it call whatever you had as ->revlidate() + (for symlinks that
 233had ->revalidate()) add calls in ->follow_link()/->readlink().
 234
 235---
 236
 237**mandatory**
 238
 239->d_parent changes are not protected by BKL anymore.  Read access is safe
 240if at least one of the following is true:
 241
 242        * filesystem has no cross-directory rename()
 243        * we know that parent had been locked (e.g. we are looking at
 244          ->d_parent of ->lookup() argument).
 245        * we are called from ->rename().
 246        * the child's ->d_lock is held
 247
 248Audit your code and add locking if needed.  Notice that any place that is
 249not protected by the conditions above is risky even in the old tree - you
 250had been relying on BKL and that's prone to screwups.  Old tree had quite
 251a few holes of that kind - unprotected access to ->d_parent leading to
 252anything from oops to silent memory corruption.
 253
 254---
 255
 256**mandatory**
 257
 258FS_NOMOUNT is gone.  If you use it - just set SB_NOUSER in flags
 259(see rootfs for one kind of solution and bdev/socket/pipe for another).
 260
 261---
 262
 263**recommended**
 264
 265Use bdev_read_only(bdev) instead of is_read_only(kdev).  The latter
 266is still alive, but only because of the mess in drivers/s390/block/dasd.c.
 267As soon as it gets fixed is_read_only() will die.
 268
 269---
 270
 271**mandatory**
 272
 273->permission() is called without BKL now. Grab it on entry, drop upon
 274return - that will guarantee the same locking you used to have.  If
 275your method or its parts do not need BKL - better yet, now you can
 276shift lock_kernel() and unlock_kernel() so that they would protect
 277exactly what needs to be protected.
 278
 279---
 280
 281**mandatory**
 282
 283->statfs() is now called without BKL held.  BKL should have been
 284shifted into individual fs sb_op functions where it's not clear that
 285it's safe to remove it.  If you don't need it, remove it.
 286
 287---
 288
 289**mandatory**
 290
 291is_read_only() is gone; use bdev_read_only() instead.
 292
 293---
 294
 295**mandatory**
 296
 297destroy_buffers() is gone; use invalidate_bdev().
 298
 299---
 300
 301**mandatory**
 302
 303fsync_dev() is gone; use fsync_bdev().  NOTE: lvm breakage is
 304deliberate; as soon as struct block_device * is propagated in a reasonable
 305way by that code fixing will become trivial; until then nothing can be
 306done.
 307
 308**mandatory**
 309
 310block truncatation on error exit from ->write_begin, and ->direct_IO
 311moved from generic methods (block_write_begin, cont_write_begin,
 312nobh_write_begin, blockdev_direct_IO*) to callers.  Take a look at
 313ext2_write_failed and callers for an example.
 314
 315**mandatory**
 316
 317->truncate is gone.  The whole truncate sequence needs to be
 318implemented in ->setattr, which is now mandatory for filesystems
 319implementing on-disk size changes.  Start with a copy of the old inode_setattr
 320and vmtruncate, and the reorder the vmtruncate + foofs_vmtruncate sequence to
 321be in order of zeroing blocks using block_truncate_page or similar helpers,
 322size update and on finally on-disk truncation which should not fail.
 323setattr_prepare (which used to be inode_change_ok) now includes the size checks
 324for ATTR_SIZE and must be called in the beginning of ->setattr unconditionally.
 325
 326**mandatory**
 327
 328->clear_inode() and ->delete_inode() are gone; ->evict_inode() should
 329be used instead.  It gets called whenever the inode is evicted, whether it has
 330remaining links or not.  Caller does *not* evict the pagecache or inode-associated
 331metadata buffers; the method has to use truncate_inode_pages_final() to get rid
 332of those. Caller makes sure async writeback cannot be running for the inode while
 333(or after) ->evict_inode() is called.
 334
 335->drop_inode() returns int now; it's called on final iput() with
 336inode->i_lock held and it returns true if filesystems wants the inode to be
 337dropped.  As before, generic_drop_inode() is still the default and it's been
 338updated appropriately.  generic_delete_inode() is also alive and it consists
 339simply of return 1.  Note that all actual eviction work is done by caller after
 340->drop_inode() returns.
