[$] Page allocation for address-space isolation

Address-space isolation may well be, as Brendan Jackman said at the
beginning of his memory-management-track session at the 2025 Linux Storage,
Filesystem, Memory-Management, and BPF Summit, 'some security
bullshit'. But it also holds the potential to protect the kernel from
a wide range of vulnerabilities, both known and unknown, while reducing the
impact of existing mitigations. Implementing address-space isolation with
reasonable performance, though, is going to require some significant
changes. Jackman was there to get feedback from the memory-management
community on how those changes should be implemented.