Documentation/filesystems/configfs/configfs.txt

   1
   2 configfs - Userspace-driven kernel object configuration.
   3
   4 Joel Becker <joel.becker@oracle.com>
   5
   6 Updated: 31 March 2005
   7
   8 Copyright (c) 2005 Oracle Corporation,
   9         Joel Becker <joel.becker@oracle.com>
  10
  11
  12 [What is configfs?]
  13
  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.
  18
  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.
  26
  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.
  34
  35 Both sysfs and configfs can and should exist together on the same
  36 system.  One is not a replacement for the other.
  37
  38 [Using configfs]
  39
  40 configfs can be compiled as a module or into the kernel.  You can access
  41 it by doing
  42
  43         mount -t configfs none /config
  44
  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.
  50
  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.  Don't mix more than one attribute in one attribute file.
  55
  56 There are two types of configfs attributes:
  57
  58 * Normal attributes, which similar to sysfs attributes, are small ASCII text
  59 files, with a maximum size of one page (PAGE_SIZE, 4096 on i386).  Preferably
  60 only one value per file should be used, and the same caveats from sysfs apply.
  61 Configfs expects write(2) to store the entire buffer at once.  When writing to
  62 normal configfs attributes, userspace processes should first read the entire
  63 file, modify the portions they wish to change, and then write the entire
  64 buffer back.
  65
  66 * Binary attributes, which are somewhat similar to sysfs binary attributes,
  67 but with a few slight changes to semantics.  The PAGE_SIZE limitation does not
  68 apply, but the whole binary item must fit in single kernel vmalloc'ed buffer.
  69 The write(2) calls from user space are buffered, and the attributes'
  70 write_bin_attribute method will be invoked on the final close, therefore it is
  71 imperative for user-space to check the return code of close(2) in order to
  72 verify that the operation finished successfully.
  73 To avoid a malicious user OOMing the kernel, there's a per-binary attribute
  74 maximum buffer value.
  75
  76 When an item needs to be destroyed, remove it with rmdir(2).  An
  77 item cannot be destroyed if any other item has a link to it (via
  78 symlink(2)).  Links can be removed via unlink(2).
  79
  80 [Configuring FakeNBD: an Example]
  81
  82 Imagine there's a Network Block Device (NBD) driver that allows you to
  83 access remote block devices.  Call it FakeNBD.  FakeNBD uses configfs
  84 for its configuration.  Obviously, there will be a nice program that
  85 sysadmins use to configure FakeNBD, but somehow that program has to tell
  86 the driver about it.  Here's where configfs comes in.
  87
  88 When the FakeNBD driver is loaded, it registers itself with configfs.
  89 readdir(3) sees this just fine:
  90
  91         # ls /config
  92         fakenbd
  93
  94 A fakenbd connection can be created with mkdir(2).  The name is
  95 arbitrary, but likely the tool will make some use of the name.  Perhaps
  96 it is a uuid or a disk name:
  97
  98         # mkdir /config/fakenbd/disk1
  99         # ls /config/fakenbd/disk1
 100         target device rw
 101
 102 The target attribute contains the IP address of the server FakeNBD will
 103 connect to.  The device attribute is the device on the server.
 104 Predictably, the rw attribute determines whether the connection is
 105 read-only or read-write.
 106
 107         # echo 10.0.0.1 > /config/fakenbd/disk1/target
 108         # echo /dev/sda1 > /config/fakenbd/disk1/device
 109         # echo 1 > /config/fakenbd/disk1/rw
 110
 111 That's it.  That's all there is.  Now the device is configured, via the
 112 shell no less.
 113
 114 [Coding With configfs]
 115
 116 Every object in configfs is a config_item.  A config_item reflects an
 117 object in the subsystem.  It has attributes that match values on that
 118 object.  configfs handles the filesystem representation of that object
 119 and its attributes, allowing the subsystem to ignore all but the
 120 basic show/store interaction.
 121
 122 Items are created and destroyed inside a config_group.  A group is a
 123 collection of items that share the same attributes and operations.
 124 Items are created by mkdir(2) and removed by rmdir(2), but configfs
 125 handles that.  The group has a set of operations to perform these tasks
 126
 127 A subsystem is the top level of a client module.  During initialization,
 128 the client module registers the subsystem with configfs, the subsystem
 129 appears as a directory at the top of the configfs filesystem.  A
 130 subsystem is also a config_group, and can do everything a config_group
 131 can.
