path: root/Documentation/filesystems/porting.rst

====================
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**

The foo_inode_info should always be allocated through alloc_inode_sb() rather
than kmem_cache_alloc() or kmalloc() related to set up the inode reclaim context
correctly.

---

**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.rst.

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, 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_state_read_once(inode) & I_NEW) {
		err = read_inode_from_disk(inode);
		if (err < 0) {