Quoting sukadev@us.ibm.com (sukadev@us.ibm.com):
> From: Sukadev Bhattiprolu <sukadev@us.ibm.com>
> Subject: [RFC][PATCH 3/4]: Enable multiple mounts of /dev/pts
> 
> To support multiple PTY namespaces, we should be allow multiple mounts of
> /dev/pts, once within each PTY namespace.
> 
> This patch removes the get_sb_single() in devpts_get_sb() and uses test and
> set sb interfaces to allow remounting /dev/pts.  The patch also removes the
> globals, 'devpts_root' and uses current_pts_mnt() to access 'devpts_mnt'
> 
> Changelog:
> 	- Version 0: Based on earlier versions from Serge Hallyn and
> 	  Matt Helsley.
> 
> Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>

Signed-off-by: Serge Hallyn <serue@us.ibm.com>

Though more of this may be Matt's than mine.


> ---
>  fs/devpts/inode.c |  120 +++++++++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 101 insertions(+), 19 deletions(-)
> 
> Index: linux-2.6.24/fs/devpts/inode.c
> ===================================================================
> --- linux-2.6.24.orig/fs/devpts/inode.c	2008-02-05 17:30:52.000000000 -0800
> +++ linux-2.6.24/fs/devpts/inode.c	2008-02-05 19:16:39.000000000 -0800
> @@ -34,7 +34,10 @@ static inline struct idr *current_pts_ns
>  }
> 
>  static struct vfsmount *devpts_mnt;
> -static struct dentry *devpts_root;
> +static inline struct vfsmount *current_pts_ns_mnt(void)
> +{
> +	return devpts_mnt;
> +}
> 
>  static struct {
>  	int setuid;
> @@ -130,7 +133,7 @@ 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);
> +	s->s_root = d_alloc_root(inode);
>  	if (s->s_root)
>  		return 0;
>  	
> @@ -140,10 +143,53 @@ fail:
>  	return -ENOMEM;
>  }
> 
> +/*
> + * We use test and set super-block operations to help determine whether we
> + * need a new super-block for this namespace. get_sb() walks the list of
> + * existing devpts supers, comparing them with the @data ptr. Since we
> + * passed 'current's namespace as the @data pointer we can compare the
> + * namespace pointer in the super-block's 's_fs_info'.  If the test is
> + * TRUE then get_sb() returns a new active reference to the super block.
> + * Otherwise, it helps us build an active reference to a new one.
> + */
> +
> +static int devpts_test_sb(struct super_block *sb, void *data)
> +{
> +	return sb->s_fs_info == data;
> +}
> +
> +static int devpts_set_sb(struct super_block *sb, void *data)
> +{
> +	sb->s_fs_info = data;
> +	return set_anon_super(sb, NULL);
> +}
> +
>  static int devpts_get_sb(struct file_system_type *fs_type,
>  	int flags, const char *dev_name, void *data, struct vfsmount *mnt)
>  {
> -	return get_sb_single(fs_type, flags, data, devpts_fill_super, mnt);
> +	struct super_block *sb;
> +	int err;
> +
> +	/* hereafter we're very simlar to get_sb_nodev */
> +	sb = sget(fs_type, devpts_test_sb, devpts_set_sb, data);
> +	if (IS_ERR(sb))
> +		return PTR_ERR(sb);
> +
> +	if (sb->s_root)
> +		return simple_set_mnt(mnt, sb);
> +
> +	sb->s_flags = flags;
> +	err = devpts_fill_super(sb, data, flags & MS_SILENT ? 1 : 0);
> +	if (err) {
> +		up_write(&sb->s_umount);
> +		deactivate_super(sb);
> +		return err;
> +	}
> +
> +	sb->s_flags |= MS_ACTIVE;
> +	devpts_mnt = mnt;
> +
> +	return simple_set_mnt(mnt, sb);
>  }
> 
