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    SubjectRe: [PATCH v4 09/10] powerpc/mm: Convert to default topdown mmap layout
    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

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