[PATCH] configfs: User-driven configuration filesystem

	- description of the CIFS filesystem

coda.txt

	- description of the CODA filesystem.

configfs/

	- directory containing configfs documentation and example code.

cramfs.txt

	- info on the cram filesystem for small storage (ROMs etc)

devfs/

configfs - Userspace-driven kernel object configuation.

Joel Becker <joel.becker@oracle.com>

Updated: 31 March 2005

Copyright (c) 2005 Oracle Corporation,

	Joel Becker <joel.becker@oracle.com>

[What is configfs?]

configfs is a ram-based filesystem that provides the converse of

sysfs's functionality.  Where sysfs is a filesystem-based view of

kernel objects, configfs is a filesystem-based manager of kernel

objects, or config_items.

With sysfs, an object is created in kernel (for example, when a device

is discovered) and it is registered with sysfs.  Its attributes then

appear in sysfs, allowing userspace to read the attributes via

readdir(3)/read(2).  It may allow some attributes to be modified via

write(2).  The important point is that the object is created and

destroyed in kernel, the kernel controls the lifecycle of the sysfs

representation, and sysfs is merely a window on all this.

A configfs config_item is created via an explicit userspace operation:

mkdir(2).  It is destroyed via rmdir(2).  The attributes appear at

mkdir(2) time, and can be read or modified via read(2) and write(2).

As with sysfs, readdir(3) queries the list of items and/or attributes.

symlink(2) can be used to group items together.  Unlike sysfs, the

lifetime of the representation is completely driven by userspace.  The

kernel modules backing the items must respond to this.

Both sysfs and configfs can and should exist together on the same

system.  One is not a replacement for the other.

[Using configfs]

configfs can be compiled as a module or into the kernel.  You can access

it by doing

	mount -t configfs none /config

The configfs tree will be empty unless client modules are also loaded.

These are modules that register their item types with configfs as

subsystems.  Once a client subsystem is loaded, it will appear as a

subdirectory (or more than one) under /config.  Like sysfs, the

configfs tree is always there, whether mounted on /config or not.

An item is created via mkdir(2).  The item's attributes will also

appear at this time.  readdir(3) can determine what the attributes are,

read(2) can query their default values, and write(2) can store new

values.  Like sysfs, attributes should be ASCII text files, preferably

with only one value per file.  The same efficiency caveats from sysfs

apply.  Don't mix more than one attribute in one attribute file.

Like sysfs, configfs expects write(2) to store the entire buffer at

once.  When writing to configfs attributes, userspace processes should

first read the entire file, modify the portions they wish to change, and

then write the entire buffer back.  Attribute files have a maximum size

of one page (PAGE_SIZE, 4096 on i386).

When an item needs to be destroyed, remove it with rmdir(2).  An

item cannot be destroyed if any other item has a link to it (via

symlink(2)).  Links can be removed via unlink(2).

[Configuring FakeNBD: an Example]

Imagine there's a Network Block Device (NBD) driver that allows you to

access remote block devices.  Call it FakeNBD.  FakeNBD uses configfs

for its configuration.  Obviously, there will be a nice program that

sysadmins use to configure FakeNBD, but somehow that program has to tell

the driver about it.  Here's where configfs comes in.

When the FakeNBD driver is loaded, it registers itself with configfs.

readdir(3) sees this just fine:

	# ls /config

	fakenbd

A fakenbd connection can be created with mkdir(2).  The name is

arbitrary, but likely the tool will make some use of the name.  Perhaps

it is a uuid or a disk name:

	# mkdir /config/fakenbd/disk1

	# ls /config/fakenbd/disk1

	target device rw

The target attribute contains the IP address of the server FakeNBD will

connect to.  The device attribute is the device on the server.

Predictably, the rw attribute determines whether the connection is

read-only or read-write.

	# echo 10.0.0.1 > /config/fakenbd/disk1/target

	# echo /dev/sda1 > /config/fakenbd/disk1/device

	# echo 1 > /config/fakenbd/disk1/rw

That's it.  That's all there is.  Now the device is configured, via the

shell no less.

[Coding With configfs]

Every object in configfs is a config_item.  A config_item reflects an

object in the subsystem.  It has attributes that match values on that

object.  configfs handles the filesystem representation of that object

and its attributes, allowing the subsystem to ignore all but the

basic show/store interaction.

Items are created and destroyed inside a config_group.  A group is a

collection of items that share the same attributes and operations.

