In the caller of map_pte(), we may modify the pvmw->pte after acquiring
the pvmw->ptl, so convert it to using pte_offset_map_rw_nolock(). At
this time, the pte_same() check is not performed after the pvmw->ptl held,
so we should get pmdval and do pmd_same() check to ensure the stability of
pvmw->pmd.

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
---
 mm/page_vma_mapped.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index ae5cc42aa2087..6410f29b37c1b 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -13,9 +13,11 @@ static inline bool not_found(struct page_vma_mapped_walk *pvmw)
 	return false;
 }
 
-static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
+static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
+		    spinlock_t **ptlp)
 {
 	pte_t ptent;
+	pmd_t pmdval;
 
 	if (pvmw->flags & PVMW_SYNC) {
 		/* Use the stricter lookup */
@@ -25,6 +27,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
 		return !!pvmw->pte;
 	}
 
+again:
 	/*
 	 * It is important to return the ptl corresponding to pte,
 	 * in case *pvmw->pmd changes underneath us; so we need to
@@ -32,10 +35,11 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
 	 * proceeds to loop over next ptes, and finds a match later.
 	 * Though, in most cases, page lock already protects this.
 	 */
-	pvmw->pte = pte_offset_map_nolock(pvmw->vma->vm_mm, pvmw->pmd,
-					  pvmw->address, ptlp);
+	pvmw->pte = pte_offset_map_rw_nolock(pvmw->vma->vm_mm, pvmw->pmd,
+					     pvmw->address, &pmdval, ptlp);
 	if (!pvmw->pte)
 		return false;
+	*pmdvalp = pmdval;
 
 	ptent = ptep_get(pvmw->pte);
 
@@ -67,8 +71,13 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
 	} else if (!pte_present(ptent)) {
 		return false;
 	}
+	spin_lock(*ptlp);
+	if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pvmw->pmd)))) {
+		pte_unmap_unlock(pvmw->pte, *ptlp);
+		goto again;
+	}
 	pvmw->ptl = *ptlp;
-	spin_lock(pvmw->ptl);
+
 	return true;
 }
 
@@ -278,7 +287,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 			step_forward(pvmw, PMD_SIZE);
 			continue;
 		}
-		if (!map_pte(pvmw, &ptl)) {
+		if (!map_pte(pvmw, &pmde, &ptl)) {
 			if (!pvmw->pte)
 				goto restart;
 			goto next_pte;
@@ -307,6 +316,12 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 		if (!pvmw->ptl) {
 			pvmw->ptl = ptl;
 			spin_lock(pvmw->ptl);
+			if (unlikely(!pmd_same(pmde, pmdp_get_lockless(pvmw->pmd)))) {
+				pte_unmap_unlock(pvmw->pte, pvmw->ptl);
+				pvmw->ptl = NULL;
+				pvmw->pte = NULL;
+				goto restart;
+			}
 		}
 		goto this_pte;
 	} while (pvmw->address < end);
-- 
2.20.1

> On Sep 24, 2024, at 14:11, Qi Zheng <zhengqi.arch@bytedance.com> wrote:
> 
> In the caller of map_pte(), we may modify the pvmw->pte after acquiring
> the pvmw->ptl, so convert it to using pte_offset_map_rw_nolock(). At
> this time, the pte_same() check is not performed after the pvmw->ptl held,
> so we should get pmdval and do pmd_same() check to ensure the stability of
> pvmw->pmd.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
> mm/page_vma_mapped.c | 25 ++++++++++++++++++++-----
> 1 file changed, 20 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index ae5cc42aa2087..6410f29b37c1b 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -13,9 +13,11 @@ static inline bool not_found(struct page_vma_mapped_walk *pvmw)
>    return false;
> }
> 
> -static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
> +static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
> +            spinlock_t **ptlp)
> {
>    pte_t ptent;
> +    pmd_t pmdval;

Why declare a new variable? Can’t we use *pmdvalp instead?

