Adding upstream version 4.19.249.upstream/4.19.249upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 636 insertions, 0 deletions

diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
new file mode 100644
index 000000000..041b0ded8
--- /dev/null
+++ b/Documentation/filesystems/porting
@@ -0,0 +1,636 @@
+Changes since 2.5.0:
+
+---
+[recommended]
+
+New helpers: sb_bread(), sb_getblk(), sb_find_get_block(), set_bh(),
+	sb_set_blocksize() and sb_min_blocksize().
+
+Use them.
+
+(sb_find_get_block() replaces 2.4's get_hash_table())
+
+---
+[recommended]
+
+New methods: ->alloc_inode() and ->destroy_inode().
+
+Remove inode->u.foo_inode_i
+Declare
+	struct foo_inode_info {
+		/* fs-private stuff */
+		struct inode vfs_inode;
+	};
+	static inline struct foo_inode_info *FOO_I(struct inode *inode)
+	{
+		return list_entry(inode, struct foo_inode_info, vfs_inode);
+	}
+
+Use FOO_I(inode) instead of &inode->u.foo_inode_i;
+
+Add foo_alloc_inode() and foo_destroy_inode() - the former should allocate
+foo_inode_info and return the address of ->vfs_inode, the latter should free
+FOO_I(inode) (see in-tree filesystems for examples).
+
+Make them ->alloc_inode and ->destroy_inode in your super_operations.
+
+Keep in mind that now you need explicit initialization of private data
+typically between calling iget_locked() and unlocking the inode.
+
+At some point that will become mandatory.
+
+---
+[mandatory]
+
+Change of file_system_type method (->read_super to ->get_sb)
+
+->read_super() is no more.  Ditto for DECLARE_FSTYPE and DECLARE_FSTYPE_DEV.
+
+Turn your foo_read_super() into a function that would return 0 in case of
+success and negative number in case of error (-EINVAL unless you have more
+informative error value to report).  Call it foo_fill_super().  Now declare
+
+int foo_get_sb(struct file_system_type *fs_type,
+	int flags, const char *dev_name, void *data, struct vfsmount *mnt)
+{
+	return get_sb_bdev(fs_type, flags, dev_name, data, foo_fill_super,
+			   mnt);
+}
+
+(or similar with s/bdev/nodev/ or s/bdev/single/, depending on the kind of
+filesystem).
+
+Replace DECLARE_FSTYPE... with explicit initializer and have ->get_sb set as
+foo_get_sb.
+
+---
+[mandatory]
+
+Locking change: ->s_vfs_rename_sem is taken only by cross-directory renames.
+Most likely there is no need to change anything, but if you relied on
+global exclusion between renames for some internal purpose - you need to
+change your internal locking.  Otherwise exclusion warranties remain the
+same (i.e. parents and victim are locked, etc.).
+
+---
+[informational]
+
+Now we have the exclusion between ->lookup() and directory removal (by
+->rmdir() and ->rename()).  If you used to need that exclusion and do
+it by internal locking (most of filesystems couldn't care less) - you
+can relax your locking.
+
+---
+[mandatory]
+
+->lookup(), ->truncate(), ->create(), ->unlink(), ->mknod(), ->mkdir(),
+->rmdir(), ->link(), ->lseek(), ->symlink(), ->rename()
+and ->readdir() are called without BKL now.  Grab it on entry, drop upon return
+- that will guarantee the same locking you used to have.  If your method or its
+parts do not need BKL - better yet, now you can shift lock_kernel() and
+unlock_kernel() so that they would protect exactly what needs to be
+protected.
+
+---
+[mandatory]
+
+BKL is also moved from around sb operations. BKL should have been shifted into
+individual fs sb_op functions.  If you don't need it, remove it.
+
+---
+[informational]
+
+check for ->link() target not being a directory is done by callers.  Feel
+free to drop it...
+
+---
+[informational]
+
+->link() callers hold ->i_mutex on the object we are linking to.  Some of your
+problems might be over...
+
+---
+[mandatory]
+
+new file_system_type method - kill_sb(superblock).  If you are converting
+an existing filesystem, set it according to ->fs_flags:
+	FS_REQUIRES_DEV		-	kill_block_super
+	FS_LITTER		-	kill_litter_super
+	neither			-	kill_anon_super
+FS_LITTER is gone - just remove it from fs_flags.
