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2 configfs - Userspace-driven kernel object configuration.
4 Joel Becker <joel.becker@oracle.com>
6 Updated: 31 March 2005
8 Copyright (c) 2005 Oracle Corporation,
9 Joel Becker <joel.becker@oracle.com>
12 [What is configfs?]
14 configfs is a ram-based filesystem that provides the converse of
15 sysfs's functionality. Where sysfs is a filesystem-based view of
16 kernel objects, configfs is a filesystem-based manager of kernel
17 objects, or config_items.
19 With sysfs, an object is created in kernel (for example, when a device
20 is discovered) and it is registered with sysfs. Its attributes then
21 appear in sysfs, allowing userspace to read the attributes via
22 readdir(3)/read(2). It may allow some attributes to be modified via
23 write(2). The important point is that the object is created and
24 destroyed in kernel, the kernel controls the lifecycle of the sysfs
25 representation, and sysfs is merely a window on all this.
27 A configfs config_item is created via an explicit userspace operation:
28 mkdir(2). It is destroyed via rmdir(2). The attributes appear at
29 mkdir(2) time, and can be read or modified via read(2) and write(2).
30 As with sysfs, readdir(3) queries the list of items and/or attributes.
31 symlink(2) can be used to group items together. Unlike sysfs, the
32 lifetime of the representation is completely driven by userspace. The
33 kernel modules backing the items must respond to this.
35 Both sysfs and configfs can and should exist together on the same
36 system. One is not a replacement for the other.
38 [Using configfs]
40 configfs can be compiled as a module or into the kernel. You can access
41 it by doing
43 mount -t configfs none /config
45 The configfs tree will be empty unless client modules are also loaded.
46 These are modules that register their item types with configfs as
47 subsystems. Once a client subsystem is loaded, it will appear as a
48 subdirectory (or more than one) under /config. Like sysfs, the
49 configfs tree is always there, whether mounted on /config or not.
51 An item is created via mkdir(2). The item's attributes will also
52 appear at this time. readdir(3) can determine what the attributes are,
53 read(2) can query their default values, and write(2) can store new
54 values. Like sysfs, attributes should be ASCII text files, preferably
55 with only one value per file. The same efficiency caveats from sysfs
56 apply. Don't mix more than one attribute in one attribute file.
58 Like sysfs, configfs expects write(2) to store the entire buffer at
59 once. When writing to configfs attributes, userspace processes should
60 first read the entire file, modify the portions they wish to change, and
61 then write the entire buffer back. Attribute files have a maximum size
62 of one page (PAGE_SIZE, 4096 on i386).
64 When an item needs to be destroyed, remove it with rmdir(2). An
65 item cannot be destroyed if any other item has a link to it (via
66 symlink(2)). Links can be removed via unlink(2).
68 [Configuring FakeNBD: an Example]
70 Imagine there's a Network Block Device (NBD) driver that allows you to
71 access remote block devices. Call it FakeNBD. FakeNBD uses configfs
72 for its configuration. Obviously, there will be a nice program that
73 sysadmins use to configure FakeNBD, but somehow that program has to tell
74 the driver about it. Here's where configfs comes in.
76 When the FakeNBD driver is loaded, it registers itself with configfs.
77 readdir(3) sees this just fine:
79 # ls /config
80 fakenbd
82 A fakenbd connection can be created with mkdir(2). The name is
83 arbitrary, but likely the tool will make some use of the name. Perhaps
84 it is a uuid or a disk name:
86 # mkdir /config/fakenbd/disk1
87 # ls /config/fakenbd/disk1
88 target device rw
90 The target attribute contains the IP address of the server FakeNBD will
91 connect to. The device attribute is the device on the server.
92 Predictably, the rw attribute determines whether the connection is
93 read-only or read-write.
95 # echo 10.0.0.1 > /config/fakenbd/disk1/target
96 # echo /dev/sda1 > /config/fakenbd/disk1/device
97 # echo 1 > /config/fakenbd/disk1/rw
99 That's it. That's all there is. Now the device is configured, via the
100 shell no less.
102 [Coding With configfs]
104 Every object in configfs is a config_item. A config_item reflects an
105 object in the subsystem. It has attributes that match values on that
106 object. configfs handles the filesystem representation of that object
107 and its attributes, allowing the subsystem to ignore all but the
108 basic show/store interaction.