Items are created by mkdir(2) and removed by rmdir(2), but configfs

handles that.  The group has a set of operations to perform these tasks

A subsystem is the top level of a client module.  During initialization,

the client module registers the subsystem with configfs, the subsystem

appears as a directory at the top of the configfs filesystem.  A

subsystem is also a config_group, and can do everything a config_group

can.

[struct config_item]

	struct config_item {

		char                    *ci_name;

		char                    ci_namebuf[UOBJ_NAME_LEN];

		struct kref             ci_kref;

		struct list_head        ci_entry;

		struct config_item      *ci_parent;

		struct config_group     *ci_group;

		struct config_item_type *ci_type;

		struct dentry           *ci_dentry;

	};

	void config_item_init(struct config_item *);

	void config_item_init_type_name(struct config_item *,

					const char *name,

					struct config_item_type *type);

	struct config_item *config_item_get(struct config_item *);

	void config_item_put(struct config_item *);

Generally, struct config_item is embedded in a container structure, a

structure that actually represents what the subsystem is doing.  The

config_item portion of that structure is how the object interacts with

configfs.

Whether statically defined in a source file or created by a parent

config_group, a config_item must have one of the _init() functions

called on it.  This initializes the reference count and sets up the

appropriate fields.

All users of a config_item should have a reference on it via

config_item_get(), and drop the reference when they are done via

config_item_put().

By itself, a config_item cannot do much more than appear in configfs.

Usually a subsystem wants the item to display and/or store attributes,

among other things.  For that, it needs a type.

[struct config_item_type]

	struct configfs_item_operations {

		void (*release)(struct config_item *);

		ssize_t (*show_attribute)(struct config_item *,

					  struct configfs_attribute *,

					  char *);

		ssize_t (*store_attribute)(struct config_item *,

					   struct configfs_attribute *,

					   const char *, size_t);

		int (*allow_link)(struct config_item *src,

				  struct config_item *target);

		int (*drop_link)(struct config_item *src,

				 struct config_item *target);

	};

	struct config_item_type {

		struct module                           *ct_owner;

		struct configfs_item_operations         *ct_item_ops;

		struct configfs_group_operations        *ct_group_ops;

		struct configfs_attribute               **ct_attrs;

	};

The most basic function of a config_item_type is to define what

operations can be performed on a config_item.  All items that have been

allocated dynamically will need to provide the ct_item_ops->release()

method.  This method is called when the config_item's reference count

reaches zero.  Items that wish to display an attribute need to provide

the ct_item_ops->show_attribute() method.  Similarly, storing a new

attribute value uses the store_attribute() method.

[struct configfs_attribute]

	struct configfs_attribute {

		char                    *ca_name;

		struct module           *ca_owner;

		mode_t                  ca_mode;

	};

When a config_item wants an attribute to appear as a file in the item's

configfs directory, it must define a configfs_attribute describing it.

It then adds the attribute to the NULL-terminated array

config_item_type->ct_attrs.  When the item appears in configfs, the

attribute file will appear with the configfs_attribute->ca_name

filename.  configfs_attribute->ca_mode specifies the file permissions.

If an attribute is readable and the config_item provides a

ct_item_ops->show_attribute() method, that method will be called

whenever userspace asks for a read(2) on the attribute.  The converse

will happen for write(2).

[struct config_group]

A config_item cannot live in a vaccum.  The only way one can be created

is via mkdir(2) on a config_group.  This will trigger creation of a

child item.

	struct config_group {

		struct config_item		cg_item;

		struct list_head		cg_children;

		struct configfs_subsystem 	*cg_subsys;

		struct config_group		**default_groups;

	};

	void config_group_init(struct config_group *group);

	void config_group_init_type_name(struct config_group *group,

					 const char *name,

					 struct config_item_type *type);

The config_group structure contains a config_item.  Properly configuring

that item means that a group can behave as an item in its own right.

However, it can do more: it can create child items or groups.  This is

accomplished via the group operations specified on the group's

config_item_type.

	struct configfs_group_operations {

		struct config_item *(*make_item)(struct config_group *group,

						 const char *name);

		struct config_group *(*make_group)(struct config_group *group,

						   const char *name);

		int (*commit_item)(struct config_item *item);

		void (*drop_item)(struct config_group *group,

				  struct config_item *item);

	};

A group creates child items by providing the

ct_group_ops->make_item() method.  If provided, this method is called from mkdir(2) in the group's directory.  The subsystem allocates a new

config_item (or more likely, its container structure), initializes it,

and returns it to configfs.  Configfs will then populate the filesystem

tree to reflect the new item.