 341
 342As before, clear_inode() must be called exactly once on each call of
 343->evict_inode() (as it used to be for each call of ->delete_inode()).  Unlike
 344before, if you are using inode-associated metadata buffers (i.e.
 345mark_buffer_dirty_inode()), it's your responsibility to call
 346invalidate_inode_buffers() before clear_inode().
 347
 348NOTE: checking i_nlink in the beginning of ->write_inode() and bailing out
 349if it's zero is not *and* *never* *had* *been* enough.  Final unlink() and iput()
 350may happen while the inode is in the middle of ->write_inode(); e.g. if you blindly
 351free the on-disk inode, you may end up doing that while ->write_inode() is writing
 352to it.
 353
 354---
 355
 356**mandatory**
 357
 358.d_delete() now only advises the dcache as to whether or not to cache
 359unreferenced dentries, and is now only called when the dentry refcount goes to
 3600. Even on 0 refcount transition, it must be able to tolerate being called 0,
 3611, or more times (eg. constant, idempotent).
 362
 363---
 364
 365**mandatory**
 366
 367.d_compare() calling convention and locking rules are significantly
 368changed. Read updated documentation in Documentation/filesystems/vfs.rst (and
 369look at examples of other filesystems) for guidance.
 370
 371---
 372
 373**mandatory**
 374
 375.d_hash() calling convention and locking rules are significantly
 376changed. Read updated documentation in Documentation/filesystems/vfs.rst (and
 377look at examples of other filesystems) for guidance.
 378
 379---
 380
 381**mandatory**
 382
 383dcache_lock is gone, replaced by fine grained locks. See fs/dcache.c
 384for details of what locks to replace dcache_lock with in order to protect
 385particular things. Most of the time, a filesystem only needs ->d_lock, which
 386protects *all* the dcache state of a given dentry.
 387
 388---
 389
 390**mandatory**
 391
 392Filesystems must RCU-free their inodes, if they can have been accessed
 393via rcu-walk path walk (basically, if the file can have had a path name in the
 394vfs namespace).
 395
 396Even though i_dentry and i_rcu share storage in a union, we will
 397initialize the former in inode_init_always(), so just leave it alone in
 398the callback.  It used to be necessary to clean it there, but not anymore
 399(starting at 3.2).
 400
 401---
 402
 403**recommended**
 404
 405vfs now tries to do path walking in "rcu-walk mode", which avoids
 406atomic operations and scalability hazards on dentries and inodes (see
 407Documentation/filesystems/path-lookup.txt). d_hash and d_compare changes
 408(above) are examples of the changes required to support this. For more complex
 409filesystem callbacks, the vfs drops out of rcu-walk mode before the fs call, so
 410no changes are required to the filesystem. However, this is costly and loses
 411the benefits of rcu-walk mode. We will begin to add filesystem callbacks that
 412are rcu-walk aware, shown below. Filesystems should take advantage of this
 413where possible.
 414
 415---
 416
 417**mandatory**
 418
 419d_revalidate is a callback that is made on every path element (if
 420the filesystem provides it), which requires dropping out of rcu-walk mode. This
 421may now be called in rcu-walk mode (nd->flags & LOOKUP_RCU). -ECHILD should be
 422returned if the filesystem cannot handle rcu-walk. See
 423Documentation/filesystems/vfs.rst for more details.
 424
 425permission is an inode permission check that is called on many or all
 426directory inodes on the way down a path walk (to check for exec permission). It
 427must now be rcu-walk aware (mask & MAY_NOT_BLOCK).  See
 428Documentation/filesystems/vfs.rst for more details.
 429
 430---
 431
 432**mandatory**
 433
 434In ->fallocate() you must check the mode option passed in.  If your
 435filesystem does not support hole punching (deallocating space in the middle of a
 436file) you must return -EOPNOTSUPP if FALLOC_FL_PUNCH_HOLE is set in mode.
 437Currently you can only have FALLOC_FL_PUNCH_HOLE with FALLOC_FL_KEEP_SIZE set,
 438so the i_size should not change when hole punching, even when puching the end of
 439a file off.
 440
 441---
 442
 443**mandatory**
 444
 445->get_sb() is gone.  Switch to use of ->mount().  Typically it's just
 446a matter of switching from calling ``get_sb_``... to ``mount_``... and changing
 447the function type.  If you were doing it manually, just switch from setting
 448->mnt_root to some pointer to returning that pointer.  On errors return
 449ERR_PTR(...).
