[RFC] Remove kern_mount() in init_devpts_fs()

[sukadev at us.ibm.com]

Is the kern_mount() of devpts really needed or can we simply register 
the filesystem type and wait for an user-space mount before being able
to create PTYs ?

This is just an RFC patch that removes the kern_mount() and the 'devpts_mnt'
and 'devpts_root' global variables and uses a 'devpts_sb' to store the single
super block associated with devpts.

Removing the kern_mount() and relying on user-space mount could simplify
cloning of PTS namespaces.


---
 fs/devpts/inode.c |   49 +++++++++++++++++++++++++++++--------------------
 1 file changed, 29 insertions(+), 20 deletions(-)

Index: linux-2.6.24/fs/devpts/inode.c
===================================================================
--- linux-2.6.24.orig/fs/devpts/inode.c	2008-02-22 14:23:53.000000000 -0800
+++ linux-2.6.24/fs/devpts/inode.c	2008-02-25 16:00:17.000000000 -0800
@@ -23,9 +23,6 @@
 
 #define DEVPTS_SUPER_MAGIC 0x1cd1
 
-static struct vfsmount *devpts_mnt;
-static struct dentry *devpts_root;
-
 static struct {
 	int setuid;
 	int setgid;
@@ -97,6 +94,7 @@ static const struct super_operations dev
 	.remount_fs	= devpts_remount,
 };
 
+static struct super_block *devpts_sb;
 static int
 devpts_fill_super(struct super_block *s, void *data, int silent)
 {
@@ -120,9 +118,11 @@ devpts_fill_super(struct super_block *s,
 	inode->i_fop = &simple_dir_operations;
 	inode->i_nlink = 2;
 
-	devpts_root = s->s_root = d_alloc_root(inode);
-	if (s->s_root)
+	s->s_root = d_alloc_root(inode);
+	if (s->s_root) {
+		devpts_sb = s;
 		return 0;
+	}
 	
 	printk("devpts: get root dentry failed\n");
 	iput(inode);
@@ -136,11 +136,17 @@ static int devpts_get_sb(struct file_sys
 	return get_sb_single(fs_type, flags, data, devpts_fill_super, mnt);
 }
 
+static void devpts_kill_sb(struct super_block *sb)
+{
+	devpts_sb = NULL;
+	kill_anon_super(sb);
+}
+
 static struct file_system_type devpts_fs_type = {
 	.owner		= THIS_MODULE,
 	.name		= "devpts",
 	.get_sb		= devpts_get_sb,
-	.kill_sb	= kill_anon_super,
+	.kill_sb	= devpts_kill_sb,
 };
 
 /*
@@ -151,7 +157,12 @@ static struct file_system_type devpts_fs
 static struct dentry *get_node(int num)
 {
 	char s[12];
-	struct dentry *root = devpts_root;
+	struct dentry *root;
+
+	if (!devpts_sb)
+		return NULL;
+
+	root = devpts_sb->s_root;
 	mutex_lock(&root->d_inode->i_mutex);
 	return lookup_one_len(s, root, sprintf(s, "%d", num));
 }
@@ -162,7 +173,12 @@ int devpts_pty_new(struct tty_struct *tt
 	struct tty_driver *driver = tty->driver;
 	dev_t device = MKDEV(driver->major, driver->minor_start+number);
 	struct dentry *dentry;
-	struct inode *inode = new_inode(devpts_mnt->mnt_sb);
+	struct inode *inode;
+
+	if (!devpts_sb)
+		return -ENOSYS;
+
+	inode = new_inode(devpts_sb);
 
 	/* We're supposed to be given the slave end of a pty */
 	BUG_ON(driver->type != TTY_DRIVER_TYPE_PTY);
@@ -181,10 +197,10 @@ int devpts_pty_new(struct tty_struct *tt
 	dentry = get_node(number);
 	if (!IS_ERR(dentry) && !dentry->d_inode) {
 		d_instantiate(dentry, inode);
-		fsnotify_create(devpts_root->d_inode, dentry);
+		fsnotify_create(devpts_sb->s_root->d_inode, dentry);
 	}
 
-	mutex_unlock(&devpts_root->d_inode->i_mutex);
+	mutex_unlock(&devpts_sb->s_root->d_inode->i_mutex);
 
 	return 0;
 }
@@ -201,7 +217,7 @@ struct tty_struct *devpts_get_tty(int nu
 		dput(dentry);
 	}
 
-	mutex_unlock(&devpts_root->d_inode->i_mutex);
+	mutex_unlock(&devpts_sb->s_root->d_inode->i_mutex);
 
 	return tty;
 }
@@ -219,24 +235,17 @@ void devpts_pty_kill(int number)
 		}
 		dput(dentry);
 	}
-	mutex_unlock(&devpts_root->d_inode->i_mutex);
+	mutex_unlock(&devpts_sb->s_root->d_inode->i_mutex);
 }
 
 static int __init init_devpts_fs(void)
 {
-	int err = register_filesystem(&devpts_fs_type);
-	if (!err) {
-		devpts_mnt = kern_mount(&devpts_fs_type);
-		if (IS_ERR(devpts_mnt))
-			err = PTR_ERR(devpts_mnt);
-	}
-	return err;
+	return register_filesystem(&devpts_fs_type);
 }
 
 static void __exit exit_devpts_fs(void)
 {
 	unregister_filesystem(&devpts_fs_type);
-	mntput(devpts_mnt);
 }
 
 module_init(init_devpts_fs)