> 
>    if (pvmw->flags & PVMW_SYNC) {
>        /* Use the stricter lookup */
> @@ -25,6 +27,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
>        return !!pvmw->pte;
>    }
> 
> +again:
>    /*
>     * It is important to return the ptl corresponding to pte,
>     * in case *pvmw->pmd changes underneath us; so we need to
> @@ -32,10 +35,11 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
>     * proceeds to loop over next ptes, and finds a match later.
>     * Though, in most cases, page lock already protects this.
>     */
> -    pvmw->pte = pte_offset_map_nolock(pvmw->vma->vm_mm, pvmw->pmd,
> -                      pvmw->address, ptlp);
> +    pvmw->pte = pte_offset_map_rw_nolock(pvmw->vma->vm_mm, pvmw->pmd,
> +                         pvmw->address, &pmdval, ptlp);
>    if (!pvmw->pte)
>        return false;
> +    *pmdvalp = pmdval;
> 
>    ptent = ptep_get(pvmw->pte);
> 
> @@ -67,8 +71,13 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
>    } else if (!pte_present(ptent)) {
>        return false;
>    }
> +    spin_lock(*ptlp);
> +    if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pvmw->pmd)))) {
> +        pte_unmap_unlock(pvmw->pte, *ptlp);
> +        goto again;
> +    }
>    pvmw->ptl = *ptlp;
> -    spin_lock(pvmw->ptl);
> +
>    return true;
> }
> 
> @@ -278,7 +287,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>            step_forward(pvmw, PMD_SIZE);
>            continue;
>        }
> -        if (!map_pte(pvmw, &ptl)) {
> +        if (!map_pte(pvmw, &pmde, &ptl)) {
>            if (!pvmw->pte)
>                goto restart;
>            goto next_pte;
> @@ -307,6 +316,12 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>        if (!pvmw->ptl) {
>            pvmw->ptl = ptl;
>            spin_lock(pvmw->ptl);
> +            if (unlikely(!pmd_same(pmde, pmdp_get_lockless(pvmw->pmd)))) {
> +                pte_unmap_unlock(pvmw->pte, pvmw->ptl);
> +                pvmw->ptl = NULL;
> +                pvmw->pte = NULL;
> +                goto restart;
> +            }
>        }
>        goto this_pte;
>    } while (pvmw->address < end);
> --
> 2.20.1
> 

On 2024/9/24 16:25, Muchun Song wrote:
> 
> 
>> On Sep 24, 2024, at 14:11, Qi Zheng <zhengqi.arch@bytedance.com> wrote:
>>
>> In the caller of map_pte(), we may modify the pvmw->pte after acquiring
>> the pvmw->ptl, so convert it to using pte_offset_map_rw_nolock(). At
>> this time, the pte_same() check is not performed after the pvmw->ptl held,
>> so we should get pmdval and do pmd_same() check to ensure the stability of
>> pvmw->pmd.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> ---
>> mm/page_vma_mapped.c | 25 ++++++++++++++++++++-----
>> 1 file changed, 20 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>> index ae5cc42aa2087..6410f29b37c1b 100644
>> --- a/mm/page_vma_mapped.c
>> +++ b/mm/page_vma_mapped.c
>> @@ -13,9 +13,11 @@ static inline bool not_found(struct page_vma_mapped_walk *pvmw)
>>     return false;
>> }
>>
>> -static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
>> +static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
>> +            spinlock_t **ptlp)
>> {
>>     pte_t ptent;
>> +    pmd_t pmdval;
> 
> Why declare a new variable? Can’t we use *pmdvalp instead?

It's just a coding habit, both are fine for me.

> 
>>
>>     if (pvmw->flags & PVMW_SYNC) {
>>         /* Use the stricter lookup */
>> @@ -25,6 +27,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
>>         return !!pvmw->pte;
>>     }
>>
>> +again:
>>     /*
>>      * It is important to return the ptl corresponding to pte,
>>      * in case *pvmw->pmd changes underneath us; so we need to
>> @@ -32,10 +35,11 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
>>      * proceeds to loop over next ptes, and finds a match later.
>>      * Though, in most cases, page lock already protects this.
>>      */
>> -    pvmw->pte = pte_offset_map_nolock(pvmw->vma->vm_mm, pvmw->pmd,
>> -                      pvmw->address, ptlp);
>> +    pvmw->pte = pte_offset_map_rw_nolock(pvmw->vma->vm_mm, pvmw->pmd,
>> +                         pvmw->address, &pmdval, ptlp);
>>     if (!pvmw->pte)
>>         return false;
>> +    *pmdvalp = pmdval;
>>
>>     ptent = ptep_get(pvmw->pte);
>>
>> @@ -67,8 +71,13 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
>>     } else if (!pte_present(ptent)) {
>>         return false;
>>     }
>> +    spin_lock(*ptlp);
>> +    if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pvmw->pmd)))) {
>> +        pte_unmap_unlock(pvmw->pte, *ptlp);
>> +        goto again;
>> +    }
>>     pvmw->ptl = *ptlp;
>> -    spin_lock(pvmw->ptl);
>> +
>>     return true;
>> }
>>
>> @@ -278,7 +287,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>             step_forward(pvmw, PMD_SIZE);
>>             continue;
>>         }
>> -        if (!map_pte(pvmw, &ptl)) {
>> +        if (!map_pte(pvmw, &pmde, &ptl)) {
>>             if (!pvmw->pte)
>>                 goto restart;
>>             goto next_pte;
>> @@ -307,6 +316,12 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>         if (!pvmw->ptl) {
>>             pvmw->ptl = ptl;
>>             spin_lock(pvmw->ptl);
>> +            if (unlikely(!pmd_same(pmde, pmdp_get_lockless(pvmw->pmd)))) {
>> +                pte_unmap_unlock(pvmw->pte, pvmw->ptl);
>> +                pvmw->ptl = NULL;
>> +                pvmw->pte = NULL;
>> +                goto restart;
>> +            }
>>         }
>>         goto this_pte;
>>     } while (pvmw->address < end);
>> --
>> 2.20.1
>>

