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**mandatory**
49
50The foo_inode_info should always be allocated through alloc_inode_sb() rather
51than kmem_cache_alloc() or kmalloc() related to set up the inode reclaim context
52correctly.
53
54---
55
56**mandatory**
57
58Change of file_system_type method (->read_super to ->get_sb)
59
60->read_super() is no more.  Ditto for DECLARE_FSTYPE and DECLARE_FSTYPE_DEV.
61
62Turn your foo_read_super() into a function that would return 0 in case of
63success and negative number in case of error (-EINVAL unless you have more
64informative error value to report).  Call it foo_fill_super().  Now declare::
65
66  int foo_get_sb(struct file_system_type *fs_type,
67	int flags, const char *dev_name, void *data, struct vfsmount *mnt)
68  {
69	return get_sb_bdev(fs_type, flags, dev_name, data, foo_fill_super,
70			   mnt);
71  }
72
73(or similar with s/bdev/nodev/ or s/bdev/single/, depending on the kind of
74filesystem).
75
76Replace DECLARE_FSTYPE... with explicit initializer and have ->get_sb set as
77foo_get_sb.
78
79---
80
81**mandatory**
82
83Locking change: ->s_vfs_rename_sem is taken only by cross-directory renames.
84Most likely there is no need to change anything, but if you relied on
85global exclusion between renames for some internal purpose - you need to
86change your internal locking.  Otherwise exclusion warranties remain the
87same (i.e. parents and victim are locked, etc.).
88
89---
90
91**informational**
92
93Now we have the exclusion between ->lookup() and directory removal (by
94->rmdir() and ->rename()).  If you used to need that exclusion and do
95it by internal locking (most of filesystems couldn't care less) - you
96can relax your locking.
97
98---
99
100**mandatory**
101
102->lookup(), ->truncate(), ->create(), ->unlink(), ->mknod(), ->mkdir(),
103->rmdir(), ->link(), ->lseek(), ->symlink(), ->rename()
104and ->readdir() are called without BKL now.  Grab it on entry, drop upon return
105- that will guarantee the same locking you used to have.  If your method or its
106parts do not need BKL - better yet, now you can shift lock_kernel() and
107unlock_kernel() so that they would protect exactly what needs to be
108protected.
109
110---
111
112**mandatory**
113
114BKL is also moved from around sb operations. BKL should have been shifted into
115individual fs sb_op functions.  If you don't need it, remove it.
116
117---
118
119**informational**
120
121check for ->link() target not being a directory is done by callers.  Feel
122free to drop it...
123
124---
125
126**informational**
127
128->link() callers hold ->i_mutex on the object we are linking to.  Some of your
129problems might be over...
130
131---
132
133**mandatory**
134
135new file_system_type method - kill_sb(superblock).  If you are converting
136an existing filesystem, set it according to ->fs_flags::
137
138	FS_REQUIRES_DEV		-	kill_block_super
139	FS_LITTER		-	kill_litter_super
140	neither			-	kill_anon_super
141
142FS_LITTER is gone - just remove it from fs_flags.
143
144---
145
146**mandatory**
147
148FS_SINGLE is gone (actually, that had happened back when ->get_sb()
149went in - and hadn't been documented ;-/).  Just remove it from fs_flags
150(and see ->get_sb() entry for other actions).
151
152---
153
154**mandatory**
155
156->setattr() is called without BKL now.  Caller _always_ holds ->i_mutex, so
157watch for ->i_mutex-grabbing code that might be used by your ->setattr().
158Callers of notify_change() need ->i_mutex now.
159
160---
161
162**recommended**
163
164New super_block field ``struct export_operations *s_export_op`` for
165explicit support for exporting, e.g. via NFS.  The structure is fully
166documented at its declaration in include/linux/fs.h, and in
167Documentation/filesystems/nfs/exporting.rst.
168
169Briefly it allows for the definition of decode_fh and encode_fh operations
170to encode and decode filehandles, and allows the filesystem to use
171a standard helper function for decode_fh, and provide file-system specific
172support for this helper, particularly get_parent.
173
174It is planned that this will be required for exporting once the code
175settles down a bit.
176
177**mandatory**
178
179s_export_op is now required for exporting a filesystem.
180isofs, ext2, ext3, resierfs, fat
181can be used as examples of very different filesystems.
