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Aufs pseudo-link (plink) represents a virtual hardlink across the
branches. To implement the plink maintenance mode, aufs uses procfs.
See also the document in this commit.

There is an external user-space utility called 'auplink' in
aufs-util.git, which has these features.
- 'list' shows the pseudo-linked inode numbers and filenames.
- 'cpup' copies-up all pseudo-link to the writable branch.
- 'flush' calls 'cpup', and then 'mount -o remount,clean_plink=inum'
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>

For details, see previous commit.
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>

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 is a very old debugging routine for rw_semaphore I was using
privately and less meaningful to other people. It was (probably) before
LOCKDEP feature was introduced, but now it is based upon LOCKDEP. This
is compiled when CONFIG_AUFS_DEBUG is enabled.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

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