Re: [PATCH mm-unstable v18 04/14] mm/khugepaged: generalize __collapse_huge_page_* for mTHP support

[Lorenzo Stoakes <ljs@kernel.org>]

On Fri, May 22, 2026 at 08:59:59AM -0600, Nico Pache wrote:
> generalize the order of the __collapse_huge_page_* and collapse_max_*
> functions to support future mTHP collapse.
>
> The current mechanism for determining collapse with the
> khugepaged_max_ptes_none value is not designed with mTHP in mind. This
> raises a key design issue: if we support user defined max_pte_none values
> (even those scaled by order), a collapse of a lower order can introduces
> an feedback loop, or "creep", when max_ptes_none is set to a value greater
> than HPAGE_PMD_NR / 2. [1]
>
> With this configuration, a successful collapse to order N will populate
> enough pages to satisfy the collapse condition on order N+1 on the next
> scan. This leads to unnecessary work and memory churn.
>
> To fix this issue introduce a helper function that will limit mTHP
> collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1.
> This effectively supports two modes: [2]
>
> - max_ptes_none=0: never collapses if it encounters an empty PTE or a PTE
>   that maps the shared zeropage. Consequently, no memory bloat.
> - max_ptes_none=511 (on 4k pagesz): Always collapse to the highest
>   available mTHP order.
>
> This removes the possibility of "creep", and a warning will be emitted if
> any non-supported max_ptes_none value is configured with mTHP enabled.
> Any intermediate value will default mTHP collapse to max_ptes_none=0.
>
> mTHP collapse will not honor the khugepaged_max_ptes_shared or
> khugepaged_max_ptes_swap parameters, and will fail if it encounters a
> shared or swapped entry.
>
> No functional changes in this patch; however it defines future behavior
> for mTHP collapse.
>
> [1] - https://lore.kernel.org/all/e46ab3ab-a3d7-4fb7-9970-d0704bd5d05a@arm.com
> [2] - https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@redhat.com
>
> Reviewed-by: Lance Yang <lance.yang@linux.dev>
> Co-developed-by: Dev Jain <dev.jain@arm.com>
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> Signed-off-by: Nico Pache <npache@redhat.com>

One small nit below, and with the fixpatch assumed applied, LGTM so:

