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

[Muchun Song <muchun.song@linux.dev>]

> 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