 132
 133 [struct config_item]
 134
 135         struct config_item {
 136                 char                    *ci_name;
 137                 char                    ci_namebuf[UOBJ_NAME_LEN];
 138                 struct kref             ci_kref;
 139                 struct list_head        ci_entry;
 140                 struct config_item      *ci_parent;
 141                 struct config_group     *ci_group;
 142                 struct config_item_type *ci_type;
 143                 struct dentry           *ci_dentry;
 144         };
 145
 146         void config_item_init(struct config_item *);
 147         void config_item_init_type_name(struct config_item *,
 148                                         const char *name,
 149                                         struct config_item_type *type);
 150         struct config_item *config_item_get(struct config_item *);
 151         void config_item_put(struct config_item *);
 152
 153 Generally, struct config_item is embedded in a container structure, a
 154 structure that actually represents what the subsystem is doing.  The
 155 config_item portion of that structure is how the object interacts with
 156 configfs.
 157
 158 Whether statically defined in a source file or created by a parent
 159 config_group, a config_item must have one of the _init() functions
 160 called on it.  This initializes the reference count and sets up the
 161 appropriate fields.
 162
 163 All users of a config_item should have a reference on it via
 164 config_item_get(), and drop the reference when they are done via
 165 config_item_put().
 166
 167 By itself, a config_item cannot do much more than appear in configfs.
 168 Usually a subsystem wants the item to display and/or store attributes,
 169 among other things.  For that, it needs a type.
 170
 171 [struct config_item_type]
 172
 173         struct configfs_item_operations {
 174                 void (*release)(struct config_item *);
 175                 int (*allow_link)(struct config_item *src,
 176                                   struct config_item *target);
 177                 int (*drop_link)(struct config_item *src,
 178                                  struct config_item *target);
 179         };
 180
 181         struct config_item_type {
 182                 struct module                           *ct_owner;
 183                 struct configfs_item_operations         *ct_item_ops;
 184                 struct configfs_group_operations        *ct_group_ops;
 185                 struct configfs_attribute               **ct_attrs;
 186                 struct configfs_bin_attribute           **ct_bin_attrs;
 187         };
 188
 189 The most basic function of a config_item_type is to define what
 190 operations can be performed on a config_item.  All items that have been
 191 allocated dynamically will need to provide the ct_item_ops->release()
 192 method.  This method is called when the config_item's reference count
 193 reaches zero.
 194
 195 [struct configfs_attribute]
 196
 197         struct configfs_attribute {
 198                 char                    *ca_name;
 199                 struct module           *ca_owner;
 200                 umode_t                  ca_mode;
 201                 ssize_t (*show)(struct config_item *, char *);
 202                 ssize_t (*store)(struct config_item *, const char *, size_t);
 203         };
 204
 205 When a config_item wants an attribute to appear as a file in the item's
 206 configfs directory, it must define a configfs_attribute describing it.
 207 It then adds the attribute to the NULL-terminated array
 208 config_item_type->ct_attrs.  When the item appears in configfs, the
 209 attribute file will appear with the configfs_attribute->ca_name
 210 filename.  configfs_attribute->ca_mode specifies the file permissions.
 211
 212 If an attribute is readable and provides a ->show method, that method will
 213 be called whenever userspace asks for a read(2) on the attribute.  If an
 214 attribute is writable and provides a ->store  method, that method will be
 215 be called whenever userspace asks for a write(2) on the attribute.
 216
 217 [struct configfs_bin_attribute]
 218
 219         struct configfs_attribute {
 220                 struct configfs_attribute       cb_attr;
 221                 void                            *cb_private;
 222                 size_t                          cb_max_size;
 223         };
 224
 225 The binary attribute is used when the one needs to use binary blob to
 226 appear as the contents of a file in the item's configfs directory.
 227 To do so add the binary attribute to the NULL-terminated array
 228 config_item_type->ct_bin_attrs, and the item appears in configfs, the
 229 attribute file will appear with the configfs_bin_attribute->cb_attr.ca_name
 230 filename.  configfs_bin_attribute->cb_attr.ca_mode specifies the file
 231 permissions.