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>

> ---
>  mm/khugepaged.c | 121 +++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 88 insertions(+), 33 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 116f39518948..e98ba5b15163 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -353,30 +353,52 @@ static bool pte_none_or_zero(pte_t pte)
>   * the shared zeropage for the given collapse operation.
>   * @cc: The collapse control struct
>   * @vma: The vma to check for userfaultfd
> + * @order: The folio order being collapsed to
>   *
>   * 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)
> +		struct vm_area_struct *vma, unsigned int order)
>  {
> +	unsigned int max_ptes_none = khugepaged_max_ptes_none;
> +
>  	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;
> +	/* for PMD collapse, respect the user defined maximum */
> +	if (is_pmd_order(order))
> +		return max_ptes_none;
> +	/*
> +	 * for mTHP collapse with the sysctl value set to KHUGEPAGED_MAX_PTES_LIMIT,
> +	 * scale the maximum number of PTEs to the order of the collapse.
> +	 */
> +	if (max_ptes_none == KHUGEPAGED_MAX_PTES_LIMIT)
> +		return (1 << order) - 1;
> +	if (!max_ptes_none)
> +		return 0;
> +	/*
> +	 * For mTHP collapse of values other than 0 or KHUGEPAGED_MAX_PTES_LIMIT,
> +	 * emit a warning and return 0.
> +	 */
> +	pr_warn_once("mTHP collapse does not support max_ptes_none values"
> +		     " other than 0 or %u, defaulting to 0.\n",
> +		     KHUGEPAGED_MAX_PTES_LIMIT);
> +	return 0;
>  }
>
>  /**
>   * collapse_max_ptes_shared - Calculate maximum allowed PTEs that map shared
>   * anonymous pages for the given collapse operation.
>   * @cc: The collapse control struct
> + * @order: The folio order being collapsed to
>   *
>   * 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)
> +static unsigned int collapse_max_ptes_shared(struct collapse_control *cc,
> +		unsigned int order)
>  {
>  	/*
>  	 * For MADV_COLLAPSE, do not restrict the number of PTEs that map shared
> @@ -384,6 +406,13 @@ static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
>  	 */
>  	if (!cc->is_khugepaged)
>  		return HPAGE_PMD_NR;
> +	/*
> +	 * for mTHP collapse do not allow collapsing anonymous memory pages that
> +	 * are shared between processes.
> +	 */
> +	if (!is_pmd_order(order))
> +		return 0;
> +	/* for PMD collapse, respect the user defined maximum */
>  	return khugepaged_max_ptes_shared;
>  }
>
> @@ -391,11 +420,13 @@ static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
>   * 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
> + * @order: The folio order being collapsed to
>   *
>   * 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)
> +static unsigned int collapse_max_ptes_swap(struct collapse_control *cc,
> +		unsigned int order)
>  {
>  	/*
>  	 * For MADV_COLLAPSE, do not restrict the number PTEs entries or
> @@ -403,6 +434,10 @@ static unsigned int collapse_max_ptes_swap(struct collapse_control *cc)
>  	 */
>  	if (!cc->is_khugepaged)
>  		return HPAGE_PMD_NR;
> +	/* for mTHP collapse do not allow any non-present PTEs or pagecache entries */
> +	if (!is_pmd_order(order))
> +		return 0;
> +	/* for PMD collapse, respect the user defined maximum */
>  	return khugepaged_max_ptes_swap;
>  }
>
> @@ -596,10 +631,11 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
>
>  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)
> +		unsigned int order, 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);
> +	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma, order);
> +	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, order);
> +	const unsigned long nr_pages = 1UL << order;
>  	struct page *page = NULL;
>  	struct folio *folio = NULL;
>  	unsigned long addr = start_addr;
> @@ -607,7 +643,7 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  	int none_or_zero = 0, shared = 0, referenced = 0;
>  	enum scan_result result = SCAN_FAIL;
>
> -	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> +	for (_pte = pte; _pte < pte + nr_pages;
>  	     _pte++, addr += PAGE_SIZE) {
>  		pte_t pteval = ptep_get(_pte);
>  		if (pte_none_or_zero(pteval)) {
> @@ -740,18 +776,18 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  }
>
>  static void __collapse_huge_page_copy_succeeded(pte_t *pte,
> -						struct vm_area_struct *vma,
> -						unsigned long address,
> -						spinlock_t *ptl,
> -						struct list_head *compound_pagelist)
> +		struct vm_area_struct *vma, unsigned long address,
> +		spinlock_t *ptl, unsigned int order,
> +		struct list_head *compound_pagelist)
>  {
> -	unsigned long end = address + HPAGE_PMD_SIZE;
> +	const unsigned long nr_pages = 1UL << order;
> +	unsigned long end = address + (PAGE_SIZE << order);

Nit: could be address + PAGE_SIZE * nr_pages, be a little nicer as you already
did the << order operation above.

