Re: [PATCH v3 4/9] cachefiles: use tmpfile_open() helper

[Al Viro <viro@zeniv.linux.org.uk>]

On Wed, Sep 21, 2022 at 10:26:12AM +0200, Christian Brauner wrote:
> -       /* From now path refers to the tmpfile */
> +
> +       /* prepare tmp path */
> +       path.mnt = cache->mnt;
>         path.dentry = file->f_path.dentry;

Do we even want that struct path from that point on?  Look:

	d_backing_inode(path.dentry) is a weird way to spell file_inode(file).

	cachefiles_mark_inode_in_use() is an overkill here - it *can't* fail
here, so all we want is
	inode_lock(inode);
	inode->i_flags |= S_KERNEL_FILE;
	trace_cachefiles_mark_active(object, inode);
	inode_unlock(inode);
where inode is, again, file_inode(file).

	cachefiles_do_unmark_inode_in_use() uses only inode.

	vfs_truncate() could use &file->f_path, but there's a potentially
nastier problem - theoretically, there are filesystems where we might
want struct file available for operation, especially for opened-and-unlinked
equivalents.  In any case, &file->f_path would do just as well as its copy.