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