> On Sep 24, 2024, at 16:33, Qi Zheng <zhengqi.arch@bytedance.com> wrote:
> 
> 
> 
> On 2024/9/24 16:25, Muchun Song wrote:
>>> On Sep 24, 2024, at 14:11, Qi Zheng <zhengqi.arch@bytedance.com> wrote:
>>> 
>>> In the caller of map_pte(), we may modify the pvmw->pte after acquiring
>>> the pvmw->ptl, so convert it to using pte_offset_map_rw_nolock(). At
>>> this time, the pte_same() check is not performed after the pvmw->ptl held,
>>> so we should get pmdval and do pmd_same() check to ensure the stability of
>>> pvmw->pmd.
>>> 
>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>>> ---
>>> mm/page_vma_mapped.c | 25 ++++++++++++++++++++-----
>>> 1 file changed, 20 insertions(+), 5 deletions(-)
>>> 
>>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>>> index ae5cc42aa2087..6410f29b37c1b 100644
>>> --- a/mm/page_vma_mapped.c
>>> +++ b/mm/page_vma_mapped.c
>>> @@ -13,9 +13,11 @@ static inline bool not_found(struct page_vma_mapped_walk *pvmw)
>>>    return false;
>>> }
>>> 
>>> -static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
>>> +static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
>>> +            spinlock_t **ptlp)
>>> {
>>>    pte_t ptent;
>>> +    pmd_t pmdval;
>> Why declare a new variable? Can’t we use *pmdvalp instead?
> 
> It's just a coding habit, both are fine for me.

Agree. But sometime it could make code look a little simpler.

> 
>>> 
>>>    if (pvmw->flags & PVMW_SYNC) {
>>>        /* Use the stricter lookup */
>>> @@ -25,6 +27,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
>>>        return !!pvmw->pte;
>>>    }
>>> 
>>> +again:
>>>    /*
>>>     * It is important to return the ptl corresponding to pte,
>>>     * in case *pvmw->pmd changes underneath us; so we need to
>>> @@ -32,10 +35,11 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
>>>     * proceeds to loop over next ptes, and finds a match later.
>>>     * Though, in most cases, page lock already protects this.
>>>     */
>>> -    pvmw->pte = pte_offset_map_nolock(pvmw->vma->vm_mm, pvmw->pmd,
>>> -                      pvmw->address, ptlp);
>>> +    pvmw->pte = pte_offset_map_rw_nolock(pvmw->vma->vm_mm, pvmw->pmd,
>>> +                         pvmw->address, &pmdval, ptlp);
>>>    if (!pvmw->pte)
>>>        return false;
>>> +    *pmdvalp = pmdval;

For instance, here, it is unnecessary if pmdvalp is passed directly to
pte_offset_map_rw_nolock.

>>> 
>>>    ptent = ptep_get(pvmw->pte);
>>> 
>>> @@ -67,8 +71,13 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
>>>    } else if (!pte_present(ptent)) {
>>>        return false;
>>>    }
>>> +    spin_lock(*ptlp);
>>> +    if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pvmw->pmd)))) {
>>> +        pte_unmap_unlock(pvmw->pte, *ptlp);
>>> +        goto again;
>>> +    }
>>>    pvmw->ptl = *ptlp;
>>> -    spin_lock(pvmw->ptl);
>>> +
>>>    return true;
>>> }
>>> 
>>> @@ -278,7 +287,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>>            step_forward(pvmw, PMD_SIZE);
>>>            continue;
>>>        }
>>> -        if (!map_pte(pvmw, &ptl)) {
>>> +        if (!map_pte(pvmw, &pmde, &ptl)) {
>>>            if (!pvmw->pte)
>>>                goto restart;
>>>            goto next_pte;
>>> @@ -307,6 +316,12 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>>        if (!pvmw->ptl) {
>>>            pvmw->ptl = ptl;
>>>            spin_lock(pvmw->ptl);
>>> +            if (unlikely(!pmd_same(pmde, pmdp_get_lockless(pvmw->pmd)))) {
>>> +                pte_unmap_unlock(pvmw->pte, pvmw->ptl);
>>> +                pvmw->ptl = NULL;
>>> +                pvmw->pte = NULL;
>>> +                goto restart;
>>> +            }
>>>        }
>>>        goto this_pte;
>>>    } while (pvmw->address < end);
>>> --
>>> 2.20.1

