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This feature automatically handles MVDOWN in other commits.
In user-space, a daemon monitors the free space of the branch and issues
MVDOWN ioctl automatically when necessary. The main role is in
user-space and several options are implemented.
For a branch to join the FHSM circle, a new attribute 'fhsm' should be
specified.

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

Support for the options of MVDOWN feature, which allows to overwrite the
existing entry, and writing to the branch even if its permission is RO.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Provide a file descriptor corresponding the specified writable branch.
The file descriptor will be used from user-space such as FHSM and
libau.so. For details, see aufs-util.git.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Implement an internal list of opened files to allow deleting a branch
which has an opened dir. Obviously I don't like such list.

There was such list in linux as sb->s_files, but in linux-3.12 s_files
became containing just a part of the opened files, and in linux-3.13 it
was totally gone.
Aufs still needs the file list, particularly for re-setting the branch
attribute from RW to RO.
After resetting to RO, aufs should return EROFS for write. In order to
support such case, aufs keeps the late s_files and mark_files_ro()
approach.

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

Extend do_open_dir(), au_do_open_nondir() and au_do_open() to receive an
additional parameter h_file, which is an opened file object by branch
fs's ->atomic_open(). By this design/commit, aufs doesn't have to
duplicate many codes into a new aufs_atomic_open() (in later commit),
and can simply share them.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Implement several f_op functions for non-dir.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

For details, read the document in this commit.
I don't like this approach, but there is no other way currently. But it
seems that UnionMount is trying add siblings of f_dentry and d_inode for
linux-4.0 or later. It may become another light for aufs too.

The finfo object which has ever mmapped is excluded from
refreshing (based upon fi_mmapped). Otherwise we may corrupt the process
memory space.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Implement f_op->open() for non-directory.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>