question on memory mapping one page at a time

Hi all, can anyone please explain the following code, what it is doing. I want the first few statements of the code, as they are calculating page frame number. This is little confusing, thanks in advance. struct page *simple_vma_nopage(struct vm_area_struct *vma, unsigned long address, int *type) { struct page *pageptr; unsigned long offset = vma->vm_pgoff << PAGE_SHIFT; unsigned long physaddr = address - vma->vm_start + offset; unsigned long pageframe = physaddr >> PAGE_SHIFT; if (!pfn_valid(pageframe)) return NOPAGE_SIGBUS; pageptr = pfn_to_page(pageframe); get_page(pageptr); if (type) *type = VM_FAULT_MINOR; return pageptr; } -- Regards, Sri.