Re: [PATCH v2 2/7] mm: Optimize mprotect() by batch-skipping PTEs

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 4/29/25 10:53, Dev Jain wrote:
> In case of prot_numa, there are various cases in which we can skip to the
> next iteration. Since the skip condition is based on the folio and not
> the PTEs, we can skip a PTE batch.
> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>  mm/mprotect.c | 27 ++++++++++++++++++++-------
>  1 file changed, 20 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 70f59aa8c2a8..ec5d17af7650 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -91,6 +91,9 @@ static bool prot_numa_skip(struct vm_area_struct *vma, struct folio *folio,
>  	bool toptier;
>  	int nid;
>  
> +	if (folio_is_zone_device(folio) || folio_test_ksm(folio))
> +		return true;
> +

Moving these here from prot_numa_avoid_fault() could have been done
earlier, while adding prot_numa_skip() itself in the previous patch
(in case this helper is determined to be really required).

>  	/* Also skip shared copy-on-write pages */
>  	if (is_cow_mapping(vma->vm_flags) &&
>  	    (folio_maybe_dma_pinned(folio) ||
> @@ -126,8 +129,10 @@ static bool prot_numa_skip(struct vm_area_struct *vma, struct folio *folio,
>  }
>  
>  static bool prot_numa_avoid_fault(struct vm_area_struct *vma,
> -		unsigned long addr, pte_t oldpte, int target_node)
> +		unsigned long addr, pte_t *pte, pte_t oldpte, int target_node,
> +		int max_nr, int *nr)
>  {
> +	const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;

Flags are all correct.

>  	struct folio *folio;
>  	int ret;
>  
> @@ -136,12 +141,16 @@ static bool prot_numa_avoid_fault(struct vm_area_struct *vma,
>  		return true;
>  
>  	folio = vm_normal_folio(vma, addr, oldpte);
> -	if (!folio || folio_is_zone_device(folio) ||
> -	    folio_test_ksm(folio))
> +	if (!folio)
>  		return true;
> +
>  	ret = prot_numa_skip(vma, folio, target_node);
> -	if (ret)
> +	if (ret) {
> +		if (folio_test_large(folio) && max_nr != 1)

Conditional checks are all correct.

> +			*nr = folio_pte_batch(folio, addr, pte, oldpte,
> +					      max_nr, flags, NULL, NULL, NULL);
>  		return ret;
> +	}
>  	if (folio_use_access_time(folio))
>  		folio_xchg_access_time(folio,
>  			jiffies_to_msecs(jiffies));
> @@ -159,6 +168,7 @@ static long change_pte_range(struct mmu_gather *tlb,
>  	bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
>  	bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
>  	bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
> +	int nr;
>  
>  	tlb_change_page_size(tlb, PAGE_SIZE);
>  	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> @@ -173,8 +183,10 @@ static long change_pte_range(struct mmu_gather *tlb,
>  	flush_tlb_batched_pending(vma->vm_mm);
>  	arch_enter_lazy_mmu_mode();
>  	do {
> +		nr = 1;

'nr' resets each iteration.

>  		oldpte = ptep_get(pte);
>  		if (pte_present(oldpte)) {
> +			int max_nr = (end - addr) >> PAGE_SHIFT;

Small nit - 'max_nr' declaration could be moved earlier along with 'nr'.

>  			pte_t ptent;
>  
>  			/*
> @@ -182,8 +194,9 @@ static long change_pte_range(struct mmu_gather *tlb,
>  			 * pages. See similar comment in change_huge_pmd.
>  			 */
>  			if (prot_numa &&
> -			    prot_numa_avoid_fault(vma, addr,
> -						  oldpte, target_node))
> +			    prot_numa_avoid_fault(vma, addr, pte,
> +						  oldpte, target_node,
> +							  max_nr, &nr))
>  					continue;
>  
>  			oldpte = ptep_modify_prot_start(vma, addr, pte);
> @@ -300,7 +313,7 @@ static long change_pte_range(struct mmu_gather *tlb,
>  				pages++;
>  			}
>  		}
> -	} while (pte++, addr += PAGE_SIZE, addr != end);
> +	} while (pte += nr, addr += nr * PAGE_SIZE, addr != end);
>  	arch_leave_lazy_mmu_mode();
>  	pte_unmap_unlock(pte - 1, ptl);
>  

Otherwise LGTM