+
+---
+[mandatory]
+
+	FS_SINGLE is gone (actually, that had happened back when ->get_sb()
+went in - and hadn't been documented ;-/).  Just remove it from fs_flags
+(and see ->get_sb() entry for other actions).
+
+---
+[mandatory]
+
+->setattr() is called without BKL now.  Caller _always_ holds ->i_mutex, so
+watch for ->i_mutex-grabbing code that might be used by your ->setattr().
+Callers of notify_change() need ->i_mutex now.
+
+---
+[recommended]
+
+New super_block field "struct export_operations *s_export_op" for
+explicit support for exporting, e.g. via NFS.  The structure is fully
+documented at its declaration in include/linux/fs.h, and in
+Documentation/filesystems/nfs/Exporting.
+
+Briefly it allows for the definition of decode_fh and encode_fh operations
+to encode and decode filehandles, and allows the filesystem to use
+a standard helper function for decode_fh, and provide file-system specific
+support for this helper, particularly get_parent.
+
+It is planned that this will be required for exporting once the code
+settles down a bit.
+
+[mandatory]
+
+s_export_op is now required for exporting a filesystem.
+isofs, ext2, ext3, resierfs, fat
+can be used as examples of very different filesystems.
+
+---
+[mandatory]
+
+iget4() and the read_inode2 callback have been superseded by iget5_locked()
+which has the following prototype,
+
+    struct inode *iget5_locked(struct super_block *sb, unsigned long ino,
+				int (*test)(struct inode *, void *),
+				int (*set)(struct inode *, void *),
+				void *data);
+
+'test' is an additional function that can be used when the inode
+number is not sufficient to identify the actual file object. 'set'
+should be a non-blocking function that initializes those parts of a
+newly created inode to allow the test function to succeed. 'data' is
+passed as an opaque value to both test and set functions.
+
+When the inode has been created by iget5_locked(), it will be returned with the
+I_NEW flag set and will still be locked.  The filesystem then needs to finalize
+the initialization. Once the inode is initialized it must be unlocked by
+calling unlock_new_inode().
+
+The filesystem is responsible for setting (and possibly testing) i_ino
+when appropriate. There is also a simpler iget_locked function that
+just takes the superblock and inode number as arguments and does the
+test and set for you.
+
+e.g.
+	inode = iget_locked(sb, ino);
+	if (inode->i_state & I_NEW) {
+		err = read_inode_from_disk(inode);
+		if (err < 0) {
+			iget_failed(inode);
+			return err;
+		}
+		unlock_new_inode(inode);
+	}
+
+Note that if the process of setting up a new inode fails, then iget_failed()
+should be called on the inode to render it dead, and an appropriate error
+should be passed back to the caller.
+
+---
+[recommended]
+
+->getattr() finally getting used.  See instances in nfs, minix, etc.
+
+---
+[mandatory]
+
+->revalidate() is gone.  If your filesystem had it - provide ->getattr()
+and let it call whatever you had as ->revlidate() + (for symlinks that
+had ->revalidate()) add calls in ->follow_link()/->readlink().
+
+---
+[mandatory]
+
+->d_parent changes are not protected by BKL anymore.  Read access is safe
+if at least one of the following is true:
+	* filesystem has no cross-directory rename()
+	* we know that parent had been locked (e.g. we are looking at
+->d_parent of ->lookup() argument).
+	* we are called from ->rename().
+	* the child's ->d_lock is held
+Audit your code and add locking if needed.  Notice that any place that is
+not protected by the conditions above is risky even in the old tree - you
+had been relying on BKL and that's prone to screwups.  Old tree had quite
+a few holes of that kind - unprotected access to ->d_parent leading to
+anything from oops to silent memory corruption.
+
+---
+[mandatory]
+
+	FS_NOMOUNT is gone.  If you use it - just set SB_NOUSER in flags
+(see rootfs for one kind of solution and bdev/socket/pipe for another).
+
+---
+[recommended]
+
+	Use bdev_read_only(bdev) instead of is_read_only(kdev).  The latter
+is still alive, but only because of the mess in drivers/s390/block/dasd.c.
+As soon as it gets fixed is_read_only() will die.
+
+---
+[mandatory]
+
+->permission() is called without BKL now. Grab it on entry, drop upon
+return - that will guarantee the same locking you used to have.  If
+your method or its parts do not need BKL - better yet, now you can
+shift lock_kernel() and unlock_kernel() so that they would protect
+exactly what needs to be protected.
