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By the commit,
	c71fd893f614 2019-06-17 locking/rwsem: Make owner available even
		if !CONFIG_RWSEM_SPIN_ON_OWNER
rwsem->owner is always defined regardless CONFIG_RWSEM_SPIN_ON_OWNER.
Aufs simply follows it.

Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Special support for filesystems which acquires an inode mutex at final
closing a file, eg, hfsplus.
This trick is very simple and stupid, just to open the file before really
necessary open to tell hfsplus that this is not the final closing.
The caller should call au_h_open_pre() after acquiring the inode mutex,
and au_h_open_post() after releasing it.
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>

->atomic_open() is another monster (the other is ->rename() of course).
It operates look-up, create, and open in a single method.
Strictly speaking the behaviour is not atomic from the branch fs's point
of view, while the atomicity is kept in aufs's. This is a second-best
approach. For details, refer to the design document in previous commit.

A simple list 'si_aopen' can be put into aufs inode object, but I don't
think it a good idea because the number of inodes is much larger than
the number of super_block. If I put it into inode, it can be an
un-interesting memory pressure I am afraid.
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>

Support for XATTR and ACL including several branch attributes to ignore
the copy error around XATTR and ACL.

NFS always sets MS_POSIXACL regardless its mount option 'noacl.'
When MS_POSIXACL is set, generic_permission() calls check_acl() (via
acl_permission_check()) and gets -EOPNOTSUPP because the NFS branch is
mounted as 'noacl.'
In aufs, h_permission() should not call generic_permission() in this
case.
The similar thing happens in coping-up XATTR. vfs_getxattr_alloc()
returns -EOPNOTSUPP.

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

An enhancement for udba=none if possible.
The condition is same to the 'no ->d_revalidate()' patch series.
In this mode, i_op->getattr() is less important, and the default VFS
handler is enough.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Implement i_op->get/setattr().
setattr() is another trigger of the copy-up. The file may or may not be
opened. And some of sub-functions are commonly used to XATTR operations
in later commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Implement i_op->link().
As aufs supports 'pseudo-link', aufs_link() can make it without
copying-up. In the case of aufs_link() has to copy-up, the name of the
target file is used as-is, and it is pseudo-linked. In other words,
calling link(2) after the copy-up is unnecessary.

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

Implement i_op->rename().
This is a big monster and I don't like it.

In this version, the copy-up always happen.
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>

Implement i_op->unlink() with supporting logical deletion by whiteout.
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>

Implement dir.i_op->get_link().
I am afraid there may exist the case skipping updating the inode atime.
It means the aufs inode atime may be reverted by the real fs inode atime
silently.  But I don't think it a big problem.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Implement dir.i_op->lookup() and ->permission().
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>

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>

Aufs allows users' direct branch access (UDBA), ie. by-passing aufs.
Of course it will make aufs inode/dentry-caches obsolete. In order to
detect such change by the several tests, "udba=" option is introduced.

For details, see lookup.txt in later commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

To create/delete/rename files including copy-up, aufs acquires several
locks on the branch fs internally. These lock/unlock operations are
consolidated into struct au_pin in this commit.
au_pin handles
- LOCKDEP class
- re-validate/verify
- suspend/resume HNOTIFY

See also lookup.txt in later commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>