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16 configfs is a ram-based filesystem that provides the converse of
17 sysfs's functionality. Where sysfs is a filesystem-based view of
18 kernel objects, configfs is a filesystem-based manager of kernel
21 With sysfs, an object is created in kernel (for example, when a device
24 readdir(3)/read(2). It may allow some attributes to be modified via
25 write(2). The important point is that the object is created and
27 representation, and sysfs is merely a window on all this.
29 A configfs config_item is created via an explicit userspace operation:
30 mkdir(2). It is destroyed via rmdir(2). The attributes appear at
31 mkdir(2) time, and can be read or modified via read(2) and write(2).
33 symlink(2) can be used to group items together. Unlike sysfs, the
38 system. One is not a replacement for the other.
43 configfs can be compiled as a module or into the kernel. You can access
50 subsystems. Once a client subsystem is loaded, it will appear as a
54 An item is created via mkdir(2). The item's attributes will also
56 read(2) can query their default values, and write(2) can store new
62 files, with a maximum size of one page (PAGE_SIZE, 4096 on i386). Preferably
64 Configfs expects write(2) to store the entire buffer at once. When writing to
70 but with a few slight changes to semantics. The PAGE_SIZE limitation does not
72 The write(2) calls from user space are buffered, and the attributes'
74 imperative for user-space to check the return code of close(2) in order to
76 To avoid a malicious user OOMing the kernel, there's a per-binary attribute
79 When an item needs to be destroyed, remove it with rmdir(2). An
80 item cannot be destroyed if any other item has a link to it (via
81 symlink(2)). Links can be removed via unlink(2).
86 Imagine there's a Network Block Device (NBD) driver that allows you to
88 for its configuration. Obviously, there will be a nice program that
98 A fakenbd connection can be created with mkdir(2). The name is
100 it is a uuid or a disk name::
121 Every object in configfs is a config_item. A config_item reflects an
127 Items are created and destroyed inside a config_group. A group is a
129 Items are created by mkdir(2) and removed by rmdir(2), but configfs
130 handles that. The group has a set of operations to perform these tasks
132 A subsystem is the top level of a client module. During initialization,
134 appears as a directory at the top of the configfs filesystem. A
135 subsystem is also a config_group, and can do everything a config_group
161 Generally, struct config_item is embedded in a container structure, a
166 Whether statically defined in a source file or created by a parent
167 config_group, a config_item must have one of the _init() functions
171 All users of a config_item should have a reference on it via
175 By itself, a config_item cannot do much more than appear in configfs.
176 Usually a subsystem wants the item to display and/or store attributes,
177 among other things. For that, it needs a type.
200 The most basic function of a config_item_type is to define what
201 operations can be performed on a config_item. All items that have been
219 When a config_item wants an attribute to appear as a file in the item's
220 configfs directory, it must define a configfs_attribute describing it.
226 If an attribute is readable and provides a ->show method, that method will
227 be called whenever userspace asks for a read(2) on the attribute. If an
228 attribute is writable and provides a ->store method, that method will be
229 called whenever userspace asks for a write(2) on the attribute.
243 appear as the contents of a file in the item's configfs directory.
253 If binary attribute is readable and the config_item provides a
255 whenever userspace asks for a read(2) on the attribute. The converse
256 will happen for write(2). The reads/writes are bufferred so only a
263 A config_item cannot live in a vacuum. The only way one can be created
264 is via mkdir(2) on a config_group. This will trigger creation of a
281 The config_group structure contains a config_item. Properly configuring
282 that item means that a group can behave as an item in its own right.
299 A group creates child items by providing the
301 mkdir(2) in the group's directory. The subsystem allocates a new
306 If the subsystem wants the child to be a group itself, the subsystem
310 Finally, when userspace calls rmdir(2) on the item or group,
311 ct_group_ops->drop_item() is called. As a config_group is also a
312 config_item, it is not necessary for a separate drop_group() method.
314 upon item allocation. If a subsystem has no work to do, it may omit
319 drop_item() is void, and as such cannot fail. When rmdir(2)
328 down. It no longer has a reference on its parent and has no place in
329 the item hierarchy. If a client needs to do some cleanup before this
337 A config_group cannot be removed while it still has child items. This
338 is implemented in the configfs rmdir(2) code. ->drop_item() will not be
339 called, as the item has not been dropped. rmdir(2) will fail, as the
345 A subsystem must register itself, usually at module_init time. This
356 A subsystem consists of a toplevel config_group and a mutex.
357 The group is where child config_items are created. For a subsystem,
364 will be visible via configfs. At that point, mkdir(2) can be called and
372 samples/configfs/configfs_sample.c. It shows a trivial object displaying
373 and storing an attribute, and a simple group creating and destroying
380 config_items are arranged in a hierarchy due to the fact that they
381 appear in a filesystem. A subsystem is NEVER to touch the filesystem
386 A subsystem can navigate the cg_children list and the ci_parent pointer
389 protect modifications. Whenever a subsystem wants to navigate the
393 A subsystem will be prevented from acquiring the mutex while a newly
395 will not be able to acquire the mutex while a dropping item has not
399 a subsystem to trust ci_parent and cg_children while they hold the
402 Item Aggregation Via symlink(2)
405 configfs provides a simple group via the group->item parent/child
406 relationship. Often, however, a larger environment requires aggregation
408 symlink(2).
410 A config_item may provide the ct_item_ops->allow_link() and
412 symlink(2) may be called with the config_item as the source of the link.
414 symlink(2) attempt outside the configfs filesystem will be denied.
416 When symlink(2) is called, the source config_item's ->allow_link()
417 method is called with itself and a target item. If the source item
418 allows linking to target item, it returns 0. A source item may wish to
419 reject a link if it only wants links to a certain type of object (say,
422 When unlink(2) is called on the symbolic link, the source item is
424 this is a void function and cannot return failure. The subsystem is
427 A config_item cannot be removed while it links to any other item, nor
434 A new config_group may want to have two types of child config_items.
436 more explicit to have a method whereby userspace sees this divergence.
438 Rather than have a group where some items behave differently than
439 others, configfs provides a method whereby one or many subgroups are
450 A configfs subsystem specifies default groups by adding them using the
454 as the parent. No extra notification is provided. When a ->drop_item()
458 As a consequence of this, default groups cannot be removed directly via
459 rmdir(2). They also are not considered when rmdir(2) on the parent
466 example, ocfs2 mounts depend on a heartbeat region item. If that
467 region item is removed with rmdir(2), the ocfs2 mount must BUG or go
471 configfs_undepend_item(). A client driver can call
473 depended on. configfs will then return -EBUSY from rmdir(2) for that
478 they will conflict. They can block and allocate. A client driver
483 it asks for a heartbeat region item. This is done via a call into the
496 Some config_items cannot have a valid initial state. That is, no
502 Consider the FakeNBD device from above. Without a target address *and*
503 a target device, the subsystem has no idea what block device to import.
513 initialized in a way that makes sense. configfs provides this as
517 committed via rename(2). The item is moved from a directory where it
518 can be modified to a directory where it cannot.
521 committable items. When this group appears in configfs, mkdir(2) will
524 support mkdir(2) or rmdir(2) either. It only allows rename(2). The
525 "pending" directory does allow mkdir(2) and rmdir(2). An item is
532 As rmdir(2) does not work in the "live" directory, an item must be
533 shutdown, or "uncommitted". Again, this is done via rename(2), this