+
+---
+[mandatory]
+
+->statfs() is now called without BKL held.  BKL should have been
+shifted into individual fs sb_op functions where it's not clear that
+it's safe to remove it.  If you don't need it, remove it.
+
+---
+[mandatory]
+
+	is_read_only() is gone; use bdev_read_only() instead.
+
+---
+[mandatory]
+
+	destroy_buffers() is gone; use invalidate_bdev().
+
+---
+[mandatory]
+
+	fsync_dev() is gone; use fsync_bdev().  NOTE: lvm breakage is
+deliberate; as soon as struct block_device * is propagated in a reasonable
+way by that code fixing will become trivial; until then nothing can be
+done.
+
+[mandatory]
+
+	block truncatation on error exit from ->write_begin, and ->direct_IO
+moved from generic methods (block_write_begin, cont_write_begin,
+nobh_write_begin, blockdev_direct_IO*) to callers.  Take a look at
+ext2_write_failed and callers for an example.
+
+[mandatory]
+
+	->truncate is gone.  The whole truncate sequence needs to be
+implemented in ->setattr, which is now mandatory for filesystems
+implementing on-disk size changes.  Start with a copy of the old inode_setattr
+and vmtruncate, and the reorder the vmtruncate + foofs_vmtruncate sequence to
+be in order of zeroing blocks using block_truncate_page or similar helpers,
+size update and on finally on-disk truncation which should not fail.
+setattr_prepare (which used to be inode_change_ok) now includes the size checks
+for ATTR_SIZE and must be called in the beginning of ->setattr unconditionally.
+
+[mandatory]
+
+	->clear_inode() and ->delete_inode() are gone; ->evict_inode() should
+be used instead.  It gets called whenever the inode is evicted, whether it has
+remaining links or not.  Caller does *not* evict the pagecache or inode-associated
+metadata buffers; the method has to use truncate_inode_pages_final() to get rid
+of those. Caller makes sure async writeback cannot be running for the inode while
+(or after) ->evict_inode() is called.
+
+	->drop_inode() returns int now; it's called on final iput() with
+inode->i_lock held and it returns true if filesystems wants the inode to be
+dropped.  As before, generic_drop_inode() is still the default and it's been
+updated appropriately.  generic_delete_inode() is also alive and it consists
+simply of return 1.  Note that all actual eviction work is done by caller after
+->drop_inode() returns.
+
+	As before, clear_inode() must be called exactly once on each call of
+->evict_inode() (as it used to be for each call of ->delete_inode()).  Unlike
+before, if you are using inode-associated metadata buffers (i.e.
+mark_buffer_dirty_inode()), it's your responsibility to call
+invalidate_inode_buffers() before clear_inode().
+
+	NOTE: checking i_nlink in the beginning of ->write_inode() and bailing out
+if it's zero is not *and* *never* *had* *been* enough.  Final unlink() and iput()
+may happen while the inode is in the middle of ->write_inode(); e.g. if you blindly
+free the on-disk inode, you may end up doing that while ->write_inode() is writing
+to it.
+
+---
+[mandatory]
+
+	.d_delete() now only advises the dcache as to whether or not to cache
+unreferenced dentries, and is now only called when the dentry refcount goes to
+0. Even on 0 refcount transition, it must be able to tolerate being called 0,
+1, or more times (eg. constant, idempotent).
+
+---
+[mandatory]
+
+	.d_compare() calling convention and locking rules are significantly
+changed. Read updated documentation in Documentation/filesystems/vfs.txt (and
+look at examples of other filesystems) for guidance.
+
+---
+[mandatory]
+
+	.d_hash() calling convention and locking rules are significantly
+changed. Read updated documentation in Documentation/filesystems/vfs.txt (and
+look at examples of other filesystems) for guidance.
+
+---
+[mandatory]
+	dcache_lock is gone, replaced by fine grained locks. See fs/dcache.c
+for details of what locks to replace dcache_lock with in order to protect
+particular things. Most of the time, a filesystem only needs ->d_lock, which
+protects *all* the dcache state of a given dentry.
+
+--
+[mandatory]
+
+	Filesystems must RCU-free their inodes, if they can have been accessed
+via rcu-walk path walk (basically, if the file can have had a path name in the
+vfs namespace).
+
+	Even though i_dentry and i_rcu share storage in a union, we will
+initialize the former in inode_init_always(), so just leave it alone in
+the callback.  It used to be necessary to clean it there, but not anymore
+(starting at 3.2).
