Re: [RFC v2 7/9] khugepaged: add mTHP support

[Dev Jain <dev.jain@arm.com>]

On 11/02/25 6:00 am, Nico Pache wrote:
> Introduce the ability for khugepaged to collapse to different mTHP sizes.
> While scanning a PMD range for potential collapse candidates, keep track
> of pages in MIN_MTHP_ORDER chunks via a bitmap. Each bit represents a
> utilized region of order MIN_MTHP_ORDER ptes. We remove the restriction
> of max_ptes_none during the scan phase so we dont bailout early and miss
> potential mTHP candidates.
> 
> After the scan is complete we will perform binary recursion on the
> bitmap to determine which mTHP size would be most efficient to collapse
> to. max_ptes_none will be scaled by the attempted collapse order to
> determine how full a THP must be to be eligible.
> 
> If a mTHP collapse is attempted, but contains swapped out, or shared
> pages, we dont perform the collapse.
> 
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>   mm/khugepaged.c | 122 ++++++++++++++++++++++++++++++++----------------
>   1 file changed, 83 insertions(+), 39 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index c8048d9ec7fb..cd310989725b 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1127,13 +1127,14 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>   {
>   	LIST_HEAD(compound_pagelist);
>   	pmd_t *pmd, _pmd;
> -	pte_t *pte;
> +	pte_t *pte, mthp_pte;
>   	pgtable_t pgtable;
>   	struct folio *folio;
>   	spinlock_t *pmd_ptl, *pte_ptl;
>   	int result = SCAN_FAIL;
>   	struct vm_area_struct *vma;
>   	struct mmu_notifier_range range;
> +	unsigned long _address = address + offset * PAGE_SIZE;
>   	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>   
>   	/*
> @@ -1148,12 +1149,13 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>   		*mmap_locked = false;
>   	}
>   
> -	result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
> +	result = alloc_charge_folio(&folio, mm, cc, order);
>   	if (result != SCAN_SUCCEED)
>   		goto out_nolock;
>   
>   	mmap_read_lock(mm);
> -	result = hugepage_vma_revalidate(mm, address, true, &vma, cc, HPAGE_PMD_ORDER);
> +	*mmap_locked = true;
> +	result = hugepage_vma_revalidate(mm, address, true, &vma, cc, order);
>   	if (result != SCAN_SUCCEED) {
>   		mmap_read_unlock(mm);
>   		goto out_nolock;
> @@ -1171,13 +1173,14 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>   		 * released when it fails. So we jump out_nolock directly in
>   		 * that case.  Continuing to collapse causes inconsistency.
>   		 */
> -		result = __collapse_huge_page_swapin(mm, vma, address, pmd,
> -				referenced, HPAGE_PMD_ORDER);
> +		result = __collapse_huge_page_swapin(mm, vma, _address, pmd,
> +				referenced, order);
>   		if (result != SCAN_SUCCEED)
>   			goto out_nolock;
>   	}
>   
>   	mmap_read_unlock(mm);
> +	*mmap_locked = false;
>   	/*
>   	 * Prevent all access to pagetables with the exception of
>   	 * gup_fast later handled by the ptep_clear_flush and the VM
> @@ -1187,7 +1190,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>   	 * mmap_lock.
>   	 */
>   	mmap_write_lock(mm);
> -	result = hugepage_vma_revalidate(mm, address, true, &vma, cc, HPAGE_PMD_ORDER);
> +	result = hugepage_vma_revalidate(mm, address, true, &vma, cc, order);
>   	if (result != SCAN_SUCCEED)
>   		goto out_up_write;
>   	/* check if the pmd is still valid */
> @@ -1198,11 +1201,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>   	vma_start_write(vma);
>   	anon_vma_lock_write(vma->anon_vma);
>   
> -	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address,
> -				address + HPAGE_PMD_SIZE);
> +	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, _address,
> +				_address + (PAGE_SIZE << order));

As I have mentioned before, since you are isolating the PTE table in 
both cases, you need to do the mmu_notifier_* stuff for HPAGE_PMD_SIZE 
in any case. Check out patch 8 from my patchset.

