Question about object-based reverse mapping

Hi, all

I'm currently reading "Understanding the Linux Kernel (3rd)" and I
have a question about "object-based reverse mapping".

This is the Figure 17-1 on page 683 : http://imgur.com/R0OtIJh

As this figure shows, many vm_area_struct are in the anon_vma?s list
and they share some page frames.

The index field in page struct represents the offset of virtual
address within the virtual memory area. What I want to know is what
would happen when a page frame is shared by several vm_area_struct.

If two virtual memory areas share the same page frame, it seems that
page->index would be overridden because there are two vma and the
variable index would be assigned two times. How the linux kernel
prevent it happening.

Thanks.
Hao Lee