+
+--
+[recommended]
+	vfs now tries to do path walking in "rcu-walk mode", which avoids
+atomic operations and scalability hazards on dentries and inodes (see
+Documentation/filesystems/path-lookup.txt). d_hash and d_compare changes
+(above) are examples of the changes required to support this. For more complex
+filesystem callbacks, the vfs drops out of rcu-walk mode before the fs call, so
+no changes are required to the filesystem. However, this is costly and loses
+the benefits of rcu-walk mode. We will begin to add filesystem callbacks that
+are rcu-walk aware, shown below. Filesystems should take advantage of this
+where possible.
+
+--
+[mandatory]
+	d_revalidate is a callback that is made on every path element (if
+the filesystem provides it), which requires dropping out of rcu-walk mode. This
+may now be called in rcu-walk mode (nd->flags & LOOKUP_RCU). -ECHILD should be
+returned if the filesystem cannot handle rcu-walk. See
+Documentation/filesystems/vfs.txt for more details.
+
+	permission is an inode permission check that is called on many or all
+directory inodes on the way down a path walk (to check for exec permission). It
+must now be rcu-walk aware (mask & MAY_NOT_BLOCK).  See
+Documentation/filesystems/vfs.txt for more details.
+ 
+--
+[mandatory]
+	In ->fallocate() you must check the mode option passed in.  If your
+filesystem does not support hole punching (deallocating space in the middle of a
+file) you must return -EOPNOTSUPP if FALLOC_FL_PUNCH_HOLE is set in mode.
+Currently you can only have FALLOC_FL_PUNCH_HOLE with FALLOC_FL_KEEP_SIZE set,
+so the i_size should not change when hole punching, even when puching the end of
+a file off.
+
+--
+[mandatory]
+	->get_sb() is gone.  Switch to use of ->mount().  Typically it's just
+a matter of switching from calling get_sb_... to mount_... and changing the
+function type.  If you were doing it manually, just switch from setting ->mnt_root
+to some pointer to returning that pointer.  On errors return ERR_PTR(...).
+
+--
+[mandatory]
+	->permission() and generic_permission()have lost flags
+argument; instead of passing IPERM_FLAG_RCU we add MAY_NOT_BLOCK into mask.
+	generic_permission() has also lost the check_acl argument; ACL checking
+has been taken to VFS and filesystems need to provide a non-NULL ->i_op->get_acl
+to read an ACL from disk.
+
+--
+[mandatory]
+	If you implement your own ->llseek() you must handle SEEK_HOLE and
+SEEK_DATA.  You can hanle this by returning -EINVAL, but it would be nicer to
+support it in some way.  The generic handler assumes that the entire file is
+data and there is a virtual hole at the end of the file.  So if the provided
+offset is less than i_size and SEEK_DATA is specified, return the same offset.
+If the above is true for the offset and you are given SEEK_HOLE, return the end
+of the file.  If the offset is i_size or greater return -ENXIO in either case.
+
+[mandatory]
+	If you have your own ->fsync() you must make sure to call
+filemap_write_and_wait_range() so that all dirty pages are synced out properly.
+You must also keep in mind that ->fsync() is not called with i_mutex held
+anymore, so if you require i_mutex locking you must make sure to take it and
+release it yourself.
+
+--
+[mandatory]
+	d_alloc_root() is gone, along with a lot of bugs caused by code
+misusing it.  Replacement: d_make_root(inode).  The difference is,
+d_make_root() drops the reference to inode if dentry allocation fails.  
+
+--
+[mandatory]
+	The witch is dead!  Well, 2/3 of it, anyway.  ->d_revalidate() and
+->lookup() do *not* take struct nameidata anymore; just the flags.
+--
+[mandatory]
+	->create() doesn't take struct nameidata *; unlike the previous
+two, it gets "is it an O_EXCL or equivalent?" boolean argument.  Note that
+local filesystems can ignore tha argument - they are guaranteed that the
+object doesn't exist.  It's remote/distributed ones that might care...
+--
+[mandatory]
+	FS_REVAL_DOT is gone; if you used to have it, add ->d_weak_revalidate()
+in your dentry operations instead.
