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Changes since 2.5.0:

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---
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[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())

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---
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[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;

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Add foo_alloc_inode() and foo_destroy_inode() - the former should allocate
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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.

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Keep in mind that now you need explicit initialization of private data
typically between calling iget_locked() and unlocking the inode.
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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

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int foo_get_sb(struct file_system_type *fs_type,
	int flags, const char *dev_name, void *data, struct vfsmount *mnt)
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{
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	return get_sb_bdev(fs_type, flags, dev_name, data, foo_fill_super,
			   mnt);
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}

(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]

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->link() callers hold ->i_mutex on the object we are linking to.  Some of your
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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]

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->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.
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---
[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
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Documentation/filesystems/nfs/Exporting.
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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.

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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().
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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.
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	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.
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---
[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()
	* dcache_lock is held
	* 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.
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[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.
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[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.
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---
[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).
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---
[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.