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

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Message-ID: <CAAhV-H5JnmJqTZ1GnhcgODtjL5FLy8x5HNRJxs6us=gzcFon5Q@mail.gmail.com>
Date: Tue, 4 Nov 2025 16:00:51 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Tianyang Zhang <zhangtianyang@...ngson.cn>
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, 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).

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().

>
> 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;
 }

 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
>
>