confused about the balloon code

[tgh <tianguanhua@ncic.ac.cn>]

hi
 I try to understand the code of balloon ,and got confused about the 
following parts of code:

static int decrease_reservation(unsigned long nr_pages)
{
         ....
        if (!PageHighMem(page)) {
            v = phys_to_virt(pfn << PAGE_SHIFT);
            scrub_pages(v, 1);
            ret = HYPERVISOR_update_va_mapping(
                (unsigned long)v, __pte_ma(0), 0);
            BUG_ON(ret);
        }
#ifdef CONFIG_XEN_SCRUB_PAGES
        else {
            v = kmap(page);
            scrub_pages(v, 1);
            kunmap(page);
        }
#endif
     ....
}

I am confused about the scrub_pages(), what is the meaning of 
scrub_pages, when should we use CONFIG_XEN_SCRUB_PAGES
could you help me
Thanks in advance