linux-kernel - Re: [PATCH] Loongarch:Make pte/pmd_modify can set _PAGE

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Message-ID: <95e6f0a6-fd6c-a85f-5983-5a37eaf960a2@loongson.cn>
Date: Wed, 5 Nov 2025 08:56:56 +0800
From: Tianyang Zhang <zhangtianyang@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: kernel@...0n.name, akpm@...ux-foundation.org, willy@...radead.org,
 david@...hat.com, linmag7@...il.com, thuth@...hat.com, maobibo@...ngson.cn,
 apopple@...dia.com, loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org,
 Liupu Wang <wangliupu@...ngson.cn>
Subject: Re: [PATCH] Loongarch:Make pte/pmd_modify can set _PAGE_MODIFIED

Hi, Huacai

在 2025/11/4 下午4:00, Huacai Chen 写道:
> Hi, Tianyang,
>
> The subject line can be:
> LoongArch: Let {pte,pmd}_modify() record the status of _PAGE_DIRTY (If
> I'm right in the later comments).
Ok. I got it
>
> On Tue, Nov 4, 2025 at 3:30 PM Tianyang Zhang <zhangtianyang@...ngson.cn> wrote:
>> In the current pte_modify operation, _PAGE_DIRTY might be cleared. Since
>> the hardware-page-walk does not have a predefined _PAGE_MODIFIED flag,
>> this could lead to loss of valid data in certain scenarios.
>>
>> The new modification involves checking whether the original PTE has the
>> _PAGE_DIRTY flag. If it exists, the _PAGE_MODIFIED bit is set, ensuring
>> that the pte_dirty interface can return accurate information.
> The description may be wrong here. Because pte_dirty() returns
> pte_val(pte) & (_PAGE_DIRTY | _PAGE_MODIFIED).
> If _PAGE_DIRTY isn't lost, pte_dirty() is always right, no matter
> whether there is or isn't _PAGE_MODIFIED.
>
> I think the real reason is we need to set _PAGE_MODIFIED in
> pte/pmd_modify to record the status of _PAGE_DIRTY, so that we can
> recover _PAGE_DIRTY afterwards, such as in pte/pmd_mkwrite().
Ok, I will adjust the description
>
>> Co-developed-by: Liupu Wang <wangliupu@...ngson.cn>
>> Signed-off-by: Liupu Wang <wangliupu@...ngson.cn>
>> Signed-off-by: Tianyang Zhang <zhangtianyang@...ngson.cn>
>> ---
>>   arch/loongarch/include/asm/pgtable.h | 17 +++++++++++++----
>>   1 file changed, 13 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/loongarch/include/asm/pgtable.h b/arch/loongarch/include/asm/pgtable.h
>> index bd128696e96d..106abfa5183b 100644
>> --- a/arch/loongarch/include/asm/pgtable.h
>> +++ b/arch/loongarch/include/asm/pgtable.h
>> @@ -424,8 +424,13 @@ static inline unsigned long pte_accessible(struct mm_struct *mm, pte_t a)
>>
>>   static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
>>   {
>> -       return __pte((pte_val(pte) & _PAGE_CHG_MASK) |
>> -                    (pgprot_val(newprot) & ~_PAGE_CHG_MASK));
>> +       unsigned long val = (pte_val(pte) & _PAGE_CHG_MASK) |
>> +                    (pgprot_val(newprot) & ~_PAGE_CHG_MASK);
>> +
>> +       if (pte_val(pte) & _PAGE_DIRTY)
>> +               val |= _PAGE_MODIFIED;
>> +
>> +       return __pte(val);
>>   }
>>
>>   extern void __update_tlb(struct vm_area_struct *vma,
>> @@ -547,9 +552,13 @@ static inline struct page *pmd_page(pmd_t pmd)
>>
>>   static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
>>   {
>> -       pmd_val(pmd) = (pmd_val(pmd) & _HPAGE_CHG_MASK) |
>> +       unsigned long val = (pmd_val(pmd) & _HPAGE_CHG_MASK) |
>>                                  (pgprot_val(newprot) & ~_HPAGE_CHG_MASK);
>> -       return pmd;
>> +
>> +       if (pmd_val(pmd) & _PAGE_DIRTY)
>> +               val |= _PAGE_MODIFIED;
>> +
>> +       return __pmd(val);
>>   }
> A minimal modification can be:
> diff --git a/arch/loongarch/include/asm/pgtable.h
> b/arch/loongarch/include/asm/pgtable.h
> index 1f20e9280062..907ece0199e0 100644
> --- a/arch/loongarch/include/asm/pgtable.h
> +++ b/arch/loongarch/include/asm/pgtable.h
> @@ -448,8 +448,13 @@ static inline unsigned long pte_accessible(struct
> mm_struct *mm, pte_t a)
>
>   static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
>   {
> -       return __pte((pte_val(pte) & _PAGE_CHG_MASK) |
> -                    (pgprot_val(newprot) & ~_PAGE_CHG_MASK));
> +       pte_val(pte) = (pte_val(pte) & _PAGE_CHG_MASK) |
> +                       (pgprot_val(newprot) & ~_PAGE_CHG_MASK);
> +
> +       if (pte_val(pte) & _PAGE_DIRTY)
> +               pte_val(pte) |= _PAGE_MODIFIED;
> +
> +       return pte;
>   }

+       pte_val(pte) = (pte_val(pte) & _PAGE_CHG_MASK) |
+                       (pgprot_val(newprot) & ~_PAGE_CHG_MASK);

After this step, _PAGE_DIRTY may have already disappeared,
If no new variables are added, they can be modified in follow way:

  static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
  {
+       if (pte_val(pte) & _PAGE_DIRTY)
+               pte_val(pte) |= _PAGE_MODIFIED;
+
        return __pte((pte_val(pte) & _PAGE_CHG_MASK) |
	 (pgprot_val(newprot) & ~_PAGE_CHG_MASK));

  }

>
>   extern void __update_tlb(struct vm_area_struct *vma,
> @@ -583,7 +588,11 @@ static inline struct page *pmd_page(pmd_t pmd)
>   static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
>   {
>          pmd_val(pmd) = (pmd_val(pmd) & _HPAGE_CHG_MASK) |
> -                               (pgprot_val(newprot) & ~_HPAGE_CHG_MASK);
> +                       (pgprot_val(newprot) & ~_HPAGE_CHG_MASK);
> +
> +       if (pmd_val(pmd) & _PAGE_DIRTY)
> +               pmd_val(pmd) |= _PAGE_MODIFIED;
> +
>          return pmd;
>   }
>
> You needn't define a new variable.
>
>
> Huacai
>
>>   static inline pmd_t pmd_mkinvalid(pmd_t pmd)
>> --
>> 2.41.0
>>
>>
Thanks

Tianyang