Re: [PATCH v3 2/3] mm: mprotect: avoid unnecessary struct page accessing if pte_protnone()

[Zi Yan <ziy@nvidia.com>]

On 15 Oct 2025, at 8:35, Kefeng Wang wrote:

> If the pte_protnone() is true, we could avoid unnecessary struct page
> accessing and reduce cache footprint when scanning page tables for prot
> numa, the performance test of pmbench memory accessing benchmark
> should be benifit, see more commit a818f5363a0e ("autonuma: reduce cache
> footprint when scanning page tables").
>
> Reviewed-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
>  mm/mprotect.c | 30 +++++++++++++-----------------
>  1 file changed, 13 insertions(+), 17 deletions(-)
>
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index bb59a42809b8..ed44aadb7aaa 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -118,18 +118,13 @@ static int mprotect_folio_pte_batch(struct folio *folio, pte_t *ptep,
>  	return folio_pte_batch_flags(folio, NULL, ptep, &pte, max_nr_ptes, flags);
>  }
>
> -static bool prot_numa_skip(struct vm_area_struct *vma, unsigned long addr,
> -			   pte_t oldpte, pte_t *pte, int target_node,
> -			   struct folio *folio)
> +static bool prot_numa_skip(struct vm_area_struct *vma, int target_node,
> +		struct folio *folio)
>  {
>  	bool ret = true;
>  	bool toptier;
>  	int nid;
>
> -	/* Avoid TLB flush if possible */
> -	if (pte_protnone(oldpte))
> -		goto skip;
> -
>  	if (!folio)
>  		goto skip;
>
> @@ -307,23 +302,24 @@ static long change_pte_range(struct mmu_gather *tlb,
>  			struct page *page;
>  			pte_t ptent;
>
> +			/* Already in the desired state. */
> +			if (prot_numa && pte_protnone(oldpte))
> +				continue;

For a folio pte batch, instead of calculating nr_ptes to skip, this just
skip PTEs one by one. Looking at folio_pte_batch_flags(), this way looks
better.

> +
>  			page = vm_normal_page(vma, addr, oldpte);
>  			if (page)
>  				folio = page_folio(page);
> +
>  			/*
>  			 * Avoid trapping faults against the zero or KSM
>  			 * pages. See similar comment in change_huge_pmd.
>  			 */
> -			if (prot_numa) {
> -				int ret = prot_numa_skip(vma, addr, oldpte, pte,
> -							 target_node, folio);
> -				if (ret) {
> -
> -					/* determine batch to skip */
> -					nr_ptes = mprotect_folio_pte_batch(folio,
> -						  pte, oldpte, max_nr_ptes, /* flags = */ 0);
> -					continue;
> -				}
> +			if (prot_numa && prot_numa_skip(vma, target_node,
> +							folio)) {
> +				/* determine batch to skip */
> +				nr_ptes = mprotect_folio_pte_batch(folio,
> +					  pte, oldpte, max_nr_ptes, /* flags = */ 0);
> +				continue;
>  			}
>
>  			nr_ptes = mprotect_folio_pte_batch(folio, pte, oldpte, max_nr_ptes, flags);

Acked-by: Zi Yan <ziy@nvidia.com>

--
Best Regards,
Yan, Zi