>   	mmu_notifier_invalidate_range_start(&range);
>   
>   	pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
> +
>   	/*
>   	 * This removes any huge TLB entry from the CPU so we won't allow
>   	 * huge and small TLB entries for the same virtual address to
> @@ -1216,10 +1220,10 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>   	mmu_notifier_invalidate_range_end(&range);
>   	tlb_remove_table_sync_one();
>   
> -	pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
> +	pte = pte_offset_map_lock(mm, &_pmd, _address, &pte_ptl);
>   	if (pte) {
> -		result = __collapse_huge_page_isolate(vma, address, pte, cc,
> -					&compound_pagelist, HPAGE_PMD_ORDER);
> +		result = __collapse_huge_page_isolate(vma, _address, pte, cc,
> +					&compound_pagelist, order);
>   		spin_unlock(pte_ptl);
>   	} else {
>   		result = SCAN_PMD_NULL;
> @@ -1248,8 +1252,8 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>   	anon_vma_unlock_write(vma->anon_vma);
>   
>   	result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
> -					   vma, address, pte_ptl,
> -					   &compound_pagelist, HPAGE_PMD_ORDER);
> +					   vma, _address, pte_ptl,
> +					   &compound_pagelist, order);
>   	pte_unmap(pte);
>   	if (unlikely(result != SCAN_SUCCEED))
>   		goto out_up_write;
> @@ -1260,20 +1264,37 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>   	 * write.
>   	 */
>   	__folio_mark_uptodate(folio);
> -	pgtable = pmd_pgtable(_pmd);
> -
> -	_pmd = mk_huge_pmd(&folio->page, vma->vm_page_prot);
> -	_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
> -
> -	spin_lock(pmd_ptl);
> -	BUG_ON(!pmd_none(*pmd));
> -	folio_add_new_anon_rmap(folio, vma, address, RMAP_EXCLUSIVE);
> -	folio_add_lru_vma(folio, vma);
> -	pgtable_trans_huge_deposit(mm, pmd, pgtable);
> -	set_pmd_at(mm, address, pmd, _pmd);
> -	update_mmu_cache_pmd(vma, address, pmd);
> -	deferred_split_folio(folio, false);
> -	spin_unlock(pmd_ptl);

My personal opinion is that this if-else nesting looks really weird. 
This is why I separated out mTHP collapse into a separate function.


