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