# Copyright (C) 2010-2020 Junjiro R. Okajima

Dynamically customizable FS operations
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Generally FS operations (struct inode_operations, struct
address_space_operations, struct file_operations, etc.) are defined as
"static const", but it never means that FS have only one set of
operation. Some FS have multiple sets of them. For instance, ext2 has
three sets, one for XIP, for NOBH, and for normal.
Since aufs overrides and redirects these operations, sometimes aufs has
to change its behaviour according to the branch FS type. More importantly
VFS acts differently if a function (member in the struct) is set or
not. It means aufs should have several sets of operations and select one
among them according to the branch FS definition.

In order to solve this problem and not to affect the behaviour of VFS,
aufs defines these operations dynamically. For instance, aufs defines
dummy direct_IO function for struct address_space_operations, but it may
not be set to the address_space_operations actually. When the branch FS
doesn't have it, aufs doesn't set it to its address_space_operations
while the function definition itself is still alive. So the behaviour
itself will not change, and it will return an error when direct_IO is
not set.

The lifetime of these dynamically generated operation object is
maintained by aufs branch object. When the branch is removed from aufs,
the reference counter of the object is decremented. When it reaches
zero, the dynamically generated operation object will be freed.

This approach is designed to support AIO (io_submit), Direct I/O and
XIP (DAX) mainly.
Currently this approach is applied to address_space_operations for
regular files only.