Re: [PATCH -v2 2/2] arm64, tlbflush: don't TLBI broadcast if page reused in write fault

["Huang, Ying" <ying.huang@linux.alibaba.com>]

Barry Song <21cnbao@gmail.com> writes:

> On Wed, Oct 22, 2025 at 11:34 PM Huang, Ying
> <ying.huang@linux.alibaba.com> wrote:
>>
>> Barry Song <21cnbao@gmail.com> writes:
>>
>> > On Wed, Oct 22, 2025 at 10:46 PM Huang, Ying
>> > <ying.huang@linux.alibaba.com> wrote:
>> >
>> >> >
>> >> > I agree. Yet the ish barrier can still avoid the page faults during CPU0's PTL.
>> >>
>> >> IIUC, you think that dsb(ish) compared with dsb(nsh) can accelerate
>> >> memory writing (visible to other CPUs).  TBH, I suspect that this is the
>> >> case.
>> >
>> > Why? In any case, nsh is not a smp domain.
>>
>> I think dsb(ish) will be slower than dsb(nsh) in theory.  I guess that
>> dsb just wait for the memory write to be visible in the specified
>> shareability domain instead of making write faster.
>>
>> > I believe a dmb(ishst) is sufficient to ensure that the new PTE writes
>> > are visible
>>
>> dmb(ishst) (smp_wmb()) should pair with dmb(ishld) (smp_rmb()).
>>
>> > to other CPUs. I’m not quite sure why the current flush code uses dsb(ish);
>> > it seems like overkill.
>>
>> dsb(ish) here is used for tlbi(XXis) broadcast.  It waits until the page
>> table change is visible to the page table walker of the remote CPU.
>
> It seems we’re aligned on all points[1], although I’m not sure whether
> you have data comparing A and B.
>
> A:
> write pte
> don't broadcast pte
> tlbi
> don't broadcast tlbi
>
> with
>
> B:
> write pte
> broadcast pte

I suspect that pte will be broadcast, DVM broadcast isn't used for
the memory coherency IIUC.

> tlbi
> don't broadcast tlbi
>
> I guess the gain comes from "don't broadcat tlbi" ?
> With B, we should be able to share many existing code.

Ryan has some plan to reduce the code duplication with the current
solution.

> [1]
> https://lore.kernel.org/linux-mm/20251013092038.6963-1-ying.huang@linux.alibaba.com/T/#m54312d4914c69aa550bee7df36711c03a4280c52

---
Best Regards,
Huang, Ying