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diff --git a/Documentation/filesystems/configfs/configfs.txt b/Documentation/filesystems/configfs/configfs.txt new file mode 100644 index 000000000000..c4ff96b7c4e0 --- /dev/null +++ b/Documentation/filesystems/configfs/configfs.txt @@ -0,0 +1,434 @@ + +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. + + |