sysfs, kernfs: make super_blocks bind to different kernfs_roots

kernfs is being updated to allow multiple sysfs_dirent hierarchies so
that it can also be used by other users.  Currently, sysfs
super_blocks are always attached to one kernfs_root - sysfs_root - and
distinguished only by their namespace tags.

This patch adds sysfs_super_info->root and update
sysfs_fill/test_super() so that super_blocks are identified by the
combination of both the associated kernfs_root and namespace tag.
This allows mounting different kernfs hierarchies.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/sysfs/mount.c

@@ -37,6 +37,7 @@ struct sysfs_dirent *sysfs_root_sd;
 
 static int sysfs_fill_super(struct super_block *sb)
 {
+	struct sysfs_super_info *info = sysfs_info(sb);
 	struct inode *inode;
 	struct dentry *root;
 
@@ -48,7 +49,7 @@ static int sysfs_fill_super(struct super_block *sb)
 
 	/* get root inode, initialize and unlock it */
 	mutex_lock(&sysfs_mutex);
-	inode = sysfs_get_inode(sb, sysfs_root_sd);
+	inode = sysfs_get_inode(sb, info->root->sd);
 	mutex_unlock(&sysfs_mutex);
 	if (!inode) {
 		pr_debug("sysfs: could not get root inode\n");
@@ -61,8 +62,8 @@ static int sysfs_fill_super(struct super_block *sb)
 		pr_debug("%s: could not get root dentry!\n", __func__);
 		return -ENOMEM;
 	}
-	kernfs_get(sysfs_root_sd);
-	root->d_fsdata = sysfs_root_sd;
+	kernfs_get(info->root->sd);
+	root->d_fsdata = info->root->sd;
 	sb->s_root = root;
 	sb->s_d_op = &sysfs_dentry_ops;
 	return 0;
@@ -73,7 +74,7 @@ static int sysfs_test_super(struct super_block *sb, void *data)
 	struct sysfs_super_info *sb_info = sysfs_info(sb);
 	struct sysfs_super_info *info = data;
 
-	return sb_info->ns == info->ns;
+	return sb_info->root == info->root && sb_info->ns == info->ns;
 }
 
 static int sysfs_set_super(struct super_block *sb, void *data)
@@ -110,6 +111,7 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
 	if (!info)
 		return ERR_PTR(-ENOMEM);
 
+	info->root = sysfs_root;
 	info->ns = kobj_ns_grab_current(KOBJ_NS_TYPE_NET);
 
 	sb = sget(fs_type, sysfs_test_super, sysfs_set_super, flags, info);

fs/sysfs/sysfs.h

@@ -19,6 +19,12 @@
  */
 
 struct sysfs_super_info {
+	/*
+	 * The root associated with this super_block.  Each super_block is
+	 * identified by the root and ns it's associated with.
+	 */
+	struct kernfs_root	*root;
+
 	/*
 	 * Each sb is associated with one namespace tag, currently the network
 	 * namespace of the task which mounted this sysfs instance.  If multiple