182
183---
184
185**mandatory**
186
187iget4() and the read_inode2 callback have been superseded by iget5_locked()
188which has the following prototype::
189
190    struct inode *iget5_locked(struct super_block *sb, unsigned long ino,
191				int (*test)(struct inode *, void *),
192				int (*set)(struct inode *, void *),
193				void *data);
194
195'test' is an additional function that can be used when the inode
196number is not sufficient to identify the actual file object. 'set'
197should be a non-blocking function that initializes those parts of a
198newly created inode to allow the test function to succeed. 'data' is
199passed as an opaque value to both test and set functions.
200
201When the inode has been created by iget5_locked(), it will be returned with the
202I_NEW flag set and will still be locked.  The filesystem then needs to finalize
203the initialization. Once the inode is initialized it must be unlocked by
204calling unlock_new_inode().
205
206The filesystem is responsible for setting (and possibly testing) i_ino
207when appropriate. There is also a simpler iget_locked function that
208just takes the superblock and inode number as arguments and does the
209test and set for you.
210
211e.g.::
212
213	inode = iget_locked(sb, ino);
214	if (inode->i_state & I_NEW) {
215		err = read_inode_from_disk(inode);
216		if (err < 0) {
217			iget_failed(inode);
218			return err;
219		}
220		unlock_new_inode(inode);
221	}
222
223Note that if the process of setting up a new inode fails, then iget_failed()
224should be called on the inode to render it dead, and an appropriate error
225should be passed back to the caller.
226
227---
228
229**recommended**
230
231->getattr() finally getting used.  See instances in nfs, minix, etc.
232
233---
234
235**mandatory**
236
237->revalidate() is gone.  If your filesystem had it - provide ->getattr()
238and let it call whatever you had as ->revlidate() + (for symlinks that
239had ->revalidate()) add calls in ->follow_link()/->readlink().
240
241---
242
243**mandatory**
244
245->d_parent changes are not protected by BKL anymore.  Read access is safe
246if at least one of the following is true:
247
248	* filesystem has no cross-directory rename()
249	* we know that parent had been locked (e.g. we are looking at
250	  ->d_parent of ->lookup() argument).
251	* we are called from ->rename().
252	* the child's ->d_lock is held
253
254Audit your code and add locking if needed.  Notice that any place that is
255not protected by the conditions above is risky even in the old tree - you
256had been relying on BKL and that's prone to screwups.  Old tree had quite
257a few holes of that kind - unprotected access to ->d_parent leading to
258anything from oops to silent memory corruption.
259
260---
261
262**mandatory**
263
264FS_NOMOUNT is gone.  If you use it - just set SB_NOUSER in flags
265(see rootfs for one kind of solution and bdev/socket/pipe for another).
266
267---
268
269**recommended**
270
271Use bdev_read_only(bdev) instead of is_read_only(kdev).  The latter
272is still alive, but only because of the mess in drivers/s390/block/dasd.c.
273As soon as it gets fixed is_read_only() will die.
274
275---
276
277**mandatory**
278
279->permission() is called without BKL now. Grab it on entry, drop upon
280return - that will guarantee the same locking you used to have.  If
281your method or its parts do not need BKL - better yet, now you can
282shift lock_kernel() and unlock_kernel() so that they would protect
283exactly what needs to be protected.
284
285---
286
287**mandatory**
288
289->statfs() is now called without BKL held.  BKL should have been
290shifted into individual fs sb_op functions where it's not clear that
291it's safe to remove it.  If you don't need it, remove it.
292
293---
294
295**mandatory**
296
297is_read_only() is gone; use bdev_read_only() instead.
298
299---
300
301**mandatory**
302
303destroy_buffers() is gone; use invalidate_bdev().
304
305---
306
307**mandatory**
308
309fsync_dev() is gone; use fsync_bdev().  NOTE: lvm breakage is
310deliberate; as soon as struct block_device * is propagated in a reasonable
311way by that code fixing will become trivial; until then nothing can be
312done.
313
314**mandatory**
315
316block truncatation on error exit from ->write_begin, and ->direct_IO
317moved from generic methods (block_write_begin, cont_write_begin,
318nobh_write_begin, blockdev_direct_IO*) to callers.  Take a look at
319ext2_write_failed and callers for an example.