 450
 451---
 452
 453**mandatory**
 454
 455->permission() and generic_permission()have lost flags
 456argument; instead of passing IPERM_FLAG_RCU we add MAY_NOT_BLOCK into mask.
 457
 458generic_permission() has also lost the check_acl argument; ACL checking
 459has been taken to VFS and filesystems need to provide a non-NULL ->i_op->get_acl
 460to read an ACL from disk.
 461
 462---
 463
 464**mandatory**
 465
 466If you implement your own ->llseek() you must handle SEEK_HOLE and
 467SEEK_DATA.  You can hanle this by returning -EINVAL, but it would be nicer to
 468support it in some way.  The generic handler assumes that the entire file is
 469data and there is a virtual hole at the end of the file.  So if the provided
 470offset is less than i_size and SEEK_DATA is specified, return the same offset.
 471If the above is true for the offset and you are given SEEK_HOLE, return the end
 472of the file.  If the offset is i_size or greater return -ENXIO in either case.
 473
 474**mandatory**
 475
 476If you have your own ->fsync() you must make sure to call
 477filemap_write_and_wait_range() so that all dirty pages are synced out properly.
 478You must also keep in mind that ->fsync() is not called with i_mutex held
 479anymore, so if you require i_mutex locking you must make sure to take it and
 480release it yourself.
 481
 482---
 483
 484**mandatory**
 485
 486d_alloc_root() is gone, along with a lot of bugs caused by code
 487misusing it.  Replacement: d_make_root(inode).  On success d_make_root(inode)
 488allocates and returns a new dentry instantiated with the passed in inode.
 489On failure NULL is returned and the passed in inode is dropped so the reference
 490to inode is consumed in all cases and failure handling need not do any cleanup
 491for the inode.  If d_make_root(inode) is passed a NULL inode it returns NULL
 492and also requires no further error handling. Typical usage is::
 493
 494        inode = foofs_new_inode(....);
 495        s->s_root = d_make_root(inode);
 496        if (!s->s_root)
 497                /* Nothing needed for the inode cleanup */
 498                return -ENOMEM;
 499        ...
 500
 501---
 502
 503**mandatory**
 504
 505The witch is dead!  Well, 2/3 of it, anyway.  ->d_revalidate() and
 506->lookup() do *not* take struct nameidata anymore; just the flags.
 507
 508---
 509
 510**mandatory**
 511
 512->create() doesn't take ``struct nameidata *``; unlike the previous
 513two, it gets "is it an O_EXCL or equivalent?" boolean argument.  Note that
 514local filesystems can ignore tha argument - they are guaranteed that the
 515object doesn't exist.  It's remote/distributed ones that might care...
 516
 517---
 518
 519**mandatory**
 520
 521FS_REVAL_DOT is gone; if you used to have it, add ->d_weak_revalidate()
 522in your dentry operations instead.
 523
 524---
 525
 526**mandatory**
 527
 528vfs_readdir() is gone; switch to iterate_dir() instead
 529
 530---
 531
 532**mandatory**
 533
 534->readdir() is gone now; switch to ->iterate()
 535
 536**mandatory**
 537
 538vfs_follow_link has been removed.  Filesystems must use nd_set_link
 539from ->follow_link for normal symlinks, or nd_jump_link for magic
 540/proc/<pid> style links.
 541
 542---
 543
 544**mandatory**
 545
 546iget5_locked()/ilookup5()/ilookup5_nowait() test() callback used to be
 547called with both ->i_lock and inode_hash_lock held; the former is *not*
 548taken anymore, so verify that your callbacks do not rely on it (none
 549of the in-tree instances did).  inode_hash_lock is still held,
 550of course, so they are still serialized wrt removal from inode hash,
 551as well as wrt set() callback of iget5_locked().
 552
 553---
 554
 555**mandatory**
 556
 557d_materialise_unique() is gone; d_splice_alias() does everything you
 558need now.  Remember that they have opposite orders of arguments ;-/
 559
 560---
 561
 562**mandatory**
 563
 564f_dentry is gone; use f_path.dentry, or, better yet, see if you can avoid
 565it entirely.
