https://lwn.net/Articles/708526/ "x86-64 is currently limited to 256 TiB of virtual address space and 64 TiB of physical address space. We are already bumping into this limit: some vendors offers servers with 64 TiB of memory today. To overcome the limitation upcoming hardware will introduce support for 5-level paging. It is a straight-forward extension of the current page table structure adding one more layer of translation. It bumps the limits to 128 PiB of virtual address space and 4 PiB of physical address space. This "ought to be enough for anybody" ©."