320
321**mandatory**
322
323->truncate is gone.  The whole truncate sequence needs to be
324implemented in ->setattr, which is now mandatory for filesystems
325implementing on-disk size changes.  Start with a copy of the old inode_setattr
326and vmtruncate, and the reorder the vmtruncate + foofs_vmtruncate sequence to
327be in order of zeroing blocks using block_truncate_page or similar helpers,
328size update and on finally on-disk truncation which should not fail.
329setattr_prepare (which used to be inode_change_ok) now includes the size checks
330for ATTR_SIZE and must be called in the beginning of ->setattr unconditionally.
331
332**mandatory**
333
334->clear_inode() and ->delete_inode() are gone; ->evict_inode() should
335be used instead.  It gets called whenever the inode is evicted, whether it has
336remaining links or not.  Caller does *not* evict the pagecache or inode-associated
337metadata buffers; the method has to use truncate_inode_pages_final() to get rid
338of those. Caller makes sure async writeback cannot be running for the inode while
339(or after) ->evict_inode() is called.
340
341->drop_inode() returns int now; it's called on final iput() with
342inode->i_lock held and it returns true if filesystems wants the inode to be
343dropped.  As before, generic_drop_inode() is still the default and it's been
344updated appropriately.  generic_delete_inode() is also alive and it consists
345simply of return 1.  Note that all actual eviction work is done by caller after
346->drop_inode() returns.
347
348As before, clear_inode() must be called exactly once on each call of
349->evict_inode() (as it used to be for each call of ->delete_inode()).  Unlike
350before, if you are using inode-associated metadata buffers (i.e.
351mark_buffer_dirty_inode()), it's your responsibility to call
352invalidate_inode_buffers() before clear_inode().
353
354NOTE: checking i_nlink in the beginning of ->write_inode() and bailing out
355if it's zero is not *and* *never* *had* *been* enough.  Final unlink() and iput()
356may happen while the inode is in the middle of ->write_inode(); e.g. if you blindly
357free the on-disk inode, you may end up doing that while ->write_inode() is writing
358to it.
359
360---
361
362**mandatory**
363
364.d_delete() now only advises the dcache as to whether or not to cache
365unreferenced dentries, and is now only called when the dentry refcount goes to
3660. Even on 0 refcount transition, it must be able to tolerate being called 0,
3671, or more times (eg. constant, idempotent).
368
369---
370
371**mandatory**
372
373.d_compare() calling convention and locking rules are significantly
374changed. Read updated documentation in Documentation/filesystems/vfs.rst (and
375look at examples of other filesystems) for guidance.
376
377---
378
379**mandatory**
380
381.d_hash() calling convention and locking rules are significantly
382changed. Read updated documentation in Documentation/filesystems/vfs.rst (and
383look at examples of other filesystems) for guidance.
384
385---
386
387**mandatory**
388
389dcache_lock is gone, replaced by fine grained locks. See fs/dcache.c
390for details of what locks to replace dcache_lock with in order to protect
391particular things. Most of the time, a filesystem only needs ->d_lock, which
392protects *all* the dcache state of a given dentry.
393
394---
395
396**mandatory**
397
398Filesystems must RCU-free their inodes, if they can have been accessed
399via rcu-walk path walk (basically, if the file can have had a path name in the
400vfs namespace).
401
402Even though i_dentry and i_rcu share storage in a union, we will
403initialize the former in inode_init_always(), so just leave it alone in
404the callback.  It used to be necessary to clean it there, but not anymore
405(starting at 3.2).
406
407---
408
409**recommended**
410
411vfs now tries to do path walking in "rcu-walk mode", which avoids
412atomic operations and scalability hazards on dentries and inodes (see
413Documentation/filesystems/path-lookup.txt). d_hash and d_compare changes
414(above) are examples of the changes required to support this. For more complex
415filesystem callbacks, the vfs drops out of rcu-walk mode before the fs call, so
416no changes are required to the filesystem. However, this is costly and loses
417the benefits of rcu-walk mode. We will begin to add filesystem callbacks that
418are rcu-walk aware, shown below. Filesystems should take advantage of this
419where possible.
420
421---
422
423**mandatory**
424
425d_revalidate is a callback that is made on every path element (if
426the filesystem provides it), which requires dropping out of rcu-walk mode. This
427may now be called in rcu-walk mode (nd->flags & LOOKUP_RCU). -ECHILD should be
428returned if the filesystem cannot handle rcu-walk. See
429Documentation/filesystems/vfs.rst for more details.
