Re: [PATCH mm-unstable v18 03/14] mm/khugepaged: rework max_ptes_* handling with helper functions

[Lorenzo Stoakes]

On Fri, May 22, 2026 at 08:59:58AM -0600, Nico Pache wrote:
> The following cleanup reworks all the max_ptes_* handling into helper
> functions. This increases the code readability and will later be used to
> implement the mTHP handling of these variables.
>
> With these changes we abstract all the madvise_collapse() special casing
> (do not respect the sysctls) away from the functions that utilize them.
> And will be used later in this series to cleanly restrict the mTHP
> collapse behavior.
>
> No functional change is intended; however, we are now only reading the
> sysfs variables once per scan, whereas before these variables were being
> read on each loop iteration.
>
> Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> Acked-by: Usama Arif <usama.arif@xxxxxxxxx>
> Signed-off-by: Nico Pache <npache@xxxxxxxxxx>

Had a read through, this is nice, all LGTM, so:

Reviewed-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>

> ---
>  mm/khugepaged.c | 120 +++++++++++++++++++++++++++++++++---------------
>  1 file changed, 84 insertions(+), 36 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 13d82993755f..116f39518948 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -348,6 +348,64 @@ static bool pte_none_or_zero(pte_t pte)
>  	return pte_present(pte) && is_zero_pfn(pte_pfn(pte));
>  }
>
> +/**
> + * collapse_max_ptes_none - Calculate maximum allowed empty PTEs or PTEs mapping
> + * the shared zeropage for the given collapse operation.
> + * @cc: The collapse control struct
> + * @vma: The vma to check for userfaultfd
> + *
> + * Return: Maximum number of empty/shared zeropage PTEs for the collapse operation
> + */
> +static unsigned int collapse_max_ptes_none(struct collapse_control *cc,
> +		struct vm_area_struct *vma)
> +{
> +	if (vma && userfaultfd_armed(vma))
> +		return 0;
> +	/* for MADV_COLLAPSE, allow any empty/shared zeropage PTEs */
> +	if (!cc->is_khugepaged)
> +		return HPAGE_PMD_NR;
> +	/* For all other cases respect the user defined maximum */
> +	return khugepaged_max_ptes_none;
> +}
> +
> +/**
> + * collapse_max_ptes_shared - Calculate maximum allowed PTEs that map shared
> + * anonymous pages for the given collapse operation.
> + * @cc: The collapse control struct
> + *
> + * Return: Maximum number of PTEs that map shared anonymous pages for the
> + * collapse operation
> + */
> +static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
> +{
> +	/*
> +	 * For MADV_COLLAPSE, do not restrict the number of PTEs that map shared
> +	 * anonymous pages.
> +	 */
> +	if (!cc->is_khugepaged)
> +		return HPAGE_PMD_NR;
> +	return khugepaged_max_ptes_shared;
> +}
> +
> +/**
> + * collapse_max_ptes_swap - Calculate the maximum allowed non-present PTEs or the
> + * maximum allowed non-present pagecache entries for the given collapse operation.
> + * @cc: The collapse control struct
> + *
> + * Return: Maximum number of non-present PTEs or the maximum allowed non-present
> + * pagecache entries for the collapse operation.
> + */
> +static unsigned int collapse_max_ptes_swap(struct collapse_control *cc)
> +{
> +	/*
> +	 * For MADV_COLLAPSE, do not restrict the number PTEs entries or
> +	 * pagecache entries that are non-present.
> +	 */
> +	if (!cc->is_khugepaged)
> +		return HPAGE_PMD_NR;
> +	return khugepaged_max_ptes_swap;
> +}
> +
>  int hugepage_madvise(struct vm_area_struct *vma,
>  		     vm_flags_t *vm_flags, int advice)
>  {
> @@ -540,6 +598,8 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  		unsigned long start_addr, pte_t *pte, struct collapse_control *cc,
>  		struct list_head *compound_pagelist)
>  {
> +	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma);
> +	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc);
>  	struct page *page = NULL;
>  	struct folio *folio = NULL;
>  	unsigned long addr = start_addr;
> @@ -551,16 +611,12 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  	     _pte++, addr += PAGE_SIZE) {
>  		pte_t pteval = ptep_get(_pte);
>  		if (pte_none_or_zero(pteval)) {
> -			++none_or_zero;
> -			if (!userfaultfd_armed(vma) &&
> -			    (!cc->is_khugepaged ||
> -			     none_or_zero <= khugepaged_max_ptes_none)) {
