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

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from Christophe Leroy's message of December 9, 2021 8:22 pm:
> 
> 
> Le 09/12/2021 à 11:15, Nicholas Piggin a écrit :
>> 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.
>> 
> 
> You mean you would like to see the following patch before doing the 
> convert ?
> 
> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/7dabf1cbde67a346a187881d4f0bd17347e0334a.1533732583.git.christophe.leroy@c-s.fr/

Yes.

Thanks,
Nick