+--
+[mandatory]
+	vfs_readdir() is gone; switch to iterate_dir() instead
+--
+[mandatory]
+	->readdir() is gone now; switch to ->iterate()
+[mandatory]
+	vfs_follow_link has been removed.  Filesystems must use nd_set_link
+	from ->follow_link for normal symlinks, or nd_jump_link for magic
+	/proc/<pid> style links.
+--
+[mandatory]
+	iget5_locked()/ilookup5()/ilookup5_nowait() test() callback used to be
+	called with both ->i_lock and inode_hash_lock held; the former is *not*
+	taken anymore, so verify that your callbacks do not rely on it (none
+	of the in-tree instances did).  inode_hash_lock is still held,
+	of course, so they are still serialized wrt removal from inode hash,
+	as well as wrt set() callback of iget5_locked().
+--
+[mandatory]
+	d_materialise_unique() is gone; d_splice_alias() does everything you
+	need now.  Remember that they have opposite orders of arguments ;-/
+--
+[mandatory]
+	f_dentry is gone; use f_path.dentry, or, better yet, see if you can avoid
+	it entirely.
+--
+[mandatory]
+	never call ->read() and ->write() directly; use __vfs_{read,write} or
+	wrappers; instead of checking for ->write or ->read being NULL, look for
+	FMODE_CAN_{WRITE,READ} in file->f_mode.
+--
+[mandatory]
+	do _not_ use new_sync_{read,write} for ->read/->write; leave it NULL
+	instead.
+--
+[mandatory]
+	->aio_read/->aio_write are gone.  Use ->read_iter/->write_iter.
+---
+[recommended]
+	for embedded ("fast") symlinks just set inode->i_link to wherever the
+	symlink body is and use simple_follow_link() as ->follow_link().
+--
+[mandatory]
+	calling conventions for ->follow_link() have changed.  Instead of returning
+	cookie and using nd_set_link() to store the body to traverse, we return
+	the body to traverse and store the cookie using explicit void ** argument.
+	nameidata isn't passed at all - nd_jump_link() doesn't need it and
+	nd_[gs]et_link() is gone.
+--
+[mandatory]
+	calling conventions for ->put_link() have changed.  It gets inode instead of
+	dentry,  it does not get nameidata at all and it gets called only when cookie
+	is non-NULL.  Note that link body isn't available anymore, so if you need it,
+	store it as cookie.
+--
+[mandatory]
+	any symlink that might use page_follow_link_light/page_put_link() must
+	have inode_nohighmem(inode) called before anything might start playing with
+	its pagecache.  No highmem pages should end up in the pagecache of such
+	symlinks.  That includes any preseeding that might be done during symlink
+	creation.  __page_symlink() will honour the mapping gfp flags, so once
+	you've done inode_nohighmem() it's safe to use, but if you allocate and
+	insert the page manually, make sure to use the right gfp flags.
+--
+[mandatory]
+	->follow_link() is replaced with ->get_link(); same API, except that
+		* ->get_link() gets inode as a separate argument
+		* ->get_link() may be called in RCU mode - in that case NULL
+		  dentry is passed
+--
+[mandatory]
+	->get_link() gets struct delayed_call *done now, and should do
+	set_delayed_call() where it used to set *cookie.
+	->put_link() is gone - just give the destructor to set_delayed_call()
+	in ->get_link().
+--
+[mandatory]
+	->getxattr() and xattr_handler.get() get dentry and inode passed separately.
+	dentry might be yet to be attached to inode, so do _not_ use its ->d_inode
+	in the instances.  Rationale: !@#!@# security_d_instantiate() needs to be
+	called before we attach dentry to inode.
+--
+[mandatory]
+	symlinks are no longer the only inodes that do *not* have i_bdev/i_cdev/
+	i_pipe/i_link union zeroed out at inode eviction.  As the result, you can't
+	assume that non-NULL value in ->i_nlink at ->destroy_inode() implies that
+	it's a symlink.  Checking ->i_mode is really needed now.  In-tree we had
+	to fix shmem_destroy_callback() that used to take that kind of shortcut;
+	watch out, since that shortcut is no longer valid.
+--
+[mandatory]
+	->i_mutex is replaced with ->i_rwsem now.  inode_lock() et.al. work as
+	they used to - they just take it exclusive.  However, ->lookup() may be
+	called with parent locked shared.  Its instances must not
+		* use d_instantiate) and d_rehash() separately - use d_add() or
+		  d_splice_alias() instead.
+		* use d_rehash() alone - call d_add(new_dentry, NULL) instead.
