linux-kernel - Re: [PATCH -v2 1/2] mm: add spurious fault fixing support for huge pmd

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Message-ID: <e32bd132-4f55-44ec-b9f0-a9e13aa6c5a0@lucifer.local>
Date: Tue, 14 Oct 2025 16:13:27 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Huang Ying <ying.huang@...ux.alibaba.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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>, Barry Song <baohua@...nel.org>,
        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 1/2] mm: add spurious fault fixing support for huge
 pmd

On Tue, Oct 14, 2025 at 04:58:44PM +0200, David Hildenbrand wrote:
> On 14.10.25 16:49, Lorenzo Stoakes wrote:
> > On Tue, Oct 14, 2025 at 04:38:05PM +0200, David Hildenbrand wrote:
> > >
> > > >
> > > > 		/* Skip spurious TLB flush for retried page fault */
> > > > 		if (vmf->flags & FAULT_FLAG_TRIED)
> > > > 			goto unlock;
> > > > 		/*
> > > > 		 * This is needed only for protection faults but the arch code
> > > > 		 * is not yet telling us if this is a protection fault or not.
> > > > 		 * This still avoids useless tlb flushes for .text page faults
> > > > 		 * with threads.
> > > > 		 */
> > > > 		if (vmf->flags & FAULT_FLAG_WRITE)
> > > > 			flush_tlb_fix_spurious_fault(vmf->vma, vmf->address,
> > > > 						     vmf->pte);
> > > >
> > > >
> > > > So I don't see why it's so egregious to have the equivalent here, or actually
> > > > ideally to abstract the code entirely.
> > >
> > > Let's definitely not duplicate such comments whereby one instance will end
> > > up bitrotting.
> >
> > We're duplicating the code in two places, how would that bitrot happen exactly?
>
> Often we adjust/fix comments to make scenarios/conditions clearer or extend
> them to cover some new conditions.
>
> So even without any code changes people will just ignore to update other
> comments.
>
> Code you can at least test with the hope to find inconsistencies.
>
> So copying rather large comments is usually never the answer :)
>
> Well, just like copying larger chunks of code, agreed.

This is a bit moot I don't think it's worth having a big debate about.

I'm one of the biggest proponents of de-duplicating things (comments
included) and have done so extensively as you know.

My _only_ point here is that it's hard to find the comment referenced and
it's _very_ likely it'll get moved later (in fact I feel like doing that
_right now_ as that function needs refactoring).

In that case the lesser evil is to copy a 4 line comment right?

But anyway, we both agree de-duplicating the code as a whole is the right
way forward and that solves the issue so let's go with that!

>
> --
> Cheers
>
> David / dhildenb
>
>

Cheers, Lorenzo