question on memory mapping one page at a time

[Sri Ram Vemulpali]

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.