Ok. But in this case i have to ensure that the pfn is not mapped by the
kernel, right?
Will scanning mem_map to find such a pfn help?
I'm wondering if there are regions which are typically 'free' which i can
reserve and use.

Either you can look at memblock_reserve or reserve_bootmem. (this may
solve your problem).
or you alloc_page api which return a struct page, you SetPageReserved
and use page_to_pfn to get the pfn number.

thanks,
thayumanavar s.