In practice the actual available usable address space for userland is 64 TiB due to user/kernel split and the kernel maintaining a virtual mapping of the entire physical address space (minus I/O ranges) [0].
However newer incoming 5-level page intel chips [1] will allow up to 57 bits of address space, 128 PiB in theory though in practice 32 PiB of userland memory. See also [0] for discussion on practical limit for 5-page too!
[0]:https://github.com/lorenzo-stoakes/linux-mm-notes/blob/maste...