remap_pfn_range doubt

Hi All,

I am trying to understand how this remap_pfn_range is implemented. I
almost got an idea on the implementation.
However one thing is not clear, they always subtract userspace address
pfn from the physican pfn
(http://lxr.linux.no/#linux+v3.2.11/mm/memory.c#L2278) and then adding
it back http://lxr.linux.no/#linux+v3.2.11/mm/memory.c#L2284 and
calling the inside function.
I don't understand why they are not directly passing it.

Is there any specific reason for this. Could you please give some
pointers on this?

-- 
With Regards
Subin Gangadharan

I am not afraid and I am also not afraid of being afraid.