how to use mmap() on a disk file to get a kernel address?

Hi,
   I try to mmap a file in a linux kernel module. I have tried to use the
function do_mmap_pgoff. But the address returned is memory virtual address
in current process' user space, i.e., below the kernel boundary. Instead, I
want to map the file in the kernel space and get the kernel virtual address
of the mapped region. Is there any kernel API in Linux support this
operation? Or I have to reimplement a kernel mmap version ?
  But reimplement a kernel version mmap() is  not easy I guess. Do you have
any idea?
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