5-level page tables have actually been in the kernel for a couple of years now. The issue listed was definitely a concern, but was worked around by having the

5-level page tables have actually been in the kernel for a couple of years now. The issue listed was definitely a concern, but was worked around by having the kernel only allocate userspace linear addresses that aren't 48-bit-canonical in response to a mmap() call that supplies such an address as the hint argument. See the commit message in this commit for example: https://lore.kernel.org/patchwork/patch/796025/ https://lore.kernel.org/patchwork/patch/796025/ So for a program to get an address above the 56-bit limit, its memory allocator has to specifically indicate to the kernel that it supports that.