> -				continue;
> -			} else {
> +			if (++none_or_zero > max_ptes_none) {
>  				result = SCAN_EXCEED_NONE_PTE;
>  				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
>  				goto out;
>  			}
> +			continue;
>  		}
>  		if (!pte_present(pteval)) {
>  			result = SCAN_PTE_NON_PRESENT;
> @@ -591,9 +647,7 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
>
>  		/* See collapse_scan_pmd(). */
>  		if (folio_maybe_mapped_shared(folio)) {
> -			++shared;
> -			if (cc->is_khugepaged &&
> -			    shared > khugepaged_max_ptes_shared) {
> +			if (++shared > max_ptes_shared) {
>  				result = SCAN_EXCEED_SHARED_PTE;
>  				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
>  				goto out;
> @@ -1262,6 +1316,9 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
>  		struct vm_area_struct *vma, unsigned long start_addr,
>  		bool *lock_dropped, struct collapse_control *cc)
>  {
> +	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma);
> +	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc);
> +	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);
>  	pmd_t *pmd;
>  	pte_t *pte, *_pte;
>  	int none_or_zero = 0, shared = 0, referenced = 0;
> @@ -1295,36 +1352,29 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
>
>  		pte_t pteval = ptep_get(_pte);
>  		if (pte_none_or_zero(pteval)) {
> -			++none_or_zero;
> -			if (!userfaultfd_armed(vma) &&
> -			    (!cc->is_khugepaged ||
> -			     none_or_zero <= khugepaged_max_ptes_none)) {
> -				continue;
> -			} else {
> +			if (++none_or_zero > max_ptes_none) {
>  				result = SCAN_EXCEED_NONE_PTE;
>  				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
>  				goto out_unmap;
>  			}
> +			continue;
>  		}
>  		if (!pte_present(pteval)) {
> -			++unmapped;
> -			if (!cc->is_khugepaged ||
> -			    unmapped <= khugepaged_max_ptes_swap) {
> -				/*
> -				 * Always be strict with uffd-wp
> -				 * enabled swap entries.  Please see
> -				 * comment below for pte_uffd_wp().
> -				 */
> -				if (pte_swp_uffd_wp_any(pteval)) {
> -					result = SCAN_PTE_UFFD_WP;
> -					goto out_unmap;
> -				}
> -				continue;
> -			} else {
> +			if (++unmapped > max_ptes_swap) {
>  				result = SCAN_EXCEED_SWAP_PTE;
>  				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
>  				goto out_unmap;
>  			}
> +			/*
> +			 * Always be strict with uffd-wp
> +			 * enabled swap entries.  Please see
> +			 * comment below for pte_uffd_wp().
> +			 */
> +			if (pte_swp_uffd_wp_any(pteval)) {
> +				result = SCAN_PTE_UFFD_WP;
> +				goto out_unmap;
> +			}
> +			continue;
>  		}
>  		if (pte_uffd_wp(pteval)) {
>  			/*
> @@ -1367,9 +1417,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
>  		 * is shared.
>  		 */
>  		if (folio_maybe_mapped_shared(folio)) {
> -			++shared;
> -			if (cc->is_khugepaged &&
> -			    shared > khugepaged_max_ptes_shared) {
> +			if (++shared > max_ptes_shared) {
>  				result = SCAN_EXCEED_SHARED_PTE;
>  				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
>  				goto out_unmap;
> @@ -2324,6 +2372,8 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>  		unsigned long addr, struct file *file, pgoff_t start,
>  		struct collapse_control *cc)
>  {
> +	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL);
> +	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);
>  	struct folio *folio = NULL;
>  	struct address_space *mapping = file->f_mapping;
>  	XA_STATE(xas, &mapping->i_pages, start);
> @@ -2342,8 +2392,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>
>  		if (xa_is_value(folio)) {
>  			swap += 1 << xas_get_order(&xas);
> -			if (cc->is_khugepaged &&
> -			    swap > khugepaged_max_ptes_swap) {
> +			if (swap > max_ptes_swap) {
>  				result = SCAN_EXCEED_SWAP_PTE;
>  				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
>  				break;
> @@ -2414,8 +2463,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>  		cc->progress += HPAGE_PMD_NR;
>
>  	if (result == SCAN_SUCCEED) {
> -		if (cc->is_khugepaged &&
> -		    present < HPAGE_PMD_NR - khugepaged_max_ptes_none) {
> +		if (present < HPAGE_PMD_NR - max_ptes_none) {
>  			result = SCAN_EXCEED_NONE_PTE;
>  			count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
>  		} else {
> --
> 2.54.0
>