Documentation/filesystems/porting - kernel/msm - Gitiles

 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.  ->write_super() Is now called
 without BKL held.  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 MS_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 going away.  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.
 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; getting rid of those is responsibility of method, as it had
 been for ->delete_inode().
 	->drop_inode() returns int now; it's called on final iput() with inode_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.
 	clear_inode() is gone; use end_writeback() instead.  As before, it 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 end_writeback().
 	No async writeback (and thus no calls of ->write_inode()) will happen
 after end_writeback() returns, so actions that should not overlap with ->write_inode()
 (e.g. freeing on-disk inode if i_nlink is 0) ought to be done after that call.

 	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).

 	i_dentry and i_rcu share storage in a union, and the vfs expects
 i_dentry to be reinitialized before it is freed, so an:

   INIT_LIST_HEAD(&inode->i_dentry);

 must be done in the RCU callback.

 --
 [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 and check_acl are inode permission checks that are called
 on many or all directory inodes on the way down a path walk (to check for
 exec permission). These must now be rcu-walk aware (flags & IPERM_FLAG_RCU).
 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(...).
	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. ->write_super() Is now called
	without BKL held. 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 MS_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 going away. 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.
	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; getting rid of those is responsibility of method, as it had
	been for ->delete_inode().
	->drop_inode() returns int now; it's called on final iput() with inode_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.
	clear_inode() is gone; use end_writeback() instead. As before, it 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 end_writeback().
	No async writeback (and thus no calls of ->write_inode()) will happen
	after end_writeback() returns, so actions that should not overlap with ->write_inode()
	(e.g. freeing on-disk inode if i_nlink is 0) ought to be done after that call.

	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).

	i_dentry and i_rcu share storage in a union, and the vfs expects
	i_dentry to be reinitialized before it is freed, so an:

	INIT_LIST_HEAD(&inode->i_dentry);

	must be done in the RCU callback.

	--
	[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 and check_acl are inode permission checks that are called
	on many or all directory inodes on the way down a path walk (to check for
	exec permission). These must now be rcu-walk aware (flags & IPERM_FLAG_RCU).
	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(...).