sysfs, kernfs: implement kernfs_create/destroy_root()

There currently is single kernfs hierarchy in the whole system which
is used for sysfs.  kernfs needs to support multiple hierarchies to
allow other users.  This patch introduces struct kernfs_root which
serves as the root of each kernfs hierarchy and implements
kernfs_create/destroy_root().

* Each kernfs_root is associated with a root sd (sysfs_dentry).  The
  root is freed when the root sd is released and kernfs_destory_root()
  simply invokes kernfs_remove() on the root sd.  sysfs_remove_one()
  is updated to handle release of the root sd.  Note that ps_iattr
  update in sysfs_remove_one() is trivially updated for readability.

* Root sd's are now dynamically allocated using sysfs_new_dirent().
  Update sysfs_alloc_ino() so that it gives out ino from 1 so that the
  root sd still gets ino 1.

* While kernfs currently only points to the root sd, it'll soon grow
  fields which are specific to each hierarchy.  As determining a given
  sd's root will be necessary, sd->s_dir.root is added.  This backlink
  fits better as a separate field in sd; however, sd->s_dir is inside
  union with space to spare, so use it to save space and provide
  kernfs_root() accessor to determine the root sd.

* As hierarchies may be destroyed now, each mount needs to hold onto
  the hierarchy it's attached to.  Update sysfs_fill_super() and
  sysfs_kill_sb() so that they get and put the kernfs_root
  respectively.

* sysfs_root is replaced with kernfs_root which is dynamically created
  by invoking kernfs_create_root() from sysfs_init().

This patch doesn't introduce any visible behavior changes.

v2: kernfs_create_root() forgot to set @sd->priv.  Fixed.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index fd8f574ef2fe..f75548b8ed7a 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -20,6 +20,11 @@ struct vm_area_struct;
 
 struct sysfs_dirent;
 
+struct kernfs_root {
+	/* published fields */
+	struct sysfs_dirent	*sd;
+};
+
 struct sysfs_open_file {
 	/* published fields */
 	struct sysfs_dirent	*sd;
@@ -76,6 +81,9 @@ struct sysfs_dirent *kernfs_find_and_get_ns(struct sysfs_dirent *parent,
 void kernfs_get(struct sysfs_dirent *sd);
 void kernfs_put(struct sysfs_dirent *sd);
 
+struct kernfs_root *kernfs_create_root(void *priv);
+void kernfs_destroy_root(struct kernfs_root *root);
+
 struct sysfs_dirent *kernfs_create_dir_ns(struct sysfs_dirent *parent,
 					  const char *name, void *priv,
 					  const void *ns);
@@ -107,6 +115,11 @@ kernfs_find_and_get_ns(struct sysfs_dirent *parent, const char *name,
 static inline void kernfs_get(struct sysfs_dirent *sd) { }
 static inline void kernfs_put(struct sysfs_dirent *sd) { }
 
+static inline struct kernfs_root *kernfs_create_root(void *priv)
+{ return ERR_PTR(-ENOSYS); }
+
+static inline void kernfs_destroy_root(struct kernfs_root *root) { }
+
 static inline struct sysfs_dirent *
 kernfs_create_dir_ns(struct sysfs_dirent *parent, const char *name, void *priv,
 		     const void *ns)