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What is the address space of the kernel for 64 bit Linux, that is, what is the address range of the code, stack, heap and data segments used by it.
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In 64-bit Windows, the theoretical amount of virtual address space is 2^64 bytes (16 exabytes), but only a small portion of the 16-exabyte range is actually used.
The book uses the term "kernel address space" to refer to the partition of the virtual address space that is allocated for the kernel. Recently, Linux and other OSes have implemented page-table isolation (PTI) to mitigate the Meltdown security vulnerability.
The address space of a process consists of all linear addresses that the process is allowed to use. Each process sees a different set of linear addresses; the address used by one process bears no relation to the address used by another.
All of kernel memory and user process memory is stored in physical memory in the computer (or perhaps on disk if data has been swapped from memory).
On a 64-bit Linux all 64-bit addresses with the highest order bit set to 1 are reserved for the kernel. In other words, the top half of the virtual address space.
For full details see http://en.wikipedia.org/wiki/X86-64#Virtual_address_space_details
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