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 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?

> 
>    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
>