Re: [PATCH v3 10/14] mm: page_vma_mapped_walk: map_pte() use pte_offset_map_rw_nolock()

[Qi Zheng]

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@xxxxxxxxxxxxx>
> > ---
> >   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);