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

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Message-ID: <87frcn6uws.fsf@DESKTOP-5N7EMDA>
Date: Tue, 16 Sep 2025 09:36:03 +0800
From: "Huang, Ying" <ying.huang@...ux.alibaba.com>
To: David Hildenbrand <david@...hat.com>
Cc: Catalin Marinas <catalin.marinas@....com>,  Will Deacon
 <will@...nel.org>,  Andrew Morton <akpm@...ux-foundation.org>,  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>,  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: [RFC PATCH 1/2] mm: add spurious fault fixing support for huge pmd

Hi, David,

Thanks for review!

David Hildenbrand <david@...hat.com> writes:

> On 15.09.25 05:29, Huang Ying wrote:
>> In the current kernel, there is spurious fault fixing support for pte,
>> but not for huge pmd because no architectures need it. But in the
>> next patch in the series, we will change the write protection fault
>> handling logic on arm64, so that some stale huge pmd entries may
>> remain in the TLB. These entries need to be flushed via the huge pmd
>> spurious fault fixing mechanism.
>> Signed-off-by: Huang Ying <ying.huang@...ux.alibaba.com>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will@...nel.org>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: David Hildenbrand <david@...hat.com>
>> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>> Cc: Vlastimil Babka <vbabka@...e.cz>
>> Cc: Zi Yan <ziy@...dia.com>
>> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> Cc: Ryan Roberts <ryan.roberts@....com>
>> Cc: Yang Shi <yang@...amperecomputing.com>
>> Cc: "Christoph Lameter (Ampere)" <cl@...two.org>
>> Cc: Dev Jain <dev.jain@....com>
>> Cc: Barry Song <baohua@...nel.org>
>> Cc: Anshuman Khandual <anshuman.khandual@....com>
>> Cc: Yicong Yang <yangyicong@...ilicon.com>
>> Cc: Kefeng Wang <wangkefeng.wang@...wei.com>
>> Cc: Kevin Brodsky <kevin.brodsky@....com>
>> Cc: Yin Fengwei <fengwei_yin@...ux.alibaba.com>
>> Cc: linux-arm-kernel@...ts.infradead.org
>> Cc: linux-kernel@...r.kernel.org
>> Cc: linux-mm@...ck.org
>> ---
>
> [...]
>
>>     int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct
>> *src_mm,
>> @@ -1857,7 +1861,20 @@ void huge_pmd_set_accessed(struct vm_fault *vmf)
>>   	if (unlikely(!pmd_same(*vmf->pmd, vmf->orig_pmd)))
>>   		goto unlock;
>>   -	touch_pmd(vmf->vma, vmf->address, vmf->pmd, write);
>> +	if (!touch_pmd(vmf->vma, vmf->address, vmf->pmd, write)) {
>> +		/* 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.
>> +		 */
>
> Can we instead just remove these comments and simplly say "see
> handle_pte_fault()"

Sure.

>> +		if (vmf->flags & FAULT_FLAG_WRITE)
>> +			flush_tlb_fix_spurious_fault_pmd(vmf->vma, vmf->address,
>> +							 vmf->pmd);
>> +	}
>
> Okay, In the PTE case, we call flush_tlb_fix_spurious_fault() during
> write faults if ptep_set_access_flags() returned "0".
>
> You are calling flush_tlb_fix_spurious_fault_pmd() during a write
> fault when pmdp_set_access_flags() returned "0" as well.
>
> In general, LGTM, but I would just let touch_pmd() return the value of
> pmdp_set_access_flags() instead and add a quick comment for
> touch_pmd() what the return value means.

Sure.

---
Best Regards,
Huang, Ying