110 Items are created and destroyed inside a config_group. A group is a
111 collection of items that share the same attributes and operations.
112 Items are created by mkdir(2) and removed by rmdir(2), but configfs
113 handles that. The group has a set of operations to perform these tasks
115 A subsystem is the top level of a client module. During initialization,
116 the client module registers the subsystem with configfs, the subsystem
117 appears as a directory at the top of the configfs filesystem. A
118 subsystem is also a config_group, and can do everything a config_group
119 can.
121 [struct config_item]
123 struct config_item {
124 char *ci_name;
125 char ci_namebuf[UOBJ_NAME_LEN];
126 struct kref ci_kref;
127 struct list_head ci_entry;
128 struct config_item *ci_parent;
129 struct config_group *ci_group;
130 struct config_item_type *ci_type;
131 struct dentry *ci_dentry;
132 };
134 void config_item_init(struct config_item *);
135 void config_item_init_type_name(struct config_item *,
136 const char *name,
137 struct config_item_type *type);
138 struct config_item *config_item_get(struct config_item *);
139 void config_item_put(struct config_item *);
141 Generally, struct config_item is embedded in a container structure, a
142 structure that actually represents what the subsystem is doing. The
143 config_item portion of that structure is how the object interacts with
144 configfs.
146 Whether statically defined in a source file or created by a parent
147 config_group, a config_item must have one of the _init() functions
148 called on it. This initializes the reference count and sets up the
149 appropriate fields.
151 All users of a config_item should have a reference on it via
152 config_item_get(), and drop the reference when they are done via
153 config_item_put().
155 By itself, a config_item cannot do much more than appear in configfs.
156 Usually a subsystem wants the item to display and/or store attributes,
157 among other things. For that, it needs a type.
159 [struct config_item_type]
161 struct configfs_item_operations {
162 void (*release)(struct config_item *);
163 ssize_t (*show_attribute)(struct config_item *,
164 struct configfs_attribute *,
165 char *);
166 ssize_t (*store_attribute)(struct config_item *,
167 struct configfs_attribute *,
168 const char *, size_t);
169 int (*allow_link)(struct config_item *src,
170 struct config_item *target);
171 int (*drop_link)(struct config_item *src,
172 struct config_item *target);
173 };
175 struct config_item_type {
176 struct module *ct_owner;
177 struct configfs_item_operations *ct_item_ops;
178 struct configfs_group_operations *ct_group_ops;
179 struct configfs_attribute **ct_attrs;
180 };
182 The most basic function of a config_item_type is to define what
183 operations can be performed on a config_item. All items that have been
184 allocated dynamically will need to provide the ct_item_ops->release()
185 method. This method is called when the config_item's reference count
186 reaches zero. Items that wish to display an attribute need to provide
187 the ct_item_ops->show_attribute() method. Similarly, storing a new
188 attribute value uses the store_attribute() method.
190 [struct configfs_attribute]
192 struct configfs_attribute {
193 char *ca_name;
194 struct module *ca_owner;
195 mode_t ca_mode;
196 };
198 When a config_item wants an attribute to appear as a file in the item's
199 configfs directory, it must define a configfs_attribute describing it.
200 It then adds the attribute to the NULL-terminated array
201 config_item_type->ct_attrs. When the item appears in configfs, the
202 attribute file will appear with the configfs_attribute->ca_name
203 filename. configfs_attribute->ca_mode specifies the file permissions.
205 If an attribute is readable and the config_item provides a
206 ct_item_ops->show_attribute() method, that method will be called
207 whenever userspace asks for a read(2) on the attribute. The converse
208 will happen for write(2).
210 [struct config_group]
212 A config_item cannot live in a vacuum. The only way one can be created
213 is via mkdir(2) on a config_group. This will trigger creation of a
214 child item.
216 struct config_group {
217 struct config_item cg_item;
218 struct list_head cg_children;
219 struct configfs_subsystem *cg_subsys;
220 struct config_group **default_groups;
221 };
223 void config_group_init(struct config_group *group);
224 void config_group_init_type_name(struct config_group *group,
225 const char *name,
226 struct config_item_type *type);
229 The config_group structure contains a config_item. Properly configuring
230 that item means that a group can behave as an item in its own right.
231 However, it can do more: it can create child items or groups. This is
232 accomplished via the group operations specified on the group's
233 config_item_type.
