sysfs: Comment sysfs directory tagging logic

Add some in-line comments to explain the new infrastructure, which
was introduced to support sysfs directory tagging with namespaces.
I think an overall description someplace might be good too, but it
didn't really seem to fit into Documentation/filesystems/sysfs.txt,
which appears more geared toward users, rather than maintainers, of
sysfs.

(Tejun, please let me know if I can make anything clearer or failed
altogether to comment something that should be commented.)

Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

fs/sysfs/dir.c

@@ -614,6 +614,14 @@ int sysfs_create_subdir(struct kobject *kobj, const char *name,
 			  KOBJ_NS_TYPE_NONE, NULL, name, p_sd);
 }
 
+/**
+ *	sysfs_read_ns_type: return associated ns_type
+ *	@kobj: the kobject being queried
+ *
+ *	Each kobject can be tagged with exactly one namespace type
+ *	(i.e. network or user).  Return the ns_type associated with
+ *	this object if any
+ */
 static enum kobj_ns_type sysfs_read_ns_type(struct kobject *kobj)
 {
 	const struct kobj_ns_type_operations *ops;

fs/sysfs/sysfs.h

@@ -58,7 +58,7 @@ struct sysfs_dirent {
 	struct sysfs_dirent	*s_sibling;
 	const char		*s_name;
 
-	const void		*s_ns;
+	const void		*s_ns; /* namespace tag */
 	union {
 		struct sysfs_elem_dir		s_dir;
 		struct sysfs_elem_symlink	s_symlink;
@@ -82,6 +82,7 @@ struct sysfs_dirent {
 #define SYSFS_COPY_NAME			(SYSFS_DIR | SYSFS_KOBJ_LINK)
 #define SYSFS_ACTIVE_REF		(SYSFS_KOBJ_ATTR | SYSFS_KOBJ_BIN_ATTR)
 
+/* identify any namespace tag on sysfs_dirents */
 #define SYSFS_NS_TYPE_MASK		0xff00
 #define SYSFS_NS_TYPE_SHIFT		8
 
@@ -93,6 +94,10 @@ static inline unsigned int sysfs_type(struct sysfs_dirent *sd)
 	return sd->s_flags & SYSFS_TYPE_MASK;
 }
 
+/*
+ * Return any namespace tags on this dirent.
+ * enum kobj_ns_type is defined in linux/kobject.h
+ */
 static inline enum kobj_ns_type sysfs_ns_type(struct sysfs_dirent *sd)
 {
 	return (sd->s_flags & SYSFS_NS_TYPE_MASK) >> SYSFS_NS_TYPE_SHIFT;
@@ -123,6 +128,12 @@ struct sysfs_addrm_cxt {
 /*
  * mount.c
  */
+
+/*
+ * Each sb is associated with a set of namespace tags (i.e.
+ * the network namespace of the task which mounted this sysfs
+ * instance).
+ */
 struct sysfs_super_info {
 	const void *ns[KOBJ_NS_TYPES];
 };

include/linux/kobject.h

@@ -136,12 +136,23 @@ struct kobj_attribute {
 
 extern const struct sysfs_ops kobj_sysfs_ops;
 
+/*
+ * Namespace types which are used to tag kobjects and sysfs entries.
+ * Network namespace will likely be the first.
+ */
 enum kobj_ns_type {
 	KOBJ_NS_TYPE_NONE = 0,
 	KOBJ_NS_TYPES
 };
 
 struct sock;
+
+/*
+ * Callbacks so sysfs can determine namespaces
+ *   @current_ns: return calling task's namespace
+ *   @netlink_ns: return namespace to which a sock belongs (right?)
+ *   @initial_ns: return the initial namespace (i.e. init_net_ns)
+ */
 struct kobj_ns_type_operations {
 	enum kobj_ns_type type;
 	const void *(*current_ns)(void);

include/linux/sysfs.h

@@ -178,6 +178,7 @@ struct sysfs_dirent *sysfs_get(struct sysfs_dirent *sd);
 void sysfs_put(struct sysfs_dirent *sd);
 void sysfs_printk_last_file(void);
 
+/* Called to clear a ns tag when it is no longer valid */
 void sysfs_exit_ns(enum kobj_ns_type type, const void *tag);
 
 int __must_check sysfs_init(void);

lib/kobject.c

@@ -948,6 +948,17 @@ const void *kobj_ns_initial(enum kobj_ns_type type)
 	return ns;
 }
 
+/*
+ * kobj_ns_exit - invalidate a namespace tag
+ *
+ * @type: the namespace type (i.e. KOBJ_NS_TYPE_NET)
+ * @ns: the actual namespace being invalidated
+ *
+ * This is called when a tag is no longer valid.  For instance,
+ * when a network namespace exits, it uses this helper to
+ * make sure no sb's sysfs_info points to the now-invalidated
+ * netns.
+ */
 void kobj_ns_exit(enum kobj_ns_type type, const void *ns)
 {
 	sysfs_exit_ns(type, ns);