Message-ID: <599ebe0a-086a-4701-b797-dcd801ad02fb@lucifer.local>
Date: Tue, 3 Feb 2026 13:07:32 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Nico Pache <npache@...hat.com>
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Subject: Re: [PATCH mm-unstable v14 08/16] khugepaged: generalize
 collapse_huge_page for mTHP collapse

On Thu, Jan 22, 2026 at 12:28:33PM -0700, Nico Pache wrote:
> Pass an order and offset to collapse_huge_page to support collapsing anon
> memory to arbitrary orders within a PMD. order indicates what mTHP size we
> are attempting to collapse to, and offset indicates were in the PMD to
> start the collapse attempt.
>
> For non-PMD collapse we must leave the anon VMA write locked until after
> we collapse the mTHP-- in the PMD case all the pages are isolated, but in
> the mTHP case this is not true, and we must keep the lock to prevent
> changes to the VMA from occurring.
>
> Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Tested-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Signed-off-by: Nico Pache <npache@...hat.com>
> ---
>  mm/khugepaged.c | 111 +++++++++++++++++++++++++++++++-----------------
>  1 file changed, 71 insertions(+), 40 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 9b7e05827749..76cb17243793 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1151,44 +1151,54 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru
>  	return SCAN_SUCCEED;
>  }
>
> -static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long address,
> -		int referenced, int unmapped, struct collapse_control *cc)
> +static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long start_addr,
> +		int referenced, int unmapped, struct collapse_control *cc,
> +		bool *mmap_locked, unsigned int order)
>  {
>  	LIST_HEAD(compound_pagelist);
>  	pmd_t *pmd, _pmd;
> -	pte_t *pte;
> +	pte_t *pte = NULL;
>  	pgtable_t pgtable;
>  	struct folio *folio;
>  	spinlock_t *pmd_ptl, *pte_ptl;
>  	enum scan_result result = SCAN_FAIL;
>  	struct vm_area_struct *vma;
>  	struct mmu_notifier_range range;
> +	bool anon_vma_locked = false;
> +	const unsigned long nr_pages = 1UL << order;
> +	const unsigned long pmd_address = start_addr & HPAGE_PMD_MASK;
>
> -	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> +	VM_WARN_ON_ONCE(pmd_address & ~HPAGE_PMD_MASK);
>
>  	/*
>  	 * Before allocating the hugepage, release the mmap_lock read lock.
>  	 * The allocation can take potentially a long time if it involves
>  	 * sync compaction, and we do not need to hold the mmap_lock during
>  	 * that. We will recheck the vma after taking it again in write mode.
> +	 * If collapsing mTHPs we may have already released the read_lock.
>  	 */
> -	mmap_read_unlock(mm);
> +	if (*mmap_locked) {
> +		mmap_read_unlock(mm);
> +		*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, pmd_address, true, &vma, cc, order);

Why would we use the PMD address here rather than the actual start address?

Also please add /*expect_anon=*/ before the 'true' because it's hard to
understand what that references.

>  	if (result != SCAN_SUCCEED) {
>  		mmap_read_unlock(mm);
> +		*mmap_locked = false;
>  		goto out_nolock;
>  	}
>
> -	result = find_pmd_or_thp_or_none(mm, address, &pmd);
> +	result = find_pmd_or_thp_or_none(mm, pmd_address, &pmd);
>  	if (result != SCAN_SUCCEED) {
>  		mmap_read_unlock(mm);
> +		*mmap_locked = false;
>  		goto out_nolock;
>  	}
>
> @@ -1198,13 +1208,16 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
>  		 * 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);
> -		if (result != SCAN_SUCCEED)
> +		result = __collapse_huge_page_swapin(mm, vma, start_addr, pmd,
> +						     referenced, order);
> +		if (result != SCAN_SUCCEED) {
> +			*mmap_locked = false;
>  			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
> @@ -1214,20 +1227,20 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
>  	 * mmap_lock.
>  	 */
>  	mmap_write_lock(mm);
> -	result = hugepage_vma_revalidate(mm, address, true, &vma, cc,
> -					 HPAGE_PMD_ORDER);
> +	result = hugepage_vma_revalidate(mm, pmd_address, true, &vma, cc, order);
>  	if (result != SCAN_SUCCEED)
>  		goto out_up_write;
>  	/* check if the pmd is still valid */
>  	vma_start_write(vma);
> -	result = check_pmd_still_valid(mm, address, pmd);
> +	result = check_pmd_still_valid(mm, pmd_address, pmd);
>  	if (result != SCAN_SUCCEED)
>  		goto out_up_write;
>
>  	anon_vma_lock_write(vma->anon_vma);
> +	anon_vma_locked = true;
>
> -	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, start_addr,
> +				start_addr + (PAGE_SIZE << order));
>  	mmu_notifier_invalidate_range_start(&range);
>
>  	pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
> @@ -1239,24 +1252,21 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
>  	 * Parallel GUP-fast is fine since GUP-fast will back off when
>  	 * it detects PMD is changed.
>  	 */
> -	_pmd = pmdp_collapse_flush(vma, address, pmd);
> +	_pmd = pmdp_collapse_flush(vma, pmd_address, pmd);
>  	spin_unlock(pmd_ptl);
>  	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, start_addr, &pte_ptl);
>  	if (pte) {
> -		result = __collapse_huge_page_isolate(vma, address, pte, cc,
> -						      HPAGE_PMD_ORDER,
> -						      &compound_pagelist);
> +		result = __collapse_huge_page_isolate(vma, start_addr, pte, cc,
> +						      order, &compound_pagelist);
>  		spin_unlock(pte_ptl);
>  	} else {
>  		result = SCAN_NO_PTE_TABLE;
>  	}
>
>  	if (unlikely(result != SCAN_SUCCEED)) {
> -		if (pte)
> -			pte_unmap(pte);
>  		spin_lock(pmd_ptl);
>  		BUG_ON(!pmd_none(*pmd));
>  		/*
> @@ -1266,21 +1276,21 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
>  		 */
>  		pmd_populate(mm, pmd, pmd_pgtable(_pmd));
>  		spin_unlock(pmd_ptl);
> -		anon_vma_unlock_write(vma