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This is the hardest test to support UDBA (users' direct branch access).
It uses 'fsnotify' internally.  Detecting UDBA, decrements the
generation of the cached aufs objects.  In the next access to the file,
aufs detects the generation is obsoleted and tries refreshing it.
Eventually aufs cache will be updated to latest status.

The fsnotify is set on the cached dirs on the non-RR branches.
The RR (real readonly) branches will never be modified and it is
unnecessary to set fsnotify for them.

This commit is for the declarations mainly, and the body parts will be
in succeeding commits.

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

The main part is in previous commit.
This commit handles the generation of aufs objects, to make sure the
inode in the file handle is still valid.
In order not to confuse NFSD, the various operation returns ESTALE for
NFSD where it used to return EBUSY.

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

As a result of branch management, the virtual inode may point a
different real inode from it used to. And aufs has to maintain its
address_space_operations, since its definition may affect the
behaviour.
I know some people (including grsec-patch) doesn't like a non-const
address_space_operations, but in order to keep the consistency of the
behaviour, the correct address_space_operations is important.

See also the document in this commit.
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>