android_kernel_samsung_msm8976/Documentation/filesystems/porting
David Howells 454e2398be [PATCH] VFS: Permit filesystem to override root dentry on mount
Extend the get_sb() filesystem operation to take an extra argument that
permits the VFS to pass in the target vfsmount that defines the mountpoint.

The filesystem is then required to manually set the superblock and root dentry
pointers.  For most filesystems, this should be done with simple_set_mnt()
which will set the superblock pointer and then set the root dentry to the
superblock's s_root (as per the old default behaviour).

The get_sb() op now returns an integer as there's now no need to return the
superblock pointer.

This patch permits a superblock to be implicitly shared amongst several mount
points, such as can be done with NFS to avoid potential inode aliasing.  In
such a case, simple_set_mnt() would not be called, and instead the mnt_root
and mnt_sb would be set directly.

The patch also makes the following changes:

 (*) the get_sb_*() convenience functions in the core kernel now take a vfsmount
     pointer argument and return an integer, so most filesystems have to change
     very little.

 (*) If one of the convenience function is not used, then get_sb() should
     normally call simple_set_mnt() to instantiate the vfsmount. This will
     always return 0, and so can be tail-called from get_sb().

 (*) generic_shutdown_super() now calls shrink_dcache_sb() to clean up the
     dcache upon superblock destruction rather than shrink_dcache_anon().

     This is required because the superblock may now have multiple trees that
     aren't actually bound to s_root, but that still need to be cleaned up. The
     currently called functions assume that the whole tree is rooted at s_root,
     and that anonymous dentries are not the roots of trees which results in
     dentries being left unculled.

     However, with the way NFS superblock sharing are currently set to be
     implemented, these assumptions are violated: the root of the filesystem is
     simply a dummy dentry and inode (the real inode for '/' may well be
     inaccessible), and all the vfsmounts are rooted on anonymous[*] dentries
     with child trees.

     [*] Anonymous until discovered from another tree.

 (*) The documentation has been adjusted, including the additional bit of
     changing ext2_* into foo_* in the documentation.

[akpm@osdl.org: convert ipath_fs, do other stuff]
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Nathan Scott <nathans@sgi.com>
Cc: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-06-23 07:42:45 -07:00

267 lines
8.3 KiB
Text

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_destory_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 in ->read_inode() and after getting an inode from new_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_sem 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_sem, so
watch for ->i_sem-grabbing code that might be used by your ->setattr().
Callers of notify_change() need ->i_sem 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/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. read_inode has not been
called so the file system still has 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) {
read_inode_from_disk(inode);
unlock_new_inode(inode);
}
---
[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.