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

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Message-ID: <1f7c3b59-3276-4f6e-946b-5f6ac9807e93@redhat.com>
Date: Thu, 16 Oct 2025 10:59:02 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Huang, Ying" <ying.huang@...ux.alibaba.com>
Cc: 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 16.10.25 10:25, Lorenzo Stoakes wrote:
> On Thu, Oct 16, 2025 at 10:22:57AM +0800, Huang, Ying wrote:
>> Lorenzo Stoakes <lorenzo.stoakes@...cle.com> writes:
>>
>>> On Wed, Oct 15, 2025 at 04:43:14PM +0800, Huang, Ying wrote:
>>> OK this is great, let's put it all in the kdoc for the new shared spurious
>>> faulting function! :) and additionally add it to the commit message.
>>
>> Sure.  Will do it in the next version.
> 
> Thanks!
> 
>>
>> [snip]
>>
>>>>>>
>>>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>>>>>> index 32e8457ad535..341622ec80e4 100644
>>>>>> --- a/include/linux/pgtable.h
>>>>>> +++ b/include/linux/pgtable.h
>>>>>> @@ -1232,6 +1232,10 @@ static inline void arch_swap_restore(swp_entry_t entry, struct folio *folio)
>>>>>>   #define flush_tlb_fix_spurious_fault(vma, address, ptep) flush_tlb_page(vma, address)
>>>>>>   #endif
>>>>>>
>>>>>> +#ifndef flush_tlb_fix_spurious_fault_pmd
>>>>>> +#define flush_tlb_fix_spurious_fault_pmd(vma, address, ptep) do { } while (0)
>>>>>> +#endif
>>>>>
>>>>> flush_tlb_fix_spurious_fault(), when the arch doesn't declare it, defaults to
>>>>> flush_tlb_page() - why do we just do nothing in this case here?
>>>>
>>>> Because all architectures do nothing for the spurious PMD page fault
>>>> fixing until the [2/2] of this series.  Where, we make it necessary to
>>>> flush the local TLB for spurious PMD page fault fixing on arm64
>>>> architecture.
>>>>
>>>> If we follow the design of flush_tlb_fix_spurious_fault(), we need to
>>>> change all architecture implementation to do nothing in this patch to
>>>> keep the current behavior.  I don't think that it's a good idea.  Do
>>>> you agree?
>>>
>>> Yeah probably we should keep the same behaviour as before, which is
>>> obviously, prior to this series, we did nothing.
>>>
>>> I guess in the PTE case we _always_ want to flush the TLB, whereas in the
>>> PMD case we otherwise don't have any need to at the point at which the
>>> spurious flush is performed.
>>>
>>> But from your explanation above re: the stale TLB entry this _only_ needs
>>> to be done for architectures which might encounter this problem rather than
>>> needing a TLB flush in general.
>>>
>>> Given we're generalising the code and one case always flushes the TLB and
>>> the other doesn't maybe it's worth putting a comment in the generalised
>>> function mentioning this?
>>
>> I'm not sure whether it's a good idea to document architecture behaviors
>> in the general code.  The behavior may be changed architecture by
>> architecture in the future.
> 
> Right, but we are unconditionaly doing a TLB flush in the PTE case but not PMD
> so let's document that to be clear :)

Agreed! That's a big benefit of merging the code, it sticks out what is 
not common already.

-- 
Cheers

David / dhildenb