On 2024/9/24 16:39, Muchun Song wrote:
> 
> 
>> On Sep 24, 2024, at 16:33, Qi Zheng <zhengqi.arch@bytedance.com> wrote:
>>
>>
>>
>> On 2024/9/24 16:25, Muchun Song wrote:
>>>> On Sep 24, 2024, at 14:11, Qi Zheng <zhengqi.arch@bytedance.com> wrote:
>>>>
>>>> In the caller of map_pte(), we may modify the pvmw->pte after acquiring
>>>> the pvmw->ptl, so convert it to using pte_offset_map_rw_nolock(). At
>>>> this time, the pte_same() check is not performed after the pvmw->ptl held,
>>>> so we should get pmdval and do pmd_same() check to ensure the stability of
>>>> pvmw->pmd.
>>>>
>>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>>>> ---
>>>> mm/page_vma_mapped.c | 25 ++++++++++++++++++++-----
>>>> 1 file changed, 20 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>>>> index ae5cc42aa2087..6410f29b37c1b 100644
>>>> --- a/mm/page_vma_mapped.c
>>>> +++ b/mm/page_vma_mapped.c
>>>> @@ -13,9 +13,11 @@ static inline bool not_found(struct page_vma_mapped_walk *pvmw)
>>>>     return false;
>>>> }
>>>>
>>>> -static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
>>>> +static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
>>>> +            spinlock_t **ptlp)
>>>> {
>>>>     pte_t ptent;
>>>> +    pmd_t pmdval;
>>> Why declare a new variable? Can’t we use *pmdvalp instead?
>>
>> It's just a coding habit, both are fine for me.
> 
> Agree. But sometime it could make code look a little simpler.
> 
>>
>>>>
>>>>     if (pvmw->flags & PVMW_SYNC) {
>>>>         /* Use the stricter lookup */
>>>> @@ -25,6 +27,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
>>>>         return !!pvmw->pte;
>>>>     }
>>>>
>>>> +again:
>>>>     /*
>>>>      * It is important to return the ptl corresponding to pte,
>>>>      * in case *pvmw->pmd changes underneath us; so we need to
>>>> @@ -32,10 +35,11 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
>>>>      * proceeds to loop over next ptes, and finds a match later.
>>>>      * Though, in most cases, page lock already protects this.
>>>>      */
>>>> -    pvmw->pte = pte_offset_map_nolock(pvmw->vma->vm_mm, pvmw->pmd,
>>>> -                      pvmw->address, ptlp);
>>>> +    pvmw->pte = pte_offset_map_rw_nolock(pvmw->vma->vm_mm, pvmw->pmd,
>>>> +                         pvmw->address, &pmdval, ptlp);
>>>>     if (!pvmw->pte)
>>>>         return false;
>>>> +    *pmdvalp = pmdval;
> 
> For instance, here, it is unnecessary if pmdvalp is passed directly to
> pte_offset_map_rw_nolock.

OK, will use pmdvalp directly. ;)

> 
>>>>
>>>>     ptent = ptep_get(pvmw->pte);
>>>>
>>>> @@ -67,8 +71,13 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
>>>>     } else if (!pte_present(ptent)) {
>>>>         return false;
>>>>     }
>>>> +    spin_lock(*ptlp);
>>>> +    if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pvmw->pmd)))) {
>>>> +        pte_unmap_unlock(pvmw->pte, *ptlp);
>>>> +        goto again;
>>>> +    }
>>>>     pvmw->ptl = *ptlp;
>>>> -    spin_lock(pvmw->ptl);
>>>> +
>>>>     return true;
>>>> }
>>>>
>>>> @@ -278,7 +287,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>>>             step_forward(pvmw, PMD_SIZE);
>>>>             continue;
>>>>         }
>>>> -        if (!map_pte(pvmw, &ptl)) {
>>>> +        if (!map_pte(pvmw, &pmde, &ptl)) {
>>>>             if (!pvmw->pte)
>>>>                 goto restart;
>>>>             goto next_pte;
>>>> @@ -307,6 +316,12 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>>>         if (!pvmw->ptl) {
>>>>             pvmw->ptl = ptl;
>>>>             spin_lock(pvmw->ptl);
>>>> +            if (unlikely(!pmd_same(pmde, pmdp_get_lockless(pvmw->pmd)))) {
>>>> +                pte_unmap_unlock(pvmw->pte, pvmw->ptl);
>>>> +                pvmw->ptl = NULL;
>>>> +                pvmw->pte = NULL;
>>>> +                goto restart;
>>>> +            }
>>>>         }
>>>>         goto this_pte;
>>>>     } while (pvmw->address < end);
>>>> --
>>>> 2.20.1
> 
>