430
431permission is an inode permission check that is called on many or all
432directory inodes on the way down a path walk (to check for exec permission). It
433must now be rcu-walk aware (mask & MAY_NOT_BLOCK).  See
434Documentation/filesystems/vfs.rst for more details.
435
436---
437
438**mandatory**
439
440In ->fallocate() you must check the mode option passed in.  If your
441filesystem does not support hole punching (deallocating space in the middle of a
442file) you must return -EOPNOTSUPP if FALLOC_FL_PUNCH_HOLE is set in mode.
443Currently you can only have FALLOC_FL_PUNCH_HOLE with FALLOC_FL_KEEP_SIZE set,
444so the i_size should not change when hole punching, even when puching the end of
445a file off.
446
447---
448
449**mandatory**
450
451->get_sb() is gone.  Switch to use of ->mount().  Typically it's just
452a matter of switching from calling ``get_sb_``... to ``mount_``... and changing
453the function type.  If you were doing it manually, just switch from setting
454->mnt_root to some pointer to returning that pointer.  On errors return
455ERR_PTR(...).
456
457---
458
459**mandatory**
460
461->permission() and generic_permission()have lost flags
462argument; instead of passing IPERM_FLAG_RCU we add MAY_NOT_BLOCK into mask.
463
464generic_permission() has also lost the check_acl argument; ACL checking
465has been taken to VFS and filesystems need to provide a non-NULL ->i_op->get_acl
466to read an ACL from disk.
467
468---
469
470**mandatory**
471
472If you implement your own ->llseek() you must handle SEEK_HOLE and
473SEEK_DATA.  You can hanle this by returning -EINVAL, but it would be nicer to
474support it in some way.  The generic handler assumes that the entire file is
475data and there is a virtual hole at the end of the file.  So if the provided
476offset is less than i_size and SEEK_DATA is specified, return the same offset.
477If the above is true for the offset and you are given SEEK_HOLE, return the end
478of the file.  If the offset is i_size or greater return -ENXIO in either case.
479
480**mandatory**
481
482If you have your own ->fsync() you must make sure to call
483filemap_write_and_wait_range() so that all dirty pages are synced out properly.
484You must also keep in mind that ->fsync() is not called with i_mutex held
485anymore, so if you require i_mutex locking you must make sure to take it and
486release it yourself.
487
488---
489
490**mandatory**
491
492d_alloc_root() is gone, along with a lot of bugs caused by code
493misusing it.  Replacement: d_make_root(inode).  On success d_make_root(inode)
494allocates and returns a new dentry instantiated with the passed in inode.
495On failure NULL is returned and the passed in inode is dropped so the reference
496to inode is consumed in all cases and failure handling need not do any cleanup
497for the inode.  If d_make_root(inode) is passed a NULL inode it returns NULL
498and also requires no further error handling. Typical usage is::
499
500	inode = foofs_new_inode(....);
501	s->s_root = d_make_root(inode);
502	if (!s->s_root)
503		/* Nothing needed for the inode cleanup */
504		return -ENOMEM;
505	...
506
507---
508
509**mandatory**
510
511The witch is dead!  Well, 2/3 of it, anyway.  ->d_revalidate() and
512->lookup() do *not* take struct nameidata anymore; just the flags.
513
514---
515
516**mandatory**
517
518->create() doesn't take ``struct nameidata *``; unlike the previous
519two, it gets "is it an O_EXCL or equivalent?" boolean argument.  Note that
520local filesystems can ignore tha argument - they are guaranteed that the
521object doesn't exist.  It's remote/distributed ones that might care...
522
523---
524
525**mandatory**
526
527FS_REVAL_DOT is gone; if you used to have it, add ->d_weak_revalidate()
528in your dentry operations instead.
529
530---
531
532**mandatory**
533
534vfs_readdir() is gone; switch to iterate_dir() instead
535
536---
537
538**mandatory**
539
540->readdir() is gone now; switch to ->iterate()
541
542**mandatory**
543
544vfs_follow_link has been removed.  Filesystems must use nd_set_link
545from ->follow_link for normal symlinks, or nd_jump_link for magic
546/proc/<pid> style links.
