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

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Message-ID: <48f8f2d8-e6e0-ec2a-7323-11551c66dac2@loongson.cn>
Date: Thu, 6 Nov 2025 15:07:48 +0800
From: Bibo Mao <maobibo@...ngson.cn>
To: Tianyang Zhang <zhangtianyang@...ngson.cn>, chenhuacai@...nel.org,
 kernel@...0n.name, akpm@...ux-foundation.org, willy@...radead.org,
 david@...hat.com, linmag7@...il.com, thuth@...hat.com, apopple@...dia.com
Cc: 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



On 2025/11/4 下午3:30, Tianyang Zhang 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.
> 
> 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;
Since ptep_get_and_clear() is not atomic operation on LoongArch like 
other architectures, considering this scenery with HW PTW enabled:
     CPU 0:                         CPU1
                              old_pte = ptep_modify_prot_start(); 

                                   old_pte = ptep_get(ptep);

     write(buf);
*HW will set _PAGE_DIRTY bit*
                                   pte_clear(mm, address, ptep);
^^^^^^^^^^ For CPU1, bit _PAGE_DIRTY is no set in old_pte, _PAGE_DIRTY 
will be lost also. ^^^^^^^^^^^
                              pte = pte_modify(old_pte,)
                              ptep_modify_prot_commit(.., pte)

Regards
Bibo Mao
> +
> +	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);
>   }
>   
>   static inline pmd_t pmd_mkinvalid(pmd_t pmd)
>