> +	if (order == HPAGE_PMD_ORDER) {
> +		pgtable = pmd_pgtable(_pmd);
> +		_pmd = mk_huge_pmd(&folio->page, vma->vm_page_prot);
> +		_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
> +
> +		spin_lock(pmd_ptl);
> +		BUG_ON(!pmd_none(*pmd));
> +		folio_add_new_anon_rmap(folio, vma, _address, RMAP_EXCLUSIVE);
> +		folio_add_lru_vma(folio, vma);
> +		pgtable_trans_huge_deposit(mm, pmd, pgtable);
> +		set_pmd_at(mm, address, pmd, _pmd);
> +		update_mmu_cache_pmd(vma, address, pmd);
> +		deferred_split_folio(folio, false);
> +		spin_unlock(pmd_ptl);
> +	} else { //mTHP
> +		mthp_pte = mk_pte(&folio->page, vma->vm_page_prot);
> +		mthp_pte = maybe_mkwrite(pte_mkdirty(mthp_pte), vma);
> +
> +		spin_lock(pmd_ptl);
> +		folio_ref_add(folio, (1 << order) - 1);
> +		folio_add_new_anon_rmap(folio, vma, _address, RMAP_EXCLUSIVE);
> +		folio_add_lru_vma(folio, vma);
> +		spin_lock(pte_ptl);
> +		set_ptes(vma->vm_mm, _address, pte, mthp_pte, (1 << order));
> +		update_mmu_cache_range(NULL, vma, _address, pte, (1 << order));
> +		spin_unlock(pte_ptl);
> +		smp_wmb(); /* make pte visible before pmd */
> +		pmd_populate(mm, pmd, pmd_pgtable(_pmd));
> +		deferred_split_folio(folio, false);
> +		spin_unlock(pmd_ptl);
> +	}
>   
>   	folio = NULL;
>   
> @@ -1353,21 +1374,27 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>   {
>   	pmd_t *pmd;
>   	pte_t *pte, *_pte;
> +	int i;
>   	int result = SCAN_FAIL, referenced = 0;
>   	int none_or_zero = 0, shared = 0;
>   	struct page *page = NULL;
>   	struct folio *folio = NULL;
>   	unsigned long _address;
> +	unsigned long enabled_orders;
>   	spinlock_t *ptl;
>   	int node = NUMA_NO_NODE, unmapped = 0;
>   	bool writable = false;
> -
> +	int chunk_none_count = 0;
> +	int scaled_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - MIN_MTHP_ORDER);
> +	unsigned long tva_flags = cc->is_khugepaged ? TVA_ENFORCE_SYSFS : 0;
>   	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>   
>   	result = find_pmd_or_thp_or_none(mm, address, &pmd);
>   	if (result != SCAN_SUCCEED)
>   		goto out;
>   
> +	bitmap_zero(cc->mthp_bitmap, MAX_MTHP_BITMAP_SIZE);
> +	bitmap_zero(cc->mthp_bitmap_temp, MAX_MTHP_BITMAP_SIZE);
>   	memset(cc->node_load, 0, sizeof(cc->node_load));
>   	nodes_clear(cc->alloc_nmask);
>   	pte = pte_offset_map_lock(mm, pmd, address, &ptl);
> @@ -1376,8 +1403,12 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>   		goto out;
>   	}
>   
> -	for (_address = address, _pte = pte; _pte < pte + HPAGE_PMD_NR;
> -	     _pte++, _address += PAGE_SIZE) {
> +	for (i = 0; i < HPAGE_PMD_NR; i++) {
> +		if (i % MIN_MTHP_NR == 0)
> +			chunk_none_count = 0;
> +
> +		_pte = pte + i;
> +		_address = address + i * PAGE_SIZE;
>   		pte_t pteval = ptep_get(_pte);
>   		if (is_swap_pte(pteval)) {
>   			++unmapped;
> @@ -1400,16 +1431,14 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>   			}
>   		}
>   		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> +			++chunk_none_count;
>   			++none_or_zero;
> -			if (!userfaultfd_armed(vma) &&
> -			    (!cc->is_khugepaged ||
> -			     none_or_zero <= khugepaged_max_ptes_none)) {
> -				continue;
> -			} else {
> +			if (userfaultfd_armed(vma)) {
>   				result = SCAN_EXCEED_NONE_PTE;
>   				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
>   				goto out_unmap;
>   			}
> +			continue;
>   		}
>   		if (pte_uffd_wp(pteval)) {
>   			/*
> @@ -1500,7 +1529,16 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>   		     folio_test_referenced(folio) || mmu_notifier_test_young(vma->vm_mm,
>   								     address)))
>   			referenced++;
> +
> +		/*
> +		 * we are reading in MIN_MTHP_NR page chunks. if there are no empty
> +		 * pages keep track of it in the bitmap for mTHP collapsing.
> +		 */
> +		if (chunk_none_count < scaled_none &&
> +			(i + 1) % MIN_MTHP_NR == 0)
> +			bitmap_set(cc->mthp_bitmap, i / MIN_MTHP_NR, 1);
>   	}
> +
>   	if (!writable) {
>   		result = SCAN_PAGE_RO;
>   	} else if (cc->is_khugepaged &&
> @@ -1513,10 +1551,14 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>   out_unmap:
>   	pte_unmap_unlock(pte, ptl);
>   	if (result == SCAN_SUCCEED) {
> -		result = collapse_huge_page(mm, address, referenced,
> -					    unmapped, cc, mmap_locked, HPAGE_PMD_ORDER, 0);
> -		/* collapse_huge_page will return with the mmap_lock released */
> -		*mmap_locked = false;
> +		enabled_orders = thp_vma_allowable_orders(vma, vma->vm_flags,
> +			tva_flags, THP_ORDERS_ALL_ANON);
> +		result = khugepaged_scan_bitmap(mm, address, referenced, unmapped, cc,
> +			       mmap_locked, enabled_orders);
> +		if (result > 0)
> +			result = SCAN_SUCCEED;
> +		else
> +			result = SCAN_FAIL;
>   	}
>   out:
>   	trace_mm_khugepaged_scan_pmd(mm, &folio->page, writable, referenced,
> @@ -2476,11 +2518,13 @@ static int khugepaged_collapse_single_pmd(unsigned long addr, struct mm_struct *
>   			fput(file);
>   			if (result == SCAN_PTE_MAPPED_HUGEPAGE) {
>   				mmap_read_lock(mm);
> +				*mmap_locked = true;
>   				if (khugepaged_test_exit_or_disable(mm))
>   					goto end;
>   				result = collapse_pte_mapped_thp(mm, addr,
>   								 !cc->is_khugepaged);
>   				mmap_read_unlock(mm);
> +				*mmap_locked = false;
>   			}
>   		} else {
>   			result = khugepaged_scan_pmd(mm, vma, addr,