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Implement exporting via NFS.
The file handle is rather large (40 bytes at most + the file handle on a
branch).
The non-virtual filesystems can use an anonymous (disconnected) dentry
as long as the inode is identified, but aufs needs a dentry with dinfo
which is usually constructed.  So aufs has to find or generate the
normal dentry from the file handle in decoding.  Eg. in aufs, there
should never be the anonymous dentry.

In decoding the file handle, if both of the dentry and the inode which
are corresponding the file handle are still in cache, then they are
returned immediately.  Otherwise aufs has to find the cached parent dir
from the file handle.  If the parent dir is not cached either, the aufs
tries these steps.
- decode the branch fs's file handle and get the parent dir
- generate the path of the parent dir on the branch
- convert the branch path to aufs's path
- lookup the inode number under the aufs' path
The last one is the slowest case.

exportfs_decode_fh() (actually reconnect_path()) acquires mutex, and
this behaviour violates the locking order between aufs si_rwsem.  This
is not a problem since internal exportfs_decode_fh() is called for the
branch fs.
Simply use lockdep_off/on to silence the lockdep message.

See also the document in later commit.
This is compiled only when CONFIG_AUFS_EXPORT is enabled.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

The functions for
- create the copy-up target file
- copy filedata
- copy metadata

In copying filedata, I had tried splice_direct() instead of repeating
read/write. Surprisingly, I could not see a big difference. So let's
keep this approach for a while. Someday SEEK_DATA/SEEK_HOLE become more
popular, it may help optimizing this read/write.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

The internal file read/write for copy-up in kernelspace.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Actually prepare the whiteout bases on the adding writable branch.
For details, refer to previous commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

As mentioned earlier, sometimes the size of XINO file is a problem.
Aufs has a feature to truncate it asynchronously using workqueue. But it
may not be so effective in some cases, and you may want to stop
discontiguous distribution of the inode numbers on branch fs.
See also the log in another commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

The branch path can be much longer and it is not suitable to print via
/proc/mounts as a part of mount options. Aufs can show it either
separately via sysfs or /proc/mounts (as a part of mount options).
This approach affects the lifetime of aufs objects and sbinfo contains
kobject (in another commit). Theoretically user can disable
CONFIG_SYSFS, but the lifetime management is always necessary. So
supporting sysfs is split into two files, sysaufs.c and sysfs.c.
sysaufs.c is always compiled, but sysfs.c is compiled only when
CONFIG_SYSFS is enabled.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

XINO and XIB files are read and written frequently after unlinked, and
it means that the remote filesystems are not suitable for them.
Additionally aufs shows their metadata via debugfs (in later commit).
To make it easier to do this, aufs expects branch filesystems to
maintain their i_size and i_blocks. And it means some filesystem are not
suitable for XINO.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

XINO and XIB files are to maintain the inode numbers in aufs
(cf. struct.txt and aufs manual in aufs-util.git).

XINO file contains just a sequence of the inode numbers, and their
offset in the file is real_inum x sizeof(inum).  So the size is limited
by s_maxbytes of the filesystem where XINO file is located.  In order to
support the larger inum, aufs stores XINO files as an internal array.

Sometimes the size of XINO file can be a problem, ie. too big,
particularly when XINO files are located on tmpfs. In this case, another
separate patch tmpfs-ino.patch in aufs4-standalone.git is recommended
(as well as vfs-ino.patch). The patch makes tmpfs to maintain inode
number within itself and suppress its discontiguous distribution.

See also the document in next commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>