 232 The cb_private member is provided for use by the driver, while the
 233 cb_max_size member specifies the maximum amount of vmalloc buffer
 234 to be used.
 235
 236 If binary attribute is readable and the config_item provides a
 237 ct_item_ops->read_bin_attribute() method, that method will be called
 238 whenever userspace asks for a read(2) on the attribute.  The converse
 239 will happen for write(2). The reads/writes are bufferred so only a
 240 single read/write will occur; the attributes' need not concern itself
 241 with it.
 242
 243 [struct config_group]
 244
 245 A config_item cannot live in a vacuum.  The only way one can be created
 246 is via mkdir(2) on a config_group.  This will trigger creation of a
 247 child item.
 248
 249         struct config_group {
 250                 struct config_item              cg_item;
 251                 struct list_head                cg_children;
 252                 struct configfs_subsystem       *cg_subsys;
 253                 struct config_group             **default_groups;
 254         };
 255
 256         void config_group_init(struct config_group *group);
 257         void config_group_init_type_name(struct config_group *group,
 258                                          const char *name,
 259                                          struct config_item_type *type);
 260
 261
 262 The config_group structure contains a config_item.  Properly configuring
 263 that item means that a group can behave as an item in its own right.
 264 However, it can do more: it can create child items or groups.  This is
 265 accomplished via the group operations specified on the group's
 266 config_item_type.
 267
 268         struct configfs_group_operations {
 269                 struct config_item *(*make_item)(struct config_group *group,
 270                                                  const char *name);
 271                 struct config_group *(*make_group)(struct config_group *group,
 272                                                    const char *name);
 273                 int (*commit_item)(struct config_item *item);
 274                 void (*disconnect_notify)(struct config_group *group,
 275                                           struct config_item *item);
 276                 void (*drop_item)(struct config_group *group,
 277                                   struct config_item *item);
 278         };
 279
 280 A group creates child items by providing the
 281 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
 282 config_item (or more likely, its container structure), initializes it,
 283 and returns it to configfs.  Configfs will then populate the filesystem
 284 tree to reflect the new item.
 285
 286 If the subsystem wants the child to be a group itself, the subsystem
 287 provides ct_group_ops->make_group().  Everything else behaves the same,
 288 using the group _init() functions on the group.
 289
 290 Finally, when userspace calls rmdir(2) on the item or group,
 291 ct_group_ops->drop_item() is called.  As a config_group is also a
 292 config_item, it is not necessary for a separate drop_group() method.
 293 The subsystem must config_item_put() the reference that was initialized
 294 upon item allocation.  If a subsystem has no work to do, it may omit
 295 the ct_group_ops->drop_item() method, and configfs will call
 296 config_item_put() on the item on behalf of the subsystem.
 297
 298 IMPORTANT: drop_item() is void, and as such cannot fail.  When rmdir(2)
 299 is called, configfs WILL remove the item from the filesystem tree
 300 (assuming that it has no children to keep it busy).  The subsystem is
 301 responsible for responding to this.  If the subsystem has references to
 302 the item in other threads, the memory is safe.  It may take some time
 303 for the item to actually disappear from the subsystem's usage.  But it
 304 is gone from configfs.
 305
 306 When drop_item() is called, the item's linkage has already been torn
 307 down.  It no longer has a reference on its parent and has no place in
 308 the item hierarchy.  If a client needs to do some cleanup before this
 309 teardown happens, the subsystem can implement the
 310 ct_group_ops->disconnect_notify() method.  The method is called after
 311 configfs has removed the item from the filesystem view but before the
 312 item is removed from its parent group.  Like drop_item(),
 313 disconnect_notify() is void and cannot fail.  Client subsystems should
 314 not drop any references here, as they still must do it in drop_item().
 315
 316 A config_group cannot be removed while it still has child items.  This
 317 is implemented in the configfs rmdir(2) code.  ->drop_item() will not be
 318 called, as the item has not been dropped.  rmdir(2) will fail, as the
 319 directory is not empty.
 320
 321 [struct configfs_subsystem]
 322
 323 A subsystem must register itself, usually at module_init time.  This
 324 tells configfs to make the subsystem appear in the file tree.
