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: <87ms5j4444.fsf@DESKTOP-5N7EMDA>
Date: Wed, 22 Oct 2025 18:34:19 +0800
From: "Huang, Ying" <ying.huang@...ux.alibaba.com>
To: Barry Song <21cnbao@...il.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

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.

---
Best Regards,
Huang, Ying