Re: [PATCH 1/9] vfs: add i_op->dentry_open()

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 13 Mar 2013 15:16:25 +0100 Miklos Szeredi <miklos@szeredi.hu> wrote:

> From: Miklos Szeredi <mszeredi@suse.cz>
> 
> Add a new inode operation i_op->dentry_open().  This is for stacked filesystems
> that want to return a struct file from a different filesystem.
> 
> ...
>
> +/**
> + * vfs_open - open the file at the given path
> + * @path: path to open
> + * @filp: newly allocated file with f_flag initialized
> + * @cred: credentials to use
> + */
> +int vfs_open(const struct path *path, struct file *filp,
> +	     const struct cred *cred)
> +{
> +	struct inode *inode = path->dentry->d_inode;
> +
> +	if (inode->i_op->dentry_open)
> +		return inode->i_op->dentry_open(path->dentry, filp, cred);
> +	else {
> +		filp->f_path = *path;
> +		return do_dentry_open(filp, NULL, cred);
> +	}
> +}

This looks like it will add some overhead to dentry_open().  That long
walk path->dentry->d_inode->i_op->dentry_open, particularly.  Can we do
anything?  Using filp->f_inode might save a tad.

It's unfortunate and a bit ugly that ->dentry_open() and
do_dentry_open() don't have the same arguments.  One would expect and
like them to do so, and this difference raises suspicions about design
warts.

If they _did_ have the same signature then we could perhaps populate
->dentry_open with do_dentry_open by default and avoid the `if'.

The test of inode->i_op->dentry_open could do with an unlikely(), but
that won't save us :(

> +EXPORT_SYMBOL(vfs_open);

Did you consider EXPORT_SYMBOL_GPL()?