547
548---
549
550**mandatory**
551
552iget5_locked()/ilookup5()/ilookup5_nowait() test() callback used to be
553called with both ->i_lock and inode_hash_lock held; the former is *not*
554taken anymore, so verify that your callbacks do not rely on it (none
555of the in-tree instances did).  inode_hash_lock is still held,
556of course, so they are still serialized wrt removal from inode hash,
557as well as wrt set() callback of iget5_locked().
558
559---
560
561**mandatory**
562
563d_materialise_unique() is gone; d_splice_alias() does everything you
564need now.  Remember that they have opposite orders of arguments ;-/
565
566---
567
568**mandatory**
569
570f_dentry is gone; use f_path.dentry, or, better yet, see if you can avoid
571it entirely.
572
573---
574
575**mandatory**
576
577never call ->read() and ->write() directly; use __vfs_{read,write} or
578wrappers; instead of checking for ->write or ->read being NULL, look for
579FMODE_CAN_{WRITE,READ} in file->f_mode.
580
581---
582
583**mandatory**
584
585do _not_ use new_sync_{read,write} for ->read/->write; leave it NULL
586instead.
587
588---
589
590**mandatory**
591	->aio_read/->aio_write are gone.  Use ->read_iter/->write_iter.
592
593---
594
595**recommended**
596
597for embedded ("fast") symlinks just set inode->i_link to wherever the
598symlink body is and use simple_follow_link() as ->follow_link().
599
600---
601
602**mandatory**
603
604calling conventions for ->follow_link() have changed.  Instead of returning
605cookie and using nd_set_link() to store the body to traverse, we return
606the body to traverse and store the cookie using explicit void ** argument.
607nameidata isn't passed at all - nd_jump_link() doesn't need it and
608nd_[gs]et_link() is gone.
609
610---
611
612**mandatory**
613
614calling conventions for ->put_link() have changed.  It gets inode instead of
615dentry,  it does not get nameidata at all and it gets called only when cookie
616is non-NULL.  Note that link body isn't available anymore, so if you need it,
617store it as cookie.
618
619---
620
621**mandatory**
622
623any symlink that might use page_follow_link_light/page_put_link() must
624have inode_nohighmem(inode) called before anything might start playing with
625its pagecache.  No highmem pages should end up in the pagecache of such
626symlinks.  That includes any preseeding that might be done during symlink
627creation.  page_symlink() will honour the mapping gfp flags, so once
628you've done inode_nohighmem() it's safe to use, but if you allocate and
629insert the page manually, make sure to use the right gfp flags.
630
631---
632
633**mandatory**
634
635->follow_link() is replaced with ->get_link(); same API, except that
636
637	* ->get_link() gets inode as a separate argument
638	* ->get_link() may be called in RCU mode - in that case NULL
639	  dentry is passed
640
641---
642
643**mandatory**
644
645->get_link() gets struct delayed_call ``*done`` now, and should do
646set_delayed_call() where it used to set ``*cookie``.
647
648->put_link() is gone - just give the destructor to set_delayed_call()
649in ->get_link().
650
651---
652
653**mandatory**
654
655->getxattr() and xattr_handler.get() get dentry and inode passed separately.
656dentry might be yet to be attached to inode, so do _not_ use its ->d_inode
657in the instances.  Rationale: !@#!@# security_d_instantiate() needs to be
658called before we attach dentry to inode.
659
660---
661
662**mandatory**
663
664symlinks are no longer the only inodes that do *not* have i_bdev/i_cdev/
665i_pipe/i_link union zeroed out at inode eviction.  As the result, you can't
666assume that non-NULL value in ->i_nlink at ->destroy_inode() implies that
667it's a symlink.  Checking ->i_mode is really needed now.  In-tree we had
668to fix shmem_destroy_callback() that used to take that kind of shortcut;
669watch out, since that shortcut is no longer valid.
670
671---
672
673**mandatory**
674
675->i_mutex is replaced with ->i_rwsem now.  inode_lock() et.al. work as
676they used to - they just take it exclusive.  However, ->lookup() may be
677called with parent locked shared.  Its instances must not
678
679	* use d_instantiate) and d_rehash() separately - use d_add() or
680	  d_splice_alias() instead.
681	* use d_rehash() alone - call d_add(new_dentry, NULL) instead.