If the subsystem wants the child to be a group itself, the subsystem

provides ct_group_ops->make_group().  Everything else behaves the same,

using the group _init() functions on the group.

Finally, when userspace calls rmdir(2) on the item or group,

ct_group_ops->drop_item() is called.  As a config_group is also a

config_item, it is not necessary for a seperate drop_group() method.

The subsystem must config_item_put() the reference that was initialized

upon item allocation.  If a subsystem has no work to do, it may omit

the ct_group_ops->drop_item() method, and configfs will call

config_item_put() on the item on behalf of the subsystem.

IMPORTANT: drop_item() is void, and as such cannot fail.  When rmdir(2)

is called, configfs WILL remove the item from the filesystem tree

(assuming that it has no children to keep it busy).  The subsystem is

responsible for responding to this.  If the subsystem has references to

the item in other threads, the memory is safe.  It may take some time

for the item to actually disappear from the subsystem's usage.  But it

is gone from configfs.

A config_group cannot be removed while it still has child items.  This

is implemented in the configfs rmdir(2) code.  ->drop_item() will not be

called, as the item has not been dropped.  rmdir(2) will fail, as the

directory is not empty.

[struct configfs_subsystem]

A subsystem must register itself, ususally at module_init time.  This

tells configfs to make the subsystem appear in the file tree.

	struct configfs_subsystem {

		struct config_group	su_group;

		struct semaphore	su_sem;

	};

	int configfs_register_subsystem(struct configfs_subsystem *subsys);

	void configfs_unregister_subsystem(struct configfs_subsystem *subsys);

	A subsystem consists of a toplevel config_group and a semaphore.

The group is where child config_items are created.  For a subsystem,

this group is usually defined statically.  Before calling

configfs_register_subsystem(), the subsystem must have initialized the

group via the usual group _init() functions, and it must also have

initialized the semaphore.

	When the register call returns, the subsystem is live, and it

will be visible via configfs.  At that point, mkdir(2) can be called and

the subsystem must be ready for it.

[An Example]

The best example of these basic concepts is the simple_children

subsystem/group and the simple_child item in configfs_example.c  It

shows a trivial object displaying and storing an attribute, and a simple

group creating and destroying these children.

[Hierarchy Navigation and the Subsystem Semaphore]

There is an extra bonus that configfs provides.  The config_groups and

config_items are arranged in a hierarchy due to the fact that they

appear in a filesystem.  A subsystem is NEVER to touch the filesystem

parts, but the subsystem might be interested in this hierarchy.  For

this reason, the hierarchy is mirrored via the config_group->cg_children

and config_item->ci_parent structure members.

A subsystem can navigate the cg_children list and the ci_parent pointer

to see the tree created by the subsystem.  This can race with configfs'

management of the hierarchy, so configfs uses the subsystem semaphore to

protect modifications.  Whenever a subsystem wants to navigate the

hierarchy, it must do so under the protection of the subsystem

semaphore.

A subsystem will be prevented from acquiring the semaphore while a newly

allocated item has not been linked into this hierarchy.   Similarly, it

will not be able to acquire the semaphore while a dropping item has not

yet been unlinked.  This means that an item's ci_parent pointer will

never be NULL while the item is in configfs, and that an item will only

be in its parent's cg_children list for the same duration.  This allows

a subsystem to trust ci_parent and cg_children while they hold the

semaphore.

[Item Aggregation Via symlink(2)]

configfs provides a simple group via the group->item parent/child

relationship.  Often, however, a larger environment requires aggregation

outside of the parent/child connection.  This is implemented via

symlink(2).

A config_item may provide the ct_item_ops->allow_link() and

ct_item_ops->drop_link() methods.  If the ->allow_link() method exists,

symlink(2) may be called with the config_item as the source of the link.

These links are only allowed between configfs config_items.  Any

symlink(2) attempt outside the configfs filesystem will be denied.

When symlink(2) is called, the source config_item's ->allow_link()

method is called with itself and a target item.  If the source item

allows linking to target item, it returns 0.  A source item may wish to

reject a link if it only wants links to a certain type of object (say,

in its own subsystem).

