new helper: alloc_file_clone()

alloc_file_clone(old_file, mode, ops): create a new struct file with
->f_path equal to that of old_file.  pipe converted.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/file_table.c

@@ -211,6 +211,17 @@ struct file *alloc_file_pseudo(struct inode *inode, struct vfsmount *mnt,
 }
 EXPORT_SYMBOL(alloc_file_pseudo);
 
+struct file *alloc_file_clone(struct file *base, int flags,
+				const struct file_operations *fops)
+{
+	struct file *f = alloc_file(&base->f_path, flags, fops);
+	if (!IS_ERR(f)) {
+		path_get(&f->f_path);
+		f->f_mapping = base->f_mapping;
+	}
+	return f;
+}
+
 /* the real guts of fput() - releasing the last reference to file
  */
 static void __fput(struct file *file)

fs/pipe.c

@@ -761,15 +761,13 @@ int create_pipe_files(struct file **res, int flags)
 
 	f->private_data = inode->i_pipe;
 
-	res[0] = alloc_file(&f->f_path, O_RDONLY | (flags & O_NONBLOCK),
-			&pipefifo_fops);
+	res[0] = alloc_file_clone(f, O_RDONLY | (flags & O_NONBLOCK),
+				  &pipefifo_fops);
 	if (IS_ERR(res[0])) {
 		put_pipe_info(inode, inode->i_pipe);
 		fput(f);
 		return PTR_ERR(res[0]);
 	}
-
-	path_get(&f->f_path);
 	res[0]->private_data = inode->i_pipe;
 	res[1] = f;
 	return 0;

include/linux/file.h

@@ -23,6 +23,8 @@ extern struct file *alloc_file(const struct path *, int flags,
 	const struct file_operations *fop);
 extern struct file *alloc_file_pseudo(struct inode *, struct vfsmount *,
 	const char *, int flags, const struct file_operations *);
+extern struct file *alloc_file_clone(struct file *, int flags,
+	const struct file_operations *);
 
 static inline void fput_light(struct file *file, int fput_needed)
 {