xref: /linux/Documentation/filesystems/porting.rst (revision 25b532cec59ee119bf1c4081fe06fac43722f61f)

====================
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.rst (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.rst (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.rst 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.rst 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).  On success d_make_root(inode)
allocates and returns a new dentry instantiated with the passed in inode.
On failure NULL is returned and the passed in inode is dropped so the reference
to inode is consumed in all cases and failure handling need not do any cleanup
for the inode.  If d_make_root(inode) is passed a NULL inode it returns NULL
and also requires no further error handling. Typical usage is::

	inode = foofs_new_inode(....);
	s->s_root = d_make_root(inode);
	if (!s->s_root)
		/* Nothing needed for the inode cleanup */
		return -ENOMEM;
	...

---

**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**

->clone_file_range() and ->dedupe_file_range have been replaced with
->remap_file_range().  See Documentation/filesystems/vfs.rst for more
information.

---

**recommended**

->lookup() instances doing an equivalent of::

	if (IS_ERR(inode))
		return ERR_CAST(inode);
	return d_splice_alias(inode, dentry);

don't need to bother with the check - d_splice_alias() will do the
right thing when given ERR_PTR(...) as inode.  Moreover, passing NULL
inode to d_splice_alias() will also do the right thing (equivalent of
d_add(dentry, NULL); return NULL;), so that kind of special cases
also doesn't need a separate treatment.

---

**strongly recommended**

take the RCU-delayed parts of ->destroy_inode() into a new method -
->free_inode().  If ->destroy_inode() becomes empty - all the better,
just get rid of it.  Synchronous work (e.g. the stuff that can't
be done from an RCU callback, or any WARN_ON() where we want the
stack trace) *might* be movable to ->evict_inode(); however,
that goes only for the things that are not needed to balance something
done by ->alloc_inode().  IOW, if it's cleaning up the stuff that
might have accumulated over the life of in-core inode, ->evict_inode()
might be a fit.

Rules for inode destruction:

	* if ->destroy_inode() is non-NULL, it gets called
	* if ->free_inode() is non-NULL, it gets scheduled by call_rcu()
	* combination of NULL ->destroy_inode and NULL ->free_inode is
	  treated as NULL/free_inode_nonrcu, to preserve the compatibility.

Note that the callback (be it via ->free_inode() or explicit call_rcu()
in ->destroy_inode()) is *NOT* ordered wrt superblock destruction;
as the matter of fact, the superblock and all associated structures
might be already gone.  The filesystem driver is guaranteed to be still
there, but that's it.  Freeing memory in the callback is fine; doing
more than that is possible, but requires a lot of care and is best
avoided.

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**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.

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**mandatory**

d_alloc_pseudo() is internal-only; uses outside of alloc_file_pseudo() are
very suspect (and won't work in modules).  Such uses are very likely to
be misspelled d_alloc_anon().