235 struct configfs_group_operations {
236 struct config_item *(*make_item)(struct config_group *group,
237 const char *name);
238 struct config_group *(*make_group)(struct config_group *group,
239 const char *name);
240 int (*commit_item)(struct config_item *item);
241 void (*drop_item)(struct config_group *group,
242 struct config_item *item);
243 };
245 A group creates child items by providing the
246 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
247 config_item (or more likely, its container structure), initializes it,
248 and returns it to configfs. Configfs will then populate the filesystem
249 tree to reflect the new item.
251 If the subsystem wants the child to be a group itself, the subsystem
252 provides ct_group_ops->make_group(). Everything else behaves the same,
253 using the group _init() functions on the group.
255 Finally, when userspace calls rmdir(2) on the item or group,
256 ct_group_ops->drop_item() is called. As a config_group is also a
257 config_item, it is not necessary for a separate drop_group() method.
258 The subsystem must config_item_put() the reference that was initialized
259 upon item allocation. If a subsystem has no work to do, it may omit
260 the ct_group_ops->drop_item() method, and configfs will call
261 config_item_put() on the item on behalf of the subsystem.
263 IMPORTANT: drop_item() is void, and as such cannot fail. When rmdir(2)
264 is called, configfs WILL remove the item from the filesystem tree
265 (assuming that it has no children to keep it busy). The subsystem is
266 responsible for responding to this. If the subsystem has references to
267 the item in other threads, the memory is safe. It may take some time
268 for the item to actually disappear from the subsystem's usage. But it
269 is gone from configfs.
271 A config_group cannot be removed while it still has child items. This
272 is implemented in the configfs rmdir(2) code. ->drop_item() will not be
273 called, as the item has not been dropped. rmdir(2) will fail, as the
274 directory is not empty.
276 [struct configfs_subsystem]
278 A subsystem must register itself, usually at module_init time. This
279 tells configfs to make the subsystem appear in the file tree.
281 struct configfs_subsystem {
282 struct config_group su_group;
283 struct semaphore su_sem;
284 };
286 int configfs_register_subsystem(struct configfs_subsystem *subsys);
287 void configfs_unregister_subsystem(struct configfs_subsystem *subsys);
289 A subsystem consists of a toplevel config_group and a semaphore.
290 The group is where child config_items are created. For a subsystem,
291 this group is usually defined statically. Before calling
292 configfs_register_subsystem(), the subsystem must have initialized the
293 group via the usual group _init() functions, and it must also have
294 initialized the semaphore.
295 When the register call returns, the subsystem is live, and it
296 will be visible via configfs. At that point, mkdir(2) can be called and
297 the subsystem must be ready for it.
299 [An Example]
301 The best example of these basic concepts is the simple_children
302 subsystem/group and the simple_child item in configfs_example.c It
303 shows a trivial object displaying and storing an attribute, and a simple
304 group creating and destroying these children.
306 [Hierarchy Navigation and the Subsystem Semaphore]
308 There is an extra bonus that configfs provides. The config_groups and
309 config_items are arranged in a hierarchy due to the fact that they
310 appear in a filesystem. A subsystem is NEVER to touch the filesystem
311 parts, but the subsystem might be interested in this hierarchy. For
312 this reason, the hierarchy is mirrored via the config_group->cg_children
313 and config_item->ci_parent structure members.
315 A subsystem can navigate the cg_children list and the ci_parent pointer
316 to see the tree created by the subsystem. This can race with configfs'
317 management of the hierarchy, so configfs uses the subsystem semaphore to
318 protect modifications. Whenever a subsystem wants to navigate the
319 hierarchy, it must do so under the protection of the subsystem
320 semaphore.
322 A subsystem will be prevented from acquiring the semaphore while a newly
323 allocated item has not been linked into this hierarchy. Similarly, it
324 will not be able to acquire the semaphore while a dropping item has not
325 yet been unlinked. This means that an item's ci_parent pointer will
326 never be NULL while the item is in configfs, and that an item will only
327 be in its parent's cg_children list for the same duration. This allows
328 a subsystem to trust ci_parent and cg_children while they hold the
329 semaphore.
331 [Item Aggregation Via symlink(2)]
333 configfs provides a simple group via the group->item parent/child
334 relationship. Often, however, a larger environment requires aggregation
335 outside of the parent/child connection. This is implemented via
336 symlink(2).
