Re: [PATCH v5 2/7] mm: Optimize mprotect() for MM_CP_PROT_NUMA by batch-skipping PTEs

[Ryan Roberts <ryan.roberts@arm.com>]

On 18/07/2025 10:02, Dev Jain wrote:
> For the MM_CP_PROT_NUMA skipping case, observe that, if we skip an
> iteration due to the underlying folio satisfying any of the skip
> conditions, then for all subsequent ptes which map the same folio, the
> iteration will be skipped for them too. Therefore, we can optimize
> by using folio_pte_batch() to batch skip the iterations.
> 
> Use prot_numa_skip() introduced in the previous patch to determine whether
> we need to skip the iteration. Change its signature to have a double
> pointer to a folio, which will be used by mprotect_folio_pte_batch() to
> determine the number of iterations we can safely skip.
> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>

LGTM:

Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>

> ---
>  mm/mprotect.c | 55 +++++++++++++++++++++++++++++++++++++++------------
>  1 file changed, 42 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 2a9c73bd0778..97adc62c50ab 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -83,28 +83,43 @@ bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>  	return pte_dirty(pte);
>  }
>  
> +static int mprotect_folio_pte_batch(struct folio *folio, pte_t *ptep,
> +				    pte_t pte, int max_nr_ptes)
> +{
> +	/* No underlying folio, so cannot batch */
> +	if (!folio)
> +		return 1;
> +
> +	if (!folio_test_large(folio))
> +		return 1;
> +
> +	return folio_pte_batch(folio, ptep, pte, max_nr_ptes);
> +}
> +
>  static bool prot_numa_skip(struct vm_area_struct *vma, unsigned long addr,
> -			   pte_t oldpte, pte_t *pte, int target_node)
> +			   pte_t oldpte, pte_t *pte, int target_node,
> +			   struct folio **foliop)
>  {
> -	struct folio *folio;
> +	struct folio *folio = NULL;
> +	bool ret = true;
>  	bool toptier;
>  	int nid;
>  
>  	/* Avoid TLB flush if possible */
>  	if (pte_protnone(oldpte))
> -		return true;
> +		goto skip;
>  
>  	folio = vm_normal_folio(vma, addr, oldpte);
>  	if (!folio)
> -		return true;
> +		goto skip;
>  
>  	if (folio_is_zone_device(folio) || folio_test_ksm(folio))
> -		return true;
> +		goto skip;
>  
>  	/* Also skip shared copy-on-write pages */
>  	if (is_cow_mapping(vma->vm_flags) &&
>  	    (folio_maybe_dma_pinned(folio) || folio_maybe_mapped_shared(folio)))
> -		return true;
> +		goto skip;
>  
>  	/*
>  	 * While migration can move some dirty pages,
> @@ -112,7 +127,7 @@ static bool prot_numa_skip(struct vm_area_struct *vma, unsigned long addr,
>  	 * context.
>  	 */
>  	if (folio_is_file_lru(folio) && folio_test_dirty(folio))
> -		return true;
> +		goto skip;
>  
>  	/*
>  	 * Don't mess with PTEs if page is already on the node
> @@ -120,7 +135,7 @@ static bool prot_numa_skip(struct vm_area_struct *vma, unsigned long addr,
>  	 */
>  	nid = folio_nid(folio);
>  	if (target_node == nid)
> -		return true;
> +		goto skip;
>  
>  	toptier = node_is_toptier(nid);
>  
> @@ -129,11 +144,15 @@ static bool prot_numa_skip(struct vm_area_struct *vma, unsigned long addr,
>  	 * balancing is disabled
>  	 */
>  	if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_NORMAL) && toptier)
> -		return true;
> +		goto skip;
>  
> +	ret = false;
>  	if (folio_use_access_time(folio))
>  		folio_xchg_access_time(folio, jiffies_to_msecs(jiffies));
> -	return false;
> +
> +skip:
> +	*foliop = folio;
> +	return ret;
>  }
>  
>  static long change_pte_range(struct mmu_gather *tlb,
> @@ -147,6 +166,7 @@ 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_ptes;
>  
>  	tlb_change_page_size(tlb, PAGE_SIZE);
>  	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> @@ -161,8 +181,11 @@ static long change_pte_range(struct mmu_gather *tlb,
>  	flush_tlb_batched_pending(vma->vm_mm);
>  	arch_enter_lazy_mmu_mode();
>  	do {
> +		nr_ptes = 1;
>  		oldpte = ptep_get(pte);
>  		if (pte_present(oldpte)) {
> +			int max_nr_ptes = (end - addr) >> PAGE_SHIFT;
> +			struct folio *folio;
>  			pte_t ptent;
>  
>  			/*
> @@ -170,9 +193,15 @@ static long change_pte_range(struct mmu_gather *tlb,
>  			 * pages. See similar comment in change_huge_pmd.
>  			 */
>  			if (prot_numa) {
> -				if (prot_numa_skip(vma, addr, oldpte, pte,
> -						   target_node))
> +				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);
>  					continue;
> +				}
>  			}
>  
>  			oldpte = ptep_modify_prot_start(vma, addr, pte);
> @@ -289,7 +318,7 @@ static long change_pte_range(struct mmu_gather *tlb,
>  				pages++;
>  			}
>  		}
> -	} while (pte++, addr += PAGE_SIZE, addr != end);
> +	} while (pte += nr_ptes, addr += nr_ptes * PAGE_SIZE, addr != end);
>  	arch_leave_lazy_mmu_mode();
>  	pte_unmap_unlock(pte - 1, ptl);
>