Re: [PATCH] Loongarch:Make pte/pmd_modify can set _PAGE_MODIFIED

From: Bibo Mao

Date: Tue Nov 04 2025 - 20:21:05 EST



On 2025/11/5 上午9:07, Huacai Chen wrote:
On Wed, Nov 5, 2025 at 8:57 AM Tianyang Zhang <zhangtianyang@xxxxxxxxxxx> 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@xxxxxxxxxxx> 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.

Regards
Bibo Mao

But the description also needs to be updated.


Co-developed-by: Liupu Wang <wangliupu@xxxxxxxxxxx>
Signed-off-by: Liupu Wang <wangliupu@xxxxxxxxxxx>
Signed-off-by: Tianyang Zhang <zhangtianyang@xxxxxxxxxxx>
---
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