> On Sep 24, 2024, at 16:33, Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> wrote:
>
>
>
> On 2024/9/24 16:25, Muchun Song wrote:
>>> On Sep 24, 2024, at 14:11, Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>>> ---
>>> 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
