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In mainline, by the commit
	549c7297717c3 2021-01-24 fs: make helpers idmap mount aware
and its series, a new parameter 'user_ns' was added to some inode
operations.  Aufs simply follows it, but it is still testing.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

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

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>

This feature automatically handles MVDOWN in other commits.
In user-space, a daemon monitors the free space of the branch and issues
MVDOWN ioctl automatically when necessary. The main role is in
user-space and several options are implemented.
For a branch to join the FHSM circle, a new attribute 'fhsm' should be
specified.

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

These are very old options.
Since Unionfs created 'diropq' unconditionally in mkdir(2), old users
may expect the same behaviour. But there are cases where 'diropq' is
unnecessary. The aufs default behaviour is to create 'diropq' only when
it is necessary.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

This new function au_aopen_or_create() tries calling branch fs's
->atomic_open() first. If it is not set, call vfs_create() instead.
By putting this behaviour into aufs's add_simple(), many codes can be
shared.
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>

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>