linux-kernel - Re: [PATCH v3 10/14] mm: page_vma_mapped_walk: map_pte() use pte_offset_map_rw

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Message-ID: <7782bc62-a09f-465a-aa43-8179542ecc02@bytedance.com>
Date: Thu, 12 Sep 2024 17:30:10 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Muchun Song <muchun.song@...ux.dev>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
 david@...hat.com, hughd@...gle.com, willy@...radead.org, vbabka@...nel.org,
 akpm@...ux-foundation.org, rppt@...nel.org, vishal.moola@...il.com,
 peterx@...hat.com, ryan.roberts@....com, christophe.leroy2@...soprasteria.com
Subject: Re: [PATCH v3 10/14] mm: page_vma_mapped_walk: map_pte() use
 pte_offset_map_rw_nolock()



On 2024/9/5 20:07, Muchun Song wrote:
> 
> 
> On 2024/9/4 16:40, Qi Zheng 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@...edance.com>
>> ---
>>   mm/page_vma_mapped.c | 24 ++++++++++++++++++++----
>>   1 file changed, 20 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>> index ae5cc42aa2087..f1d73fd448708 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);
>> @@ -69,6 +73,12 @@ static bool map_pte(struct page_vma_mapped_walk 
>> *pvmw, spinlock_t **ptlp)
>>       }
>>       pvmw->ptl = *ptlp;
>>       spin_lock(pvmw->ptl);
>> +
>> +    if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pvmw->pmd)))) {
>> +        spin_unlock(pvmw->ptl);
> 
> Forgot to clear pvmw->ptl? Or how about moving the assignment for it
> to the place where the pmd_same check is successful?
> 
>> +        goto again;
>> +    }
>> +
> 
> Maybe here is the right place to assign pvmw->ptl.

Right, will do in the v4.

> 
> Muchun,
> Thanks.
> 
>>       return true;
>>   }
>> @@ -278,7 +288,7 @@ 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 +317,12 @@ 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);
>