Linux MM : virtual memory address space

I was going through http://linux-mm.org/HighMemory "Currently the 32 bit x86 architecture is the most popular type of computer. In this architecture, traditionally the Linux kernel has split the 4GB of virtual memory address space into 3GB for user programs and 1GB for the kernel" What about 64-bit system? Where does the code lie in linux kernel for the check? Is there any latest and updated memory management documentation for Linux kernel? Regards, Priyaranjan