 566
 567---
 568
 569**mandatory**
 570
 571never call ->read() and ->write() directly; use __vfs_{read,write} or
 572wrappers; instead of checking for ->write or ->read being NULL, look for
 573FMODE_CAN_{WRITE,READ} in file->f_mode.
 574
 575---
 576
 577**mandatory**
 578
 579do _not_ use new_sync_{read,write} for ->read/->write; leave it NULL
 580instead.
 581
 582---
 583
 584**mandatory**
 585        ->aio_read/->aio_write are gone.  Use ->read_iter/->write_iter.
 586
 587---
 588
 589**recommended**
 590
 591for embedded ("fast") symlinks just set inode->i_link to wherever the
 592symlink body is and use simple_follow_link() as ->follow_link().
 593
 594---
 595
 596**mandatory**
 597
 598calling conventions for ->follow_link() have changed.  Instead of returning
 599cookie and using nd_set_link() to store the body to traverse, we return
 600the body to traverse and store the cookie using explicit void ** argument.
 601nameidata isn't passed at all - nd_jump_link() doesn't need it and
 602nd_[gs]et_link() is gone.
 603
 604---
 605
 606**mandatory**
 607
 608calling conventions for ->put_link() have changed.  It gets inode instead of
 609dentry,  it does not get nameidata at all and it gets called only when cookie
 610is non-NULL.  Note that link body isn't available anymore, so if you need it,
 611store it as cookie.
 612
 613---
 614
 615**mandatory**
 616
 617any symlink that might use page_follow_link_light/page_put_link() must
 618have inode_nohighmem(inode) called before anything might start playing with
 619its pagecache.  No highmem pages should end up in the pagecache of such
 620symlinks.  That includes any preseeding that might be done during symlink
 621creation.  __page_symlink() will honour the mapping gfp flags, so once
 622you've done inode_nohighmem() it's safe to use, but if you allocate and
 623insert the page manually, make sure to use the right gfp flags.
 624
 625---
 626
 627**mandatory**
 628
 629->follow_link() is replaced with ->get_link(); same API, except that
 630
 631        * ->get_link() gets inode as a separate argument
 632        * ->get_link() may be called in RCU mode - in that case NULL
 633          dentry is passed
 634
 635---
 636
 637**mandatory**
 638
 639->get_link() gets struct delayed_call ``*done`` now, and should do
 640set_delayed_call() where it used to set ``*cookie``.
 641
 642->put_link() is gone - just give the destructor to set_delayed_call()
 643in ->get_link().
 644
 645---
 646
 647**mandatory**
 648
 649->getxattr() and xattr_handler.get() get dentry and inode passed separately.
 650dentry might be yet to be attached to inode, so do _not_ use its ->d_inode
 651in the instances.  Rationale: !@#!@# security_d_instantiate() needs to be
 652called before we attach dentry to inode.
 653
 654---
 655
 656**mandatory**
 657
 658symlinks are no longer the only inodes that do *not* have i_bdev/i_cdev/
 659i_pipe/i_link union zeroed out at inode eviction.  As the result, you can't
 660assume that non-NULL value in ->i_nlink at ->destroy_inode() implies that
 661it's a symlink.  Checking ->i_mode is really needed now.  In-tree we had
 662to fix shmem_destroy_callback() that used to take that kind of shortcut;
 663watch out, since that shortcut is no longer valid.
 664
 665---
 666
 667**mandatory**
 668
 669->i_mutex is replaced with ->i_rwsem now.  inode_lock() et.al. work as
 670they used to - they just take it exclusive.  However, ->lookup() may be
 671called with parent locked shared.  Its instances must not
 672
 673        * use d_instantiate) and d_rehash() separately - use d_add() or
 674          d_splice_alias() instead.
 675        * use d_rehash() alone - call d_add(new_dentry, NULL) instead.
 676        * in the unlikely case when (read-only) access to filesystem
 677          data structures needs exclusion for some reason, arrange it
 678          yourself.  None of the in-tree filesystems needed that.
 679        * rely on ->d_parent and ->d_name not changing after dentry has
 680          been fed to d_add() or d_splice_alias().  Again, none of the
 681          in-tree instances relied upon that.
 682
 683We are guaranteed that lookups of the same name in the same directory
 684will not happen in parallel ("same" in the sense of your ->d_compare()).
