Excerpts from Christophe Leroy's message of December 9, 2021 3:18 am:

Select CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT and
remove arch/powerpc/mm/mmap.c

This change provides standard randomisation of mmaps.

See commit 8b8addf891de ("x86/mm/32: Enable full randomization on i386
and X86_32") for all the benefits of mmap randomisation.

The justification seems pretty reasonable.

Comparison between powerpc implementation and the generic one:
- mmap_is_legacy() is identical.
- arch_mmap_rnd() does exactly the same allthough it's written
slightly differently.
- MIN_GAP and MAX_GAP are identical.
- mmap_base() does the same but uses STACK_RND_MASK which provides
the same values as stack_maxrandom_size().
- arch_pick_mmap_layout() is almost identical. The only difference
is that it also adds the random factor to mm->mmap_base in legacy mode.

That last point is what provides the standard randomisation of mmaps.

Thanks for describing it. Could you add random_factor to mmap_base for
the legacy path for powerpc as a 2-line change that adds the legacy
randomisation. And then this bigger patch would be closer to a no-op.

Thanks,
Nick