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

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Message-ID: <CAGsJ_4xhJSLnXOZy4kPmnif5Paq+OPN_Ww+rPk2WO4-ADSC0Yw@mail.gmail.com>
Date: Wed, 22 Oct 2025 23:52:23 +1300
From: Barry Song <21cnbao@...il.com>
To: "Huang, Ying" <ying.huang@...ux.alibaba.com>
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, Zi Yan <ziy@...dia.com>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, Ryan Roberts <ryan.roberts@....com>, 
	Yang Shi <yang@...amperecomputing.com>, "Christoph Lameter (Ampere)" <cl@...two.org>, Dev Jain <dev.jain@....com>, 
	Anshuman Khandual <anshuman.khandual@....com>, Yicong Yang <yangyicong@...ilicon.com>, 
	Kefeng Wang <wangkefeng.wang@...wei.com>, Kevin Brodsky <kevin.brodsky@....com>, 
	Yin Fengwei <fengwei_yin@...ux.alibaba.com>, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH -v2 2/2] arm64, tlbflush: don't TLBI broadcast if page
 reused in write fault

On Wed, Oct 22, 2025 at 11:34 PM Huang, Ying
<ying.huang@...ux.alibaba.com> wrote:
>
> Barry Song <21cnbao@...il.com> writes:
>
> > On Wed, Oct 22, 2025 at 10:46 PM Huang, Ying
> > <ying.huang@...ux.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
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.

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

Thanks
Barry