>  	struct folio *src, *tmp;
>  	pte_t pteval;
>  	pte_t *_pte;
>  	unsigned int nr_ptes;
>
> -	for (_pte = pte; _pte < pte + HPAGE_PMD_NR; _pte += nr_ptes,
> +	for (_pte = pte; _pte < pte + nr_pages; _pte += nr_ptes,
>  	     address += nr_ptes * PAGE_SIZE) {
>  		nr_ptes = 1;
>  		pteval = ptep_get(_pte);
> @@ -804,11 +840,10 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
>  }
>
>  static void __collapse_huge_page_copy_failed(pte_t *pte,
> -					     pmd_t *pmd,
> -					     pmd_t orig_pmd,
> -					     struct vm_area_struct *vma,
> -					     struct list_head *compound_pagelist)
> +		pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma,
> +		unsigned int order, struct list_head *compound_pagelist)
>  {
> +	const unsigned long nr_pages = 1UL << order;
>  	spinlock_t *pmd_ptl;
>
>  	/*
> @@ -824,7 +859,7 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
>  	 * Release both raw and compound pages isolated
>  	 * in __collapse_huge_page_isolate.
>  	 */
> -	release_pte_pages(pte, pte + HPAGE_PMD_NR, compound_pagelist);
> +	release_pte_pages(pte, pte + nr_pages, compound_pagelist);
>  }
>
>  /*
> @@ -844,16 +879,17 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
>   */
>  static enum scan_result __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
>  		pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma,
> -		unsigned long address, spinlock_t *ptl,
> +		unsigned long address, spinlock_t *ptl, unsigned int order,
>  		struct list_head *compound_pagelist)
>  {
> +	const unsigned long nr_pages = 1UL << order;
>  	unsigned int i;
>  	enum scan_result result = SCAN_SUCCEED;
>
>  	/*
>  	 * Copying pages' contents is subject to memory poison at any iteration.
>  	 */
> -	for (i = 0; i < HPAGE_PMD_NR; i++) {
> +	for (i = 0; i < nr_pages; i++) {
>  		pte_t pteval = ptep_get(pte + i);
>  		struct page *page = folio_page(folio, i);
>  		unsigned long src_addr = address + i * PAGE_SIZE;
> @@ -872,10 +908,10 @@ static enum scan_result __collapse_huge_page_copy(pte_t *pte, struct folio *foli
>
>  	if (likely(result == SCAN_SUCCEED))
>  		__collapse_huge_page_copy_succeeded(pte, vma, address, ptl,
> -						    compound_pagelist);
> +						    order, compound_pagelist);
>  	else
>  		__collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
> -						 compound_pagelist);
> +						 order, compound_pagelist);
>
>  	return result;
>  }
> @@ -1042,16 +1078,20 @@ static enum scan_result check_pmd_still_valid(struct mm_struct *mm,
>   * Bring missing pages in from swap, to complete THP collapse.
>   * Only done if khugepaged_scan_pmd believes it is worthwhile.
>   *
> + * For mTHP orders the function bails on the first swap entry, because
> + * faulting pages back in during collapse could re-populate PTEs that
> + * push a later scan over the threshold for a higher-order collapse.
> + *
>   * Called and returns without pte mapped or spinlocks held.
>   * Returns result: if not SCAN_SUCCEED, mmap_lock has been released.
>   */
>  static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
> -		struct vm_area_struct *vma, unsigned long start_addr, pmd_t *pmd,
> -		int referenced)
> +		struct vm_area_struct *vma, unsigned long start_addr,
> +		pmd_t *pmd, int referenced, unsigned int order)
>  {
>  	int swapped_in = 0;
>  	vm_fault_t ret = 0;
> -	unsigned long addr, end = start_addr + (HPAGE_PMD_NR * PAGE_SIZE);
> +	unsigned long addr, end = start_addr + (PAGE_SIZE << order);
>  	enum scan_result result;
>  	pte_t *pte = NULL;
>  	spinlock_t *ptl;
> @@ -1083,6 +1123,19 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
>  		    pte_present(vmf.orig_pte))
>  			continue;
>
> +		/*
> +		 * TODO: Support swapin without leading to further mTHP
> +		 * collapses. Currently bringing in new pages via swapin may
> +		 * cause a future higher order collapse on a rescan of the same
> +		 * range.
> +		 */
> +		if (!is_pmd_order(order)) {
> +			pte_unmap(pte);
> +			mmap_read_unlock(mm);
> +			result = SCAN_EXCEED_SWAP_PTE;
> +			goto out;
> +		}
> +
>  		vmf.pte = pte;
>  		vmf.ptl = ptl;
>  		ret = do_swap_page(&vmf);
> @@ -1203,7 +1256,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
>  		 * that case.  Continuing to collapse causes inconsistency.
>  		 */
>  		result = __collapse_huge_page_swapin(mm, vma, address, pmd,
> -						     referenced);
> +						     referenced, HPAGE_PMD_ORDER);
>  		if (result != SCAN_SUCCEED)
>  			goto out_nolock;
>  	}
> @@ -1251,6 +1304,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
>  	pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
>  	if (pte) {
>  		result = __collapse_huge_page_isolate(vma, address, pte, cc,
> +						      HPAGE_PMD_ORDER,
>  						      &compound_pagelist);
>  		spin_unlock(pte_ptl);
>  	} else {
> @@ -1281,6 +1335,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
>
>  	result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
>  					   vma, address, pte_ptl,
> +					   HPAGE_PMD_ORDER,
>  					   &compound_pagelist);
>  	pte_unmap(pte);
>  	if (unlikely(result != SCAN_SUCCEED))
> @@ -1316,9 +1371,9 @@ 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);
> +	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER);
> +	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, HPAGE_PMD_ORDER);
> +	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);
>  	pmd_t *pmd;
>  	pte_t *pte, *_pte;
>  	int none_or_zero = 0, shared = 0, referenced = 0;
> @@ -2372,8 +2427,8 @@ 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);
> +	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL, HPAGE_PMD_ORDER);
> +	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);
>  	struct folio *folio = NULL;
>  	struct address_space *mapping = file->f_mapping;
>  	XA_STATE(xas, &mapping->i_pages, start);
> --
> 2.54.0
>