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Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
(cherry picked from commit 56b4b776c84f364384971c4cdfd66b4a2f0696d4)

Print the current data status for debugging.
The trigger key is a module parameter and you can freely change. The
default is 'a' of course.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Simple checks when the module is loaded.
This feature is compiled when CONFIG_AUFS_DEBUG is enabled.
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>

Here are entry adding inode operations, i_op->create(), symlink(),
mkdir(), mknod(), and tmpfile().
Obviously they return EOPNOTSUPP when the target branch fs doesn't
support the operation.
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>

The structure is very similar to iinfo and dinfo (in previous commits).
This commit is for non-dir files. For a directory, see later commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

As a part of looking-up, construct a virtual inode.
After branch-management (add/del branches), the inode has to be
refreshed to represent a revealed file.
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>

Imagine a sequence such like this.
- user opens a file which exists on the lower readonly branch.
- user unlinks the file (still opened). its parent dir may be removed
  too.
- user writes something to the file.

Then aufs has to copy-up the unlinked file.
Note that the copy-up in aufs is not done in open(2) by default.
This commit handles this case by copy-up the file in write(2).
The target directory is the one which aufs created internally for this
purpose.
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>

The marker to represent that the directory is opaque (stop digging
down the branch stack) is implemented as a special whiteout.

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

The whiteout represents a logical deletion.
Although the document in this commit mentioned about rmdir(2) and
rename(2) for dir, this commit doesn't contain such functions. They will
be added in later commits.
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>

Aufs uses the workqueue both synchronously and asynchronously.
For sync-use-case, aufs uses its own specific wkq since doesn't want to
be disturbed by other tasks on the system. For async-use-case, aufs uses
the system global workqueue.
Aufs has to prevent itself to being unmounted during the async-task is
queued.

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

Print various info about aufs branch and superblock. This feature is
enabled when CONFIG_AUFS_DEBUG and the module parameter 'debug' are set.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

The generation of iinfo and dinfo inherit sbinfo's.
Also iinfo generation tracks the branch inode's generation to test the
matching after the branch management.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

The structure is very similar to aufs inode info (in previous commit).
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Print various info about aufs inode. This feature is enabled when
CONFIG_AUFS_DEBUG and the module parameter 'debug' are set.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

This parameter is available only when CONFIG_AUFS_DEBUG is enabled, and
its default value is 0. Setting 1 will enable the various verifications
and debug outputs.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>