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

[Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>]

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.

> >     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