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Message-ID: <e035d888-a7a6-0f46-fdc1-92331cc12a93@loongson.cn>
Date: Thu, 6 Nov 2025 10:10:42 +0800
From: Bibo Mao <maobibo@...ngson.cn>
To: Tianyang Zhang <zhangtianyang@...ngson.cn>,
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, 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
On 2025/11/6 上午9:55, Tianyang Zhang wrote:
> Hi ,Bibao
>
> 在 2025/11/5 上午9:18, Bibo Mao 写道:
>>
>>
>> On 2025/11/5 上午9:07, Huacai Chen wrote:
>>> On Wed, Nov 5, 2025 at 8:57 AM Tianyang Zhang
>>> <zhangtianyang@...ngson.cn> wrote:
>>>>
>>>> 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
>>> After some thinking, your original description may be right. Without
>>> this patch the scenario maybe like this:
>>> The pte is dirty _PAGE_DIRTY but without _PAGE_MODIFIED, after
>>> pte_modify() we lose _PAGE_DIRTY, then pte_dirty() returns false. So
>>> we need _PAGE_MODIFIED to record _PAGE_DIRTY here.
>> In theory pte_modify() is to modify RWX attribute. I think that it is
>> a tricky to remove _PAGE_DIRTY and add _PAGE_MODIFIED with HW PTW system.
>>
>> Also _PAGE_ACCESSED is lost with pte_modify() API, is there any
>> influence with HW PTW system, or wait until possible problems coming out.
>
> 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));
> }
> In my understand, During the pte_modify process, it is essential to
> ensure that specific bits are inherited from the original PTE rather
> than simply replaced(as set_pte),
>
> this guarantees the coherent operation of the memory management system.
>
> Since _PAGE_CHG_MASK explicitly requires preserving pte_modified, and
The problem is how _PAGE_CHG_MASK should be defined, do you check with
other architectures?
> there is an inherent correlation between pte_dirty and pte_modified,
> these attributes must be evaluated and handled accordingly.
>
> The pte_valid attribute, being a hardware property, is inherently the
> target of modification in the pte_modify interface. Therefore, it is
> reasonable not to preserve it.
On HW PTW system, _PAGE_PRESENT will control whether trigger page fault
rather than pte_valid/_PAGE_ACCESSED. For simple, do you think the
following code is ok or not ?
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 mask = _PAGE_CHG_MASK;
+
+ if (cpu_has_ptw)
+ mask |= _PAGE_DIRTY | _PAGE_ACCESSED;
+ return __pte((pte_val(pte) & mask) |
+ (pgprot_val(newprot) & ~mask));
}
Regards
Bibo Mao
>
> Thanks
>
> Tianyang
>
>>
>> Regards
>> Bibo Mao
>>>
>>> But the description also needs to be updated.
>>>
>>>>>
>>>>>> 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));
>>>>
>>>> }
>>> OK, it makes sense.
>>>
>>> Huacai
>>>>
>>>>>
>>>>> 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
>>>>
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