 325
 326         struct configfs_subsystem {
 327                 struct config_group     su_group;
 328                 struct mutex            su_mutex;
 329         };
 330
 331         int configfs_register_subsystem(struct configfs_subsystem *subsys);
 332         void configfs_unregister_subsystem(struct configfs_subsystem *subsys);
 333
 334         A subsystem consists of a toplevel config_group and a mutex.
 335 The group is where child config_items are created.  For a subsystem,
 336 this group is usually defined statically.  Before calling
 337 configfs_register_subsystem(), the subsystem must have initialized the
 338 group via the usual group _init() functions, and it must also have
 339 initialized the mutex.
 340         When the register call returns, the subsystem is live, and it
 341 will be visible via configfs.  At that point, mkdir(2) can be called and
 342 the subsystem must be ready for it.
 343
 344 [An Example]
 345
 346 The best example of these basic concepts is the simple_children
 347 subsystem/group and the simple_child item in
 348 samples/configfs/configfs_sample.c. It shows a trivial object displaying
 349 and storing an attribute, and a simple group creating and destroying
 350 these children.
 351
 352 [Hierarchy Navigation and the Subsystem Mutex]
 353
 354 There is an extra bonus that configfs provides.  The config_groups and
 355 config_items are arranged in a hierarchy due to the fact that they
 356 appear in a filesystem.  A subsystem is NEVER to touch the filesystem
 357 parts, but the subsystem might be interested in this hierarchy.  For
 358 this reason, the hierarchy is mirrored via the config_group->cg_children
 359 and config_item->ci_parent structure members.
 360
 361 A subsystem can navigate the cg_children list and the ci_parent pointer
 362 to see the tree created by the subsystem.  This can race with configfs'
 363 management of the hierarchy, so configfs uses the subsystem mutex to
 364 protect modifications.  Whenever a subsystem wants to navigate the
 365 hierarchy, it must do so under the protection of the subsystem
 366 mutex.
 367
 368 A subsystem will be prevented from acquiring the mutex while a newly
 369 allocated item has not been linked into this hierarchy.   Similarly, it
 370 will not be able to acquire the mutex while a dropping item has not
 371 yet been unlinked.  This means that an item's ci_parent pointer will
 372 never be NULL while the item is in configfs, and that an item will only
 373 be in its parent's cg_children list for the same duration.  This allows
 374 a subsystem to trust ci_parent and cg_children while they hold the
 375 mutex.
 376
 377 [Item Aggregation Via symlink(2)]
 378
 379 configfs provides a simple group via the group->item parent/child
 380 relationship.  Often, however, a larger environment requires aggregation
 381 outside of the parent/child connection.  This is implemented via
 382 symlink(2).
 383
 384 A config_item may provide the ct_item_ops->allow_link() and
 385 ct_item_ops->drop_link() methods.  If the ->allow_link() method exists,
 386 symlink(2) may be called with the config_item as the source of the link.
 387 These links are only allowed between configfs config_items.  Any
 388 symlink(2) attempt outside the configfs filesystem will be denied.
 389
 390 When symlink(2) is called, the source config_item's ->allow_link()
 391 method is called with itself and a target item.  If the source item
 392 allows linking to target item, it returns 0.  A source item may wish to
 393 reject a link if it only wants links to a certain type of object (say,
 394 in its own subsystem).
 395
 396 When unlink(2) is called on the symbolic link, the source item is
 397 notified via the ->drop_link() method.  Like the ->drop_item() method,
 398 this is a void function and cannot return failure.  The subsystem is
 399 responsible for responding to the change.
 400
 401 A config_item cannot be removed while it links to any other item, nor
 402 can it be removed while an item links to it.  Dangling symlinks are not
 403 allowed in configfs.
 404
 405 [Automatically Created Subgroups]
 406
 407 A new config_group may want to have two types of child config_items.
 408 While this could be codified by magic names in ->make_item(), it is much
 409 more explicit to have a method whereby userspace sees this divergence.
 410
 411 Rather than have a group where some items behave differently than
 412 others, configfs provides a method whereby one or many subgroups are
 413 automatically created inside the parent at its creation.  Thus,
 414 mkdir("parent") results in "parent", "parent/subgroup1", up through
 415 "parent/subgroupN".  Items of type 1 can now be created in
 416 "parent/subgroup1", and items of type N can be created in
 417 "parent/subgroupN".
 418
 419 These automatic subgroups, or default groups, do not preclude other
 420 children of the parent group.  If ct_group_ops->make_group() exists,
 421 other child groups can be created on the parent group directly.
