Re: [PATCH v4 10/13] mm: page_vma_mapped_walk: map_pte() use pte_offset_map_rw_nolock()

[Qi Zheng <zhengqi.arch@bytedance.com>]

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
>>