 685Lookups on different names in the same directory can and do happen in
 686parallel now.
 687
 688---
 689
 690**recommended**
 691
 692->iterate_shared() is added; it's a parallel variant of ->iterate().
 693Exclusion on struct file level is still provided (as well as that
 694between it and lseek on the same struct file), but if your directory
 695has been opened several times, you can get these called in parallel.
 696Exclusion between that method and all directory-modifying ones is
 697still provided, of course.
 698
 699Often enough ->iterate() can serve as ->iterate_shared() without any
 700changes - it is a read-only operation, after all.  If you have any
 701per-inode or per-dentry in-core data structures modified by ->iterate(),
 702you might need something to serialize the access to them.  If you
 703do dcache pre-seeding, you'll need to switch to d_alloc_parallel() for
 704that; look for in-tree examples.
 705
 706Old method is only used if the new one is absent; eventually it will
 707be removed.  Switch while you still can; the old one won't stay.
 708
 709---
 710
 711**mandatory**
 712
 713->atomic_open() calls without O_CREAT may happen in parallel.
 714
 715---
 716
 717**mandatory**
 718
 719->setxattr() and xattr_handler.set() get dentry and inode passed separately.
 720The xattr_handler.set() gets passed the user namespace of the mount the inode
 721is seen from so filesystems can idmap the i_uid and i_gid accordingly.
 722dentry might be yet to be attached to inode, so do _not_ use its ->d_inode
 723in the instances.  Rationale: !@#!@# security_d_instantiate() needs to be
 724called before we attach dentry to inode and !@#!@##!@$!$#!@#$!@$!@$ smack
 725->d_instantiate() uses not just ->getxattr() but ->setxattr() as well.
 726
 727---
 728
 729**mandatory**
 730
 731->d_compare() doesn't get parent as a separate argument anymore.  If you
 732used it for finding the struct super_block involved, dentry->d_sb will
 733work just as well; if it's something more complicated, use dentry->d_parent.
 734Just be careful not to assume that fetching it more than once will yield
 735the same value - in RCU mode it could change under you.
 736
 737---
 738
 739**mandatory**
 740
 741->rename() has an added flags argument.  Any flags not handled by the
 742filesystem should result in EINVAL being returned.
 743
 744---
 745
 746
 747**recommended**
 748
 749->readlink is optional for symlinks.  Don't set, unless filesystem needs
 750to fake something for readlink(2).
 751
 752---
 753
 754**mandatory**
 755
 756->getattr() is now passed a struct path rather than a vfsmount and
 757dentry separately, and it now has request_mask and query_flags arguments
 758to specify the fields and sync type requested by statx.  Filesystems not
 759supporting any statx-specific features may ignore the new arguments.
 760
 761---
 762
 763**mandatory**
 764
 765->atomic_open() calling conventions have changed.  Gone is ``int *opened``,
 766along with FILE_OPENED/FILE_CREATED.  In place of those we have
 767FMODE_OPENED/FMODE_CREATED, set in file->f_mode.  Additionally, return
 768value for 'called finish_no_open(), open it yourself' case has become
 7690, not 1.  Since finish_no_open() itself is returning 0 now, that part
 770does not need any changes in ->atomic_open() instances.
 771
 772---
 773
 774**mandatory**
 775
 776alloc_file() has become static now; two wrappers are to be used instead.
 777alloc_file_pseudo(inode, vfsmount, name, flags, ops) is for the cases
 778when dentry needs to be created; that's the majority of old alloc_file()
 779users.  Calling conventions: on success a reference to new struct file
 780is returned and callers reference to inode is subsumed by that.  On
 781failure, ERR_PTR() is returned and no caller's references are affected,
 782so the caller needs to drop the inode reference it held.
 783alloc_file_clone(file, flags, ops) does not affect any caller's references.
 784On success you get a new struct file sharing the mount/dentry with the
 785original, on failure - ERR_PTR().
 786
 787---
 788
 789**mandatory**
 790
 791->clone_file_range() and ->dedupe_file_range have been replaced with
 792->remap_file_range().  See Documentation/filesystems/vfs.rst for more
 793information.