682	* in the unlikely case when (read-only) access to filesystem
683	  data structures needs exclusion for some reason, arrange it
684	  yourself.  None of the in-tree filesystems needed that.
685	* rely on ->d_parent and ->d_name not changing after dentry has
686	  been fed to d_add() or d_splice_alias().  Again, none of the
687	  in-tree instances relied upon that.
688
689We are guaranteed that lookups of the same name in the same directory
690will not happen in parallel ("same" in the sense of your ->d_compare()).
691Lookups on different names in the same directory can and do happen in
692parallel now.
693
694---
695
696**recommended**
697
698->iterate_shared() is added; it's a parallel variant of ->iterate().
699Exclusion on struct file level is still provided (as well as that
700between it and lseek on the same struct file), but if your directory
701has been opened several times, you can get these called in parallel.
702Exclusion between that method and all directory-modifying ones is
703still provided, of course.
704
705Often enough ->iterate() can serve as ->iterate_shared() without any
706changes - it is a read-only operation, after all.  If you have any
707per-inode or per-dentry in-core data structures modified by ->iterate(),
708you might need something to serialize the access to them.  If you
709do dcache pre-seeding, you'll need to switch to d_alloc_parallel() for
710that; look for in-tree examples.
711
712Old method is only used if the new one is absent; eventually it will
713be removed.  Switch while you still can; the old one won't stay.
714
715---
716
717**mandatory**
718
719->atomic_open() calls without O_CREAT may happen in parallel.
720
721---
722
723**mandatory**
724
725->setxattr() and xattr_handler.set() get dentry and inode passed separately.
726The xattr_handler.set() gets passed the user namespace of the mount the inode
727is seen from so filesystems can idmap the i_uid and i_gid accordingly.
728dentry might be yet to be attached to inode, so do _not_ use its ->d_inode
729in the instances.  Rationale: !@#!@# security_d_instantiate() needs to be
730called before we attach dentry to inode and !@#!@##!@$!$#!@#$!@$!@$ smack
731->d_instantiate() uses not just ->getxattr() but ->setxattr() as well.
732
733---
734
735**mandatory**
736
737->d_compare() doesn't get parent as a separate argument anymore.  If you
738used it for finding the struct super_block involved, dentry->d_sb will
739work just as well; if it's something more complicated, use dentry->d_parent.
740Just be careful not to assume that fetching it more than once will yield
741the same value - in RCU mode it could change under you.
742
743---
744
745**mandatory**
746
747->rename() has an added flags argument.  Any flags not handled by the
748filesystem should result in EINVAL being returned.
749
750---
751
752
753**recommended**
754
755->readlink is optional for symlinks.  Don't set, unless filesystem needs
756to fake something for readlink(2).
757
758---
759
760**mandatory**
761
762->getattr() is now passed a struct path rather than a vfsmount and
763dentry separately, and it now has request_mask and query_flags arguments
764to specify the fields and sync type requested by statx.  Filesystems not
765supporting any statx-specific features may ignore the new arguments.
766
767---
768
769**mandatory**
770
771->atomic_open() calling conventions have changed.  Gone is ``int *opened``,
772along with FILE_OPENED/FILE_CREATED.  In place of those we have
773FMODE_OPENED/FMODE_CREATED, set in file->f_mode.  Additionally, return
774value for 'called finish_no_open(), open it yourself' case has become
7750, not 1.  Since finish_no_open() itself is returning 0 now, that part
776does not need any changes in ->atomic_open() instances.
777
778---
779
780**mandatory**
781
782alloc_file() has become static now; two wrappers are to be used instead.
783alloc_file_pseudo(inode, vfsmount, name, flags, ops) is for the cases
784when dentry needs to be created; that's the majority of old alloc_file()
785users.  Calling conventions: on success a reference to new struct file
786is returned and callers reference to inode is subsumed by that.  On
787failure, ERR_PTR() is returned and no caller's references are affected,
788so the caller needs to drop the inode reference it held.
789alloc_file_clone(file, flags, ops) does not affect any caller's references.
790On success you get a new struct file sharing the mount/dentry with the
791original, on failure - ERR_PTR().
792
793---
794
795**mandatory**
796
797->clone_file_range() and ->dedupe_file_range have been replaced with
798->remap_file_range().  See Documentation/filesystems/vfs.rst for more
799information.