>  static struct file_system_type devpts_fs_type = {
> @@ -158,10 +204,9 @@ static struct file_system_type devpts_fs
>   * to the System V naming convention
>   */
> 
> -static struct dentry *get_node(int num)
> +static struct dentry *get_node(struct dentry *root, int num)
>  {
>  	char s[12];
> -	struct dentry *root = devpts_root;
>  	mutex_lock(&root->d_inode->i_mutex);
>  	return lookup_one_len(s, root, sprintf(s, "%d", num));
>  }
> @@ -207,12 +252,28 @@ 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 dentry *root;
> +	struct vfsmount *mnt;
> +	struct inode *inode;
> +
> 
>  	/* We're supposed to be given the slave end of a pty */
>  	BUG_ON(driver->type != TTY_DRIVER_TYPE_PTY);
>  	BUG_ON(driver->subtype != PTY_TYPE_SLAVE);
> 
> +	mnt = current_pts_ns_mnt();
> +	if (!mnt)
> +		return -ENOSYS;
> +	root = mnt->mnt_root;
> +
> +	mutex_lock(&root->d_inode->i_mutex);
> +	inode = idr_find(current_pts_ns_allocated_ptys(), number);
> +	mutex_unlock(&root->d_inode->i_mutex);
> +
> +	if (inode && !IS_ERR(inode))
> +		return -EEXIST;
> +
> +	inode = new_inode(mnt->mnt_sb);
>  	if (!inode)
>  		return -ENOMEM;
> 
> @@ -222,23 +283,31 @@ int devpts_pty_new(struct tty_struct *tt
>  	inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
>  	init_special_inode(inode, S_IFCHR|config.mode, device);
>  	inode->i_private = tty;
> +	idr_replace(current_pts_ns_allocated_ptys(), inode, number);
> 
> -	dentry = get_node(number);
> +	dentry = get_node(root, number);
>  	if (!IS_ERR(dentry) && !dentry->d_inode) {
>  		d_instantiate(dentry, inode);
> -		fsnotify_create(devpts_root->d_inode, dentry);
> +		fsnotify_create(root->d_inode, dentry);
>  	}
> 
> -	mutex_unlock(&devpts_root->d_inode->i_mutex);
> +	mutex_unlock(&root->d_inode->i_mutex);
> 
>  	return 0;
>  }
> 
>  struct tty_struct *devpts_get_tty(int number)
>  {
> -	struct dentry *dentry = get_node(number);
> +	struct vfsmount *mnt;
> +	struct dentry *dentry;
>  	struct tty_struct *tty;
> 
> +	mnt = current_pts_ns_mnt();
> +	if (!mnt)
> +		return NULL;
> +
> +	dentry = get_node(mnt->mnt_root, number);
> +
>  	tty = NULL;
>  	if (!IS_ERR(dentry)) {
>  		if (dentry->d_inode)
> @@ -246,14 +315,21 @@ struct tty_struct *devpts_get_tty(int nu
>  		dput(dentry);
>  	}
> 
> -	mutex_unlock(&devpts_root->d_inode->i_mutex);
> +	mutex_unlock(&mnt->mnt_root->d_inode->i_mutex);
> 
>  	return tty;
>  }
> 
>  void devpts_pty_kill(int number)
>  {
> -	struct dentry *dentry = get_node(number);
> +	struct dentry *dentry;
> +	struct dentry *root;
> +	struct vfsmount *mnt;
> +
> +	mnt = current_pts_ns_mnt();
> +	root = mnt->mnt_root;
> +
> +	dentry = get_node(root, number);
> 
>  	if (!IS_ERR(dentry)) {
>  		struct inode *inode = dentry->d_inode;
> @@ -264,17 +340,23 @@ void devpts_pty_kill(int number)
>  		}
>  		dput(dentry);
>  	}
> -	mutex_unlock(&devpts_root->d_inode->i_mutex);
> +	mutex_unlock(&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);
> -	}
> +	struct vfsmount *mnt;
> +	int err;
> +
> +	err = register_filesystem(&devpts_fs_type);
> +	if (err)
> +		return err;
> +
> +	mnt = kern_mount_data(&devpts_fs_type, NULL);
> +	if (IS_ERR(mnt))
> +		err = PTR_ERR(mnt);
> +	else
> +		devpts_mnt = mnt;
>  	return err;
>  }
> 
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