[mirror_ubuntu-bionic-kernel.git] / Documentation / filesystems / aufs / design / 10dynop.txt


# Copyright (C) 2010-2017 Junjiro R. Okajima
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

Dynamically customizable FS operations
----------------------------------------------------------------------
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.
Commit	Line	Data
0006ebb4 SF	1
	2	# Copyright (C) 2010-2017 Junjiro R. Okajima
	3	#
	4	# This program is free software; you can redistribute it and/or modify
	5	# it under the terms of the GNU General Public License as published by
	6	# the Free Software Foundation; either version 2 of the License, or
	7	# (at your option) any later version.
	8	#
	9	# This program is distributed in the hope that it will be useful,
	10	# but WITHOUT ANY WARRANTY; without even the implied warranty of
	11	# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	12	# GNU General Public License for more details.
	13	#
	14	# You should have received a copy of the GNU General Public License
	15	# along with this program. If not, see <http://www.gnu.org/licenses/>.
	16
	17	Dynamically customizable FS operations
	18	----------------------------------------------------------------------
	19	Generally FS operations (struct inode_operations, struct
	20	address_space_operations, struct file_operations, etc.) are defined as
	21	"static const", but it never means that FS have only one set of
	22	operation. Some FS have multiple sets of them. For instance, ext2 has
	23	three sets, one for XIP, for NOBH, and for normal.
	24	Since aufs overrides and redirects these operations, sometimes aufs has
	25	to change its behaviour according to the branch FS type. More importantly
	26	VFS acts differently if a function (member in the struct) is set or
	27	not. It means aufs should have several sets of operations and select one
	28	among them according to the branch FS definition.
	29
	30	In order to solve this problem and not to affect the behaviour of VFS,
	31	aufs defines these operations dynamically. For instance, aufs defines
	32	dummy direct_IO function for struct address_space_operations, but it may
	33	not be set to the address_space_operations actually. When the branch FS
	34	doesn't have it, aufs doesn't set it to its address_space_operations
	35	while the function definition itself is still alive. So the behaviour
	36	itself will not change, and it will return an error when direct_IO is
	37	not set.
	38
	39	The lifetime of these dynamically generated operation object is
	40	maintained by aufs branch object. When the branch is removed from aufs,
	41	the reference counter of the object is decremented. When it reaches
	42	zero, the dynamically generated operation object will be freed.
	43
	44	This approach is designed to support AIO (io_submit), Direct I/O and
	45	XIP (DAX) mainly.
	46	Currently this approach is applied to address_space_operations for
	47	regular files only.