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diff --git a/Documentation/filesystems/configfs/configfs.txt b/Documentation/filesystems/configfs/configfs.txt deleted file mode 100644 index 16e606c11f40..000000000000 --- a/Documentation/filesystems/configfs/configfs.txt +++ /dev/null @@ -1,508 +0,0 @@ - -configfs - Userspace-driven kernel object configuration. - -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. Don't mix more than one attribute in one attribute file. - -There are two types of configfs attributes: - -* Normal attributes, which similar to sysfs attributes, are small ASCII text -files, with a maximum size of one page (PAGE_SIZE, 4096 on i386). Preferably -only one value per file should be used, and the same caveats from sysfs apply. -Configfs expects write(2) to store the entire buffer at once. When writing to -normal configfs attributes, userspace processes should first read the entire -file, modify the portions they wish to change, and then write the entire -buffer back. - -* Binary attributes, which are somewhat similar to sysfs binary attributes, -but with a few slight changes to semantics. The PAGE_SIZE limitation does not -apply, but the whole binary item must fit in single kernel vmalloc'ed buffer. -The write(2) calls from user space are buffered, and the attributes' -write_bin_attribute method will be invoked on the final close, therefore it is -imperative for user-space to check the return code of close(2) in order to -verify that the operation finished successfully. -To avoid a malicious user OOMing the kernel, there's a per-binary attribute -maximum buffer value. - -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 *); - int (*allow_link)(struct config_item *src, - struct config_item *target); - void (*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; - struct configfs_bin_attribute **ct_bin_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. - -[struct configfs_attribute] - - struct configfs_attribute { - char *ca_name; - struct module *ca_owner; - umode_t ca_mode; - ssize_t (*show)(struct config_item *, char *); - ssize_t (*store)(struct config_item *, const char *, size_t); - }; - -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 provides a ->show method, that method will -be called whenever userspace asks for a read(2) on the attribute. If an -attribute is writable and provides a ->store method, that method will be -be called whenever userspace asks for a write(2) on the attribute. - -[struct configfs_bin_attribute] - - struct configfs_bin_attribute { - struct configfs_attribute cb_attr; - void *cb_private; - size_t cb_max_size; - }; - -The binary attribute is used when the one needs to use binary blob to -appear as the contents of a file in the item's configfs directory. -To do so add the binary attribute to the NULL-terminated array -config_item_type->ct_bin_attrs, and the item appears in configfs, the -attribute file will appear with the configfs_bin_attribute->cb_attr.ca_name -filename. configfs_bin_attribute->cb_attr.ca_mode specifies the file -permissions. -The cb_private member is provided for use by the driver, while the -cb_max_size member specifies the maximum amount of vmalloc buffer -to be used. - -If binary attribute is readable and the config_item provides a -ct_item_ops->read_bin_attribute() method, that method will be called -whenever userspace asks for a read(2) on the attribute. The converse -will happen for write(2). The reads/writes are bufferred so only a -single read/write will occur; the attributes' need not concern itself -with it. - -[struct config_group] - -A config_item cannot live in a vacuum. 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 list_head default_groups; - struct list_head group_entry; - }; - - 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 (*disconnect_notify)(struct config_group *group, - 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 separate 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. - -When drop_item() is called, the item's linkage has already been torn -down. It no longer has a reference on its parent and has no place in -the item hierarchy. If a client needs to do some cleanup before this -teardown happens, the subsystem can implement the -ct_group_ops->disconnect_notify() method. The method is called after -configfs has removed the item from the filesystem view but before the -item is removed from its parent group. Like drop_item(), -disconnect_notify() is void and cannot fail. Client subsystems should -not drop any references here, as they still must do it in drop_item(). - -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, usually 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 mutex su_mutex; - }; - - 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 mutex. -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 mutex. - 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 -samples/configfs/configfs_sample.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 Mutex] - -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 mutex to -protect modifications. Whenever a subsystem wants to navigate the -hierarchy, it must do so under the protection of the subsystem -mutex. - -A subsystem will be prevented from acquiring the mutex while a newly -allocated item has not been linked into this hierarchy. Similarly, it -will not be able to acquire the mutex 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 -mutex. - -[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 adding them using the -configfs_add_default_group() function to the parent config_group -structure. Each added group 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. - -[Dependent Subsystems] - -Sometimes other drivers depend on particular configfs items. For -example, ocfs2 mounts depend on a heartbeat region item. If that -region item is removed with rmdir(2), the ocfs2 mount must BUG or go -readonly. Not happy. - -configfs provides two additional API calls: configfs_depend_item() and -configfs_undepend_item(). A client driver can call -configfs_depend_item() on an existing item to tell configfs that it is -depended on. configfs will then return -EBUSY from rmdir(2) for that -item. When the item is no longer depended on, the client driver calls -configfs_undepend_item() on it. - -These API cannot be called underneath any configfs callbacks, as -they will conflict. They can block and allocate. A client driver -probably shouldn't calling them of its own gumption. Rather it should -be providing an API that external subsystems call. - -How does this work? Imagine the ocfs2 mount process. When it mounts, -it asks for a heartbeat region item. This is done via a call into the -heartbeat code. Inside the heartbeat code, the region item is looked -up. Here, the heartbeat code calls configfs_depend_item(). If it -succeeds, then heartbeat knows the region is safe to give to ocfs2. -If it fails, it was being torn down anyway, and heartbeat can gracefully -pass up an error. - -[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 attributes 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 receives 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. - - |