800
801---
802
803**recommended**
804
805->lookup() instances doing an equivalent of::
806
807	if (IS_ERR(inode))
808		return ERR_CAST(inode);
809	return d_splice_alias(inode, dentry);
810
811don't need to bother with the check - d_splice_alias() will do the
812right thing when given ERR_PTR(...) as inode.  Moreover, passing NULL
813inode to d_splice_alias() will also do the right thing (equivalent of
814d_add(dentry, NULL); return NULL;), so that kind of special cases
815also doesn't need a separate treatment.
816
817---
818
819**strongly recommended**
820
821take the RCU-delayed parts of ->destroy_inode() into a new method -
822->free_inode().  If ->destroy_inode() becomes empty - all the better,
823just get rid of it.  Synchronous work (e.g. the stuff that can't
824be done from an RCU callback, or any WARN_ON() where we want the
825stack trace) *might* be movable to ->evict_inode(); however,
826that goes only for the things that are not needed to balance something
827done by ->alloc_inode().  IOW, if it's cleaning up the stuff that
828might have accumulated over the life of in-core inode, ->evict_inode()
829might be a fit.
830
831Rules for inode destruction:
832
833	* if ->destroy_inode() is non-NULL, it gets called
834	* if ->free_inode() is non-NULL, it gets scheduled by call_rcu()
835	* combination of NULL ->destroy_inode and NULL ->free_inode is
836	  treated as NULL/free_inode_nonrcu, to preserve the compatibility.
837
838Note that the callback (be it via ->free_inode() or explicit call_rcu()
839in ->destroy_inode()) is *NOT* ordered wrt superblock destruction;
840as the matter of fact, the superblock and all associated structures
841might be already gone.  The filesystem driver is guaranteed to be still
842there, but that's it.  Freeing memory in the callback is fine; doing
843more than that is possible, but requires a lot of care and is best
844avoided.
845
846---
847
848**mandatory**
849
850DCACHE_RCUACCESS is gone; having an RCU delay on dentry freeing is the
851default.  DCACHE_NORCU opts out, and only d_alloc_pseudo() has any
852business doing so.
853
854---
855
856**mandatory**
857
858d_alloc_pseudo() is internal-only; uses outside of alloc_file_pseudo() are
859very suspect (and won't work in modules).  Such uses are very likely to
860be misspelled d_alloc_anon().
861
862---
863
864**mandatory**
865
866[should've been added in 2016] stale comment in finish_open() nonwithstanding,
867failure exits in ->atomic_open() instances should *NOT* fput() the file,
868no matter what.  Everything is handled by the caller.
869
870---
871
872**mandatory**
873
874clone_private_mount() returns a longterm mount now, so the proper destructor of
875its result is kern_unmount() or kern_unmount_array().
876
877---
878
879**mandatory**
880
881zero-length bvec segments are disallowed, they must be filtered out before
882passed on to an iterator.
883
884---
885
886**mandatory**
887
888For bvec based itererators bio_iov_iter_get_pages() now doesn't copy bvecs but
889uses the one provided. Anyone issuing kiocb-I/O should ensure that the bvec and
890page references stay until I/O has completed, i.e. until ->ki_complete() has
891been called or returned with non -EIOCBQUEUED code.
892
893---
894
895**mandatory**
896
897mnt_want_write_file() can now only be paired with mnt_drop_write_file(),
898whereas previously it could be paired with mnt_drop_write() as well.
899
900---
901
902**mandatory**
903
904iov_iter_copy_from_user_atomic() is gone; use copy_page_from_iter_atomic().
905The difference is copy_page_from_iter_atomic() advances the iterator and
906you don't need iov_iter_advance() after it.  However, if you decide to use
907only a part of obtained data, you should do iov_iter_revert().
908
909---
910
911**mandatory**
912
913Calling conventions for file_open_root() changed; now it takes struct path *
914instead of passing mount and dentry separately.  For callers that used to
915pass <mnt, mnt->mnt_root> pair (i.e. the root of given mount), a new helper
916is provided - file_open_root_mnt().  In-tree users adjusted.
917
918---
919
920**mandatory**
921
922no_llseek is gone; don't set .llseek to that - just leave it NULL instead.
923Checks for "does that file have llseek(2), or should it fail with ESPIPE"
924should be done by looking at FMODE_LSEEK in file->f_mode.
925