When unlink(2) is called on the symbolic link, the source item is

notified via the ->drop_link() method.  Like the ->drop_item() method,

this is a void function and cannot return failure.  The subsystem is

responsible for responding to the change.

A config_item cannot be removed while it links to any other item, nor

can it be removed while an item links to it.  Dangling symlinks are not

allowed in configfs.

[Automatically Created Subgroups]

A new config_group may want to have two types of child config_items.

While this could be codified by magic names in ->make_item(), it is much

more explicit to have a method whereby userspace sees this divergence.

Rather than have a group where some items behave differently than

others, configfs provides a method whereby one or many subgroups are

automatically created inside the parent at its creation.  Thus,

mkdir("parent) results in "parent", "parent/subgroup1", up through

"parent/subgroupN".  Items of type 1 can now be created in

"parent/subgroup1", and items of type N can be created in

"parent/subgroupN".

These automatic subgroups, or default groups, do not preclude other

children of the parent group.  If ct_group_ops->make_group() exists,

other child groups can be created on the parent group directly.

A configfs subsystem specifies default groups by filling in the

NULL-terminated array default_groups on the config_group structure.

Each group in that array is populated in the configfs tree at the same

time as the parent group.  Similarly, they are removed at the same time

as the parent.  No extra notification is provided.  When a ->drop_item()

method call notifies the subsystem the parent group is going away, it

also means every default group child associated with that parent group.

As a consequence of this, default_groups cannot be removed directly via

rmdir(2).  They also are not considered when rmdir(2) on the parent

group is checking for children.

[Committable Items]

NOTE: Committable items are currently unimplemented.

Some config_items cannot have a valid initial state.  That is, no

default values can be specified for the item's attributes such that the

item can do its work.  Userspace must configure one or more attributes,

after which the subsystem can start whatever entity this item

represents.

Consider the FakeNBD device from above.  Without a target address *and*

a target device, the subsystem has no idea what block device to import.

The simple example assumes that the subsystem merely waits until all the

appropriate attributes are configured, and then connects.  This will,

indeed, work, but now every attribute store must check if the attributes

are initialized.  Every attribute store must fire off the connection if

that condition is met.

Far better would be an explicit action notifying the subsystem that the

config_item is ready to go.  More importantly, an explicit action allows

the subsystem to provide feedback as to whether the attibutes are

initialized in a way that makes sense.  configfs provides this as

committable items.

configfs still uses only normal filesystem operations.  An item is

committed via rename(2).  The item is moved from a directory where it

can be modified to a directory where it cannot.

Any group that provides the ct_group_ops->commit_item() method has

committable items.  When this group appears in configfs, mkdir(2) will

not work directly in the group.  Instead, the group will have two

subdirectories: "live" and "pending".  The "live" directory does not

support mkdir(2) or rmdir(2) either.  It only allows rename(2).  The

"pending" directory does allow mkdir(2) and rmdir(2).  An item is

created in the "pending" directory.  Its attributes can be modified at

will.  Userspace commits the item by renaming it into the "live"

directory.  At this point, the subsystem recieves the ->commit_item()

callback.  If all required attributes are filled to satisfaction, the

method returns zero and the item is moved to the "live" directory.

As rmdir(2) does not work in the "live" directory, an item must be

shutdown, or "uncommitted".  Again, this is done via rename(2), this

time from the "live" directory back to the "pending" one.  The subsystem

is notified by the ct_group_ops->uncommit_object() method.

T:	git kernel.org:/pub/scm/linux/kernel/git/sfrench/cifs-2.6.git

S:	Supported	

CONFIGFS

P:	Joel Becker

M:	Joel Becker <joel.becker@oracle.com>

S:	Supported

CIRRUS LOGIC GENERIC FBDEV DRIVER

P:	Jeff Garzik

M:	jgarzik@pobox.com

	  If unsure, say N.

config CONFIGFS_FS

	tristate "Userspace-driven configuration filesystem (EXPERIMENTAL)"

	depends on EXPERIMENTAL

	help

	  configfs is a ram-based filesystem that provides the converse

	  of sysfs's functionality. Where sysfs is a filesystem-based

	  view of kernel objects, configfs is a filesystem-based manager

	  of kernel objects, or config_items.

	  Both sysfs and configfs can and should exist together on the

	  same system. One is not a replacement for the other.

	  If unsure, say N.

endmenu

menu "Miscellaneous filesystems"