338 A config_item may provide the ct_item_ops->allow_link() and
339 ct_item_ops->drop_link() methods. If the ->allow_link() method exists,
340 symlink(2) may be called with the config_item as the source of the link.
341 These links are only allowed between configfs config_items. Any
342 symlink(2) attempt outside the configfs filesystem will be denied.
344 When symlink(2) is called, the source config_item's ->allow_link()
345 method is called with itself and a target item. If the source item
346 allows linking to target item, it returns 0. A source item may wish to
347 reject a link if it only wants links to a certain type of object (say,
348 in its own subsystem).
350 When unlink(2) is called on the symbolic link, the source item is
351 notified via the ->drop_link() method. Like the ->drop_item() method,
352 this is a void function and cannot return failure. The subsystem is
353 responsible for responding to the change.
355 A config_item cannot be removed while it links to any other item, nor
356 can it be removed while an item links to it. Dangling symlinks are not
357 allowed in configfs.
359 [Automatically Created Subgroups]
361 A new config_group may want to have two types of child config_items.
362 While this could be codified by magic names in ->make_item(), it is much
363 more explicit to have a method whereby userspace sees this divergence.
365 Rather than have a group where some items behave differently than
366 others, configfs provides a method whereby one or many subgroups are
367 automatically created inside the parent at its creation. Thus,
368 mkdir("parent) results in "parent", "parent/subgroup1", up through
369 "parent/subgroupN". Items of type 1 can now be created in
370 "parent/subgroup1", and items of type N can be created in
371 "parent/subgroupN".
373 These automatic subgroups, or default groups, do not preclude other
374 children of the parent group. If ct_group_ops->make_group() exists,
375 other child groups can be created on the parent group directly.
377 A configfs subsystem specifies default groups by filling in the
378 NULL-terminated array default_groups on the config_group structure.
379 Each group in that array is populated in the configfs tree at the same
380 time as the parent group. Similarly, they are removed at the same time
381 as the parent. No extra notification is provided. When a ->drop_item()
382 method call notifies the subsystem the parent group is going away, it
383 also means every default group child associated with that parent group.
385 As a consequence of this, default_groups cannot be removed directly via
386 rmdir(2). They also are not considered when rmdir(2) on the parent
387 group is checking for children.
389 [Committable Items]
391 NOTE: Committable items are currently unimplemented.
393 Some config_items cannot have a valid initial state. That is, no
394 default values can be specified for the item's attributes such that the
395 item can do its work. Userspace must configure one or more attributes,
396 after which the subsystem can start whatever entity this item
397 represents.
399 Consider the FakeNBD device from above. Without a target address *and*
400 a target device, the subsystem has no idea what block device to import.
401 The simple example assumes that the subsystem merely waits until all the
402 appropriate attributes are configured, and then connects. This will,
403 indeed, work, but now every attribute store must check if the attributes
404 are initialized. Every attribute store must fire off the connection if
405 that condition is met.
407 Far better would be an explicit action notifying the subsystem that the
408 config_item is ready to go. More importantly, an explicit action allows
409 the subsystem to provide feedback as to whether the attributes are
410 initialized in a way that makes sense. configfs provides this as
411 committable items.
413 configfs still uses only normal filesystem operations. An item is
414 committed via rename(2). The item is moved from a directory where it
415 can be modified to a directory where it cannot.
417 Any group that provides the ct_group_ops->commit_item() method has
418 committable items. When this group appears in configfs, mkdir(2) will
419 not work directly in the group. Instead, the group will have two
420 subdirectories: "live" and "pending". The "live" directory does not
421 support mkdir(2) or rmdir(2) either. It only allows rename(2). The
422 "pending" directory does allow mkdir(2) and rmdir(2). An item is
423 created in the "pending" directory. Its attributes can be modified at
424 will. Userspace commits the item by renaming it into the "live"
425 directory. At this point, the subsystem receives the ->commit_item()
426 callback. If all required attributes are filled to satisfaction, the
427 method returns zero and the item is moved to the "live" directory.
429 As rmdir(2) does not work in the "live" directory, an item must be
430 shutdown, or "uncommitted". Again, this is done via rename(2), this
431 time from the "live" directory back to the "pending" one. The subsystem
432 is notified by the ct_group_ops->uncommit_object() method.