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.