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->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.

  	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(...).

  
  --
  [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.