identify a vitual address

Hi,
I have to implement a function that returns the physical address given the
virtual address. I use a method similar to follow_page() implementation. In
previous kernels I could distinguish kernel addresses from user space
addresses by checking if the address is higher or lower than PAGE_OFFSET and
for kernel space addresses I used init_mm's page tables while for user space
addresses I used the process's page tables. Now that the address space of
kernel and user can overlap my question is how to distinguish between them?
One more question. Why does follow_page() not exported? There are many
devices who do DMA and need this translation.

Thanks
Eli
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When the going gets tough the tough get going