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Generally aufs hides the name of whiteouts. But in some cases, to show
them is very useful for users. For instance, creating a new middle layer
(branch) by merging existing layers.

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

Sometimes the aufs policy to respect the branch fs's permission bits
makes users confused. IE. the direcotry permission bits on the top branch
allows users to read, but the lower branch prohibts. This option may be
useful for such case.
See also the document in this commit.
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>

This is very similar to file operations, including re-open after branch
management. The major part of readdir(3) is split into another object
called VDIR (in previous commit).
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Implement i_op->rmdir() with supporting logical deletion by whiteout
including all children.

As struct.txt in previous commit described, the target dir is renamed to
a whiteout-ed temporary unique name in rmdir(2), and then removed
asynchronously by the system global workqueue.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

This commit is just to prepare for the succeeding commit, and split to
suppress the size of a single commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Now aufs becomes mountable with very few features.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

In order to prevent firing the notify event from aufs itself, hnotify
feature is suspend/resume-able. They are combined with mutex lock/unlock
for the parent dir.

See also previous commits.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

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