+		* in the unlikely case when (read-only) access to filesystem
+		  data structures needs exclusion for some reason, arrange it
+		  yourself.  None of the in-tree filesystems needed that.
+		* rely on ->d_parent and ->d_name not changing after dentry has
+		  been fed to d_add() or d_splice_alias().  Again, none of the
+		  in-tree instances relied upon that.
+	We are guaranteed that lookups of the same name in the same directory
+	will not happen in parallel ("same" in the sense of your ->d_compare()).
+	Lookups on different names in the same directory can and do happen in
+	parallel now.
+--
+[recommended]
+	->iterate_shared() is added; it's a parallel variant of ->iterate().
+	Exclusion on struct file level is still provided (as well as that
+	between it and lseek on the same struct file), but if your directory
+	has been opened several times, you can get these called in parallel.
+	Exclusion between that method and all directory-modifying ones is
+	still provided, of course.
+
+	Often enough ->iterate() can serve as ->iterate_shared() without any
+	changes - it is a read-only operation, after all.  If you have any
+	per-inode or per-dentry in-core data structures modified by ->iterate(),
+	you might need something to serialize the access to them.  If you
+	do dcache pre-seeding, you'll need to switch to d_alloc_parallel() for
+	that; look for in-tree examples.
+
+	Old method is only used if the new one is absent; eventually it will
+	be removed.  Switch while you still can; the old one won't stay.
+--
+[mandatory]
+	->atomic_open() calls without O_CREAT may happen in parallel.
+--
+[mandatory]
+	->setxattr() and xattr_handler.set() get dentry and inode passed separately.
+	dentry might be yet to be attached to inode, so do _not_ use its ->d_inode
+	in the instances.  Rationale: !@#!@# security_d_instantiate() needs to be
+	called before we attach dentry to inode and !@#!@##!@$!$#!@#$!@$!@$ smack
+	->d_instantiate() uses not just ->getxattr() but ->setxattr() as well.
+--
+[mandatory]
+	->d_compare() doesn't get parent as a separate argument anymore.  If you
+	used it for finding the struct super_block involved, dentry->d_sb will
+	work just as well; if it's something more complicated, use dentry->d_parent.
+	Just be careful not to assume that fetching it more than once will yield
+	the same value - in RCU mode it could change under you.
+--
+[mandatory]
+	->rename() has an added flags argument.  Any flags not handled by the
+        filesystem should result in EINVAL being returned.
+--
+[recommended]
+	->readlink is optional for symlinks.  Don't set, unless filesystem needs
+	to fake something for readlink(2).
+--
+[mandatory]
+	->getattr() is now passed a struct path rather than a vfsmount and
+	dentry separately, and it now has request_mask and query_flags arguments
+	to specify the fields and sync type requested by statx.  Filesystems not
+	supporting any statx-specific features may ignore the new arguments.
+--
+[mandatory]
+	->atomic_open() calling conventions have changed.  Gone is int *opened,
+	along with FILE_OPENED/FILE_CREATED.  In place of those we have
+	FMODE_OPENED/FMODE_CREATED, set in file->f_mode.  Additionally, return
+	value for 'called finish_no_open(), open it yourself' case has become
+	0, not 1.  Since finish_no_open() itself is returning 0 now, that part
+	does not need any changes in ->atomic_open() instances.
+--
+[mandatory]
+	alloc_file() has become static now; two wrappers are to be used instead.
+	alloc_file_pseudo(inode, vfsmount, name, flags, ops) is for the cases
+	when dentry needs to be created; that's the majority of old alloc_file()
+	users.  Calling conventions: on success a reference to new struct file
+	is returned and callers reference to inode is subsumed by that.  On
+	failure, ERR_PTR() is returned and no caller's references are affected,
+	so the caller needs to drop the inode reference it held.
+	alloc_file_clone(file, flags, ops) does not affect any caller's references.
+	On success you get a new struct file sharing the mount/dentry with the
+	original, on failure - ERR_PTR().
+--
+[mandatory]
+	DCACHE_RCUACCESS is gone; having an RCU delay on dentry freeing is the
+	default.  DCACHE_NORCU opts out, and only d_alloc_pseudo() has any
+	business doing so.
+--
+[mandatory]
+
+	[should've been added in 2016] stale comment in finish_open()
+	nonwithstanding, failure exits in ->atomic_open() instances should
+	*NOT* fput() the file, no matter what.  Everything is handled by the
+	caller.