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

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

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@bytedance.com>
> ---
>   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.

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);