 794
 795---
 796
 797**recommended**
 798
 799->lookup() instances doing an equivalent of::
 800
 801        if (IS_ERR(inode))
 802                return ERR_CAST(inode);
 803        return d_splice_alias(inode, dentry);
 804
 805don't need to bother with the check - d_splice_alias() will do the
 806right thing when given ERR_PTR(...) as inode.  Moreover, passing NULL
 807inode to d_splice_alias() will also do the right thing (equivalent of
 808d_add(dentry, NULL); return NULL;), so that kind of special cases
 809also doesn't need a separate treatment.
 810
 811---
 812
 813**strongly recommended**
 814
 815take the RCU-delayed parts of ->destroy_inode() into a new method -
 816->free_inode().  If ->destroy_inode() becomes empty - all the better,
 817just get rid of it.  Synchronous work (e.g. the stuff that can't
 818be done from an RCU callback, or any WARN_ON() where we want the
 819stack trace) *might* be movable to ->evict_inode(); however,
 820that goes only for the things that are not needed to balance something
 821done by ->alloc_inode().  IOW, if it's cleaning up the stuff that
 822might have accumulated over the life of in-core inode, ->evict_inode()
 823might be a fit.
 824
 825Rules for inode destruction:
 826
 827        * if ->destroy_inode() is non-NULL, it gets called
 828        * if ->free_inode() is non-NULL, it gets scheduled by call_rcu()
 829        * combination of NULL ->destroy_inode and NULL ->free_inode is
 830          treated as NULL/free_inode_nonrcu, to preserve the compatibility.
 831
 832Note that the callback (be it via ->free_inode() or explicit call_rcu()
 833in ->destroy_inode()) is *NOT* ordered wrt superblock destruction;
 834as the matter of fact, the superblock and all associated structures
 835might be already gone.  The filesystem driver is guaranteed to be still
 836there, but that's it.  Freeing memory in the callback is fine; doing
 837more than that is possible, but requires a lot of care and is best
 838avoided.
 839
 840---
 841
 842**mandatory**
 843
 844DCACHE_RCUACCESS is gone; having an RCU delay on dentry freeing is the
 845default.  DCACHE_NORCU opts out, and only d_alloc_pseudo() has any
 846business doing so.
 847
 848---
 849
 850**mandatory**
 851
 852d_alloc_pseudo() is internal-only; uses outside of alloc_file_pseudo() are
 853very suspect (and won't work in modules).  Such uses are very likely to
 854be misspelled d_alloc_anon().
 855
 856---
 857
 858**mandatory**
 859
 860[should've been added in 2016] stale comment in finish_open() nonwithstanding,
 861failure exits in ->atomic_open() instances should *NOT* fput() the file,
 862no matter what.  Everything is handled by the caller.
 863
 864---
 865
 866**mandatory**
 867
 868clone_private_mount() returns a longterm mount now, so the proper destructor of
 869its result is kern_unmount() or kern_unmount_array().
 870
 871---
 872
 873**mandatory**
 874
 875zero-length bvec segments are disallowed, they must be filtered out before
 876passed on to an iterator.
 877
 878---
 879
 880**mandatory**
 881
 882For bvec based itererators bio_iov_iter_get_pages() now doesn't copy bvecs but
 883uses the one provided. Anyone issuing kiocb-I/O should ensure that the bvec and
 884page references stay until I/O has completed, i.e. until ->ki_complete() has
 885been called or returned with non -EIOCBQUEUED code.
 886
 887---
 888
 889**mandatory**
 890
 891mnt_want_write_file() can now only be paired with mnt_drop_write_file(),
 892whereas previously it could be paired with mnt_drop_write() as well.
 893
 894---
 895
 896**mandatory**
 897
 898iov_iter_copy_from_user_atomic() is gone; use copy_page_from_iter_atomic().
 899The difference is copy_page_from_iter_atomic() advances the iterator and
 900you don't need iov_iter_advance() after it.  However, if you decide to use
 901only a part of obtained data, you should do iov_iter_revert().
 902
 903---
 904
 905**mandatory**
 906
 907Calling conventions for file_open_root() changed; now it takes struct path *
 908instead of passing mount and dentry separately.  For callers that used to
 909pass <mnt, mnt->mnt_root> pair (i.e. the root of given mount), a new helper
 910is provided - file_open_root_mnt().  In-tree users adjusted.
 911