RISC-V Linux: flat pagemap reservation is too large on Sv39

snmalloc 0.7.4 assumes a 48-bit address space on Linux/RISC-V. On an Sv39 riscv64 Linux machine, allocator initialisation fails because the flat pagemap reservation is too large:

Failed to initialise snmalloc.
Trace/breakpoint trap (core dumped)

With the default 48-bit setting, snmalloc tries to reserve a 256 GiB pagemap:

mmap(NULL, 274877906944, PROT_NONE,
     MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0) = -1 ENOMEM

I tried two compile-time Linux/RISC-V address_bits values:

• address_bits = 38, matching the existing FreeBSD/RISC-V special case. This works on an Sv39 machine because the pagemap reservation becomes much smaller. However, it is too small to cover valid high user-space mappings on systems with a larger virtual address space, such as Sv48. In that case, snmalloc can see addresses above the configured pagemap range and fail when looking them up. #860

• address_bits = 47. This covers high user-space mappings better and avoids the too-small-pagemap-range problem, but it still fails on the Sv39 machine for SNMALLOC_CHECK_CLIENT builds. Check-mode enables random_pagemap, and FlatPagemap::init<true>() reserves the required pagemap plus an additional 4 * next_pow2(REQUIRED_SIZE) randomisation area. With 47 address bits, the required pagemap is 128 GiB, so the total reservation becomes 640 GiB:

mmap(NULL, 687194767360, PROT_NONE,
     MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0) = -1 ENOMEM

Some possible directions seem to be:

• keep a conservative Linux/RISC-V address_bits value and ensure mappings stay within that range;
• use a larger address_bits value, but make randomized pagemap initialisation fall back to non-randomized placement when the larger reservation fails;
• select the effective address-space size based on the actual runtime VA layout;
• replace or relax the flat pagemap assumption for Linux/RISC-V.

I would appreciate guidance on which approach best matches snmalloc's design expectations.

Reproduce the allocator-initialisation failure on an Sv39 Linux/RISC-V machine, including an SNMALLOC_CHECK_CLIENT build. Start by tracing the Linux/RISC-V address_bits setting and FlatPagemap::init() with random_pagemap enabled, and review #860. Done should avoid an excessive reservation on Sv39 without breaking valid high mappings on larger virtual address spaces.