 422
 423 A configfs subsystem specifies default groups by filling in the
 424 NULL-terminated array default_groups on the config_group structure.
 425 Each group in that array is populated in the configfs tree at the same
 426 time as the parent group.  Similarly, they are removed at the same time
 427 as the parent.  No extra notification is provided.  When a ->drop_item()
 428 method call notifies the subsystem the parent group is going away, it
 429 also means every default group child associated with that parent group.
 430
 431 As a consequence of this, default_groups cannot be removed directly via
 432 rmdir(2).  They also are not considered when rmdir(2) on the parent
 433 group is checking for children.
 434
 435 [Dependent Subsystems]
 436
 437 Sometimes other drivers depend on particular configfs items.  For
 438 example, ocfs2 mounts depend on a heartbeat region item.  If that
 439 region item is removed with rmdir(2), the ocfs2 mount must BUG or go
 440 readonly.  Not happy.
 441
 442 configfs provides two additional API calls: configfs_depend_item() and
 443 configfs_undepend_item().  A client driver can call
 444 configfs_depend_item() on an existing item to tell configfs that it is
 445 depended on.  configfs will then return -EBUSY from rmdir(2) for that
 446 item.  When the item is no longer depended on, the client driver calls
 447 configfs_undepend_item() on it.
 448
 449 These API cannot be called underneath any configfs callbacks, as
 450 they will conflict.  They can block and allocate.  A client driver
 451 probably shouldn't calling them of its own gumption.  Rather it should
 452 be providing an API that external subsystems call.
 453
 454 How does this work?  Imagine the ocfs2 mount process.  When it mounts,
 455 it asks for a heartbeat region item.  This is done via a call into the
 456 heartbeat code.  Inside the heartbeat code, the region item is looked
 457 up.  Here, the heartbeat code calls configfs_depend_item().  If it
 458 succeeds, then heartbeat knows the region is safe to give to ocfs2.
 459 If it fails, it was being torn down anyway, and heartbeat can gracefully
 460 pass up an error.
 461
 462 [Committable Items]
 463
 464 NOTE: Committable items are currently unimplemented.
 465
 466 Some config_items cannot have a valid initial state.  That is, no
 467 default values can be specified for the item's attributes such that the
 468 item can do its work.  Userspace must configure one or more attributes,
 469 after which the subsystem can start whatever entity this item
 470 represents.
 471
 472 Consider the FakeNBD device from above.  Without a target address *and*
 473 a target device, the subsystem has no idea what block device to import.
 474 The simple example assumes that the subsystem merely waits until all the
 475 appropriate attributes are configured, and then connects.  This will,
 476 indeed, work, but now every attribute store must check if the attributes
 477 are initialized.  Every attribute store must fire off the connection if
 478 that condition is met.
 479
 480 Far better would be an explicit action notifying the subsystem that the
 481 config_item is ready to go.  More importantly, an explicit action allows
 482 the subsystem to provide feedback as to whether the attributes are
 483 initialized in a way that makes sense.  configfs provides this as
 484 committable items.
 485
 486 configfs still uses only normal filesystem operations.  An item is
 487 committed via rename(2).  The item is moved from a directory where it
 488 can be modified to a directory where it cannot.
 489
 490 Any group that provides the ct_group_ops->commit_item() method has
 491 committable items.  When this group appears in configfs, mkdir(2) will
 492 not work directly in the group.  Instead, the group will have two
 493 subdirectories: "live" and "pending".  The "live" directory does not
 494 support mkdir(2) or rmdir(2) either.  It only allows rename(2).  The
 495 "pending" directory does allow mkdir(2) and rmdir(2).  An item is
 496 created in the "pending" directory.  Its attributes can be modified at
 497 will.  Userspace commits the item by renaming it into the "live"
 498 directory.  At this point, the subsystem receives the ->commit_item()
 499 callback.  If all required attributes are filled to satisfaction, the
 500 method returns zero and the item is moved to the "live" directory.
 501
 502 As rmdir(2) does not work in the "live" directory, an item must be
 503 shutdown, or "uncommitted".  Again, this is done via rename(2), this
 504 time from the "live" directory back to the "pending" one.  The subsystem
 505 is notified by the ct_group_ops->uncommit_object() method.
 506
 507