[PATCH v5 0/9] Merge arm64/riscv hugetlbfs contpte support

[Ryan Roberts]

On 09/05/2025 12:09, Alexandre Ghiti wrote:
> Hi Will,
> 
> On Thu, May 8, 2025 at 2:30 PM Will Deacon <will at kernel.org> wrote:
>>
>> Hi folks,
>>
>> On Mon, May 05, 2025 at 06:08:50PM +0200, Alexandre Ghiti wrote:
>>> On 29/04/2025 16:09, Ryan Roberts wrote:
>>>> On 07/04/2025 13:04, Alexandre Ghiti wrote:
>>>>> Can someone from arm64 review this? I think it's preferable to share the same
>>>>> implementation between riscv and arm64.
>>>> I've been thinking about this for a while and had some conversations internally.
>>>> This patchset has both pros and cons.
>>>>
>>>> In the pros column, it increases code reuse in an area that has had quite of few
>>>> bugs popping up lately; so this would bring more eyes and hopefully higher
>>>> quality in the long run.
>>>>
>>>> But in the cons column, we have seen HW errata in similar areas in the past and
>>>> I'm nervous that by hoisting this code to mm, we make it harder to workaround
>>>> any future errata. Additionally I can imagine that this change could make it
>>>> harder to support future Arm architecture enhancements.
>>>>
>>>> I appreciate the cons are not strong *technical* arguments but nevertheless they
>>>> are winning out in this case; My opinion is that we should keep the arm64
>>>> implementations of huge_pte_ (and contpte_ too - I know you have a separate
>>>> series for this) private to arm64.
>>>>
>>>> Sorry about that.
>>>>
>>>>> The end goal is the support of mTHP using svnapot on riscv, which we want soon,
>>>>> so if that patchset does not gain any traction, I'll just copy/paste the arm64
>>>>> implementation into riscv.
>>>> This copy/paste approach would be my preference.
>>>
>>>
>>> I have to admit that I disagree with this approach, the riscv and arm64
>>> implementations are *exactly* the same so it sounds weird to duplicate code,
>>> the pros you mention outweigh the cons.
>>>
>>> Unless I'm missing something about the erratas? To me, that's easily fixed
>>> by providing arch specific overrides no? Can you describe what sort of
>>> erratas would not fit then?

One concrete feature is the use of Arm's FEAT_BBM level 2 to avoid having to do
break-before-make and TLB maintenance when doing a fold or unfold operation.
There is a series in flight to add this support at [1]. I can see this type of
approach being extended to the hugetlb helpers in future.

I also have another series in flight at [2] that tidies up the hugetlb
implementation and does some optimizations. But the optimizations depend on
arm64-specific TLB maintenance APIs.

[1]
https://lore.kernel.org/linux-arm-kernel/20250428153514.55772-2-miko.lenczewski@arm.com/

[2]
https://lore.kernel.org/linux-arm-kernel/20250422081822.1836315-1-ryan.roberts@arm.com/

As for errata, that's obviously much more fuzzy; there have been a bunch
relating to the MMU in the recent past, and I wouldn't be shocked if more turned up.

For future architecture enchancements, I'm aware of one potential feature being
discussed for which this change would likely make it harder to implement.

>>
>> If we start with the common implementation you have here, nothing
>> prevents us from forking the code in future if the architectures diverge
>> so I'd be inclined to merge this series and see how we get on. 

OK if that's your preference, I'm ok with it. I don't have strong opinion, just
a sense that we will end up with loads of arch-specific overrides. As you say,
let's see.

Alexandre, I guess this series is quite old now and will need to incorporate the
hugtelb fixes I did last cycle? And ideally I'd like [2] to land then for that
to also be incorporated into your next version. (I'm still hopeful we can get
[2] into v6.16 and have been waiting patiently for Will to pick it up ;) ).

I guess we can worry about [1] later as that is only affected by your other series.

How does that sound?

>> However,
>> one thing I *do* think we need to ensure is that the relevant folks from
>> both arm64 (i.e. Ryan) and riscv (i.e. Alexandre) are cc'd on changes to
>> the common code. Otherwise, it's going to be a step backwards in terms
>> of maintainability.
>>>> Could we add something to MAINTAINERS so that the new file picks you both
>> up as reviewers?

That's fine with me. Lorenzo added me for some parts of MM this cycle anyway.

Thanks,
Ryan

> 
> I'm adding Lorenzo as he is cleaning the mm MAINTAINERS entries.
> 
> @Lorenzo: should we add a new section "CONTPTE" for this? FYI, hugetlb
> is the first patchset, I have another patchset to merge THP contpte
> support [1] as well so the "HUGETLB" section does not seem to be a
> good fit.
> 
> [1] https://lore.kernel.org/linux-riscv/20240508191931.46060-1-alexghiti@rivosinc.com/
> 
> Thanks,
> 
> Alex
> 
>>
>> Will