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

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Message-ID: <313d2c86-d13e-4f79-b977-81a43d685cf6@redhat.com>
Date: Tue, 14 Oct 2025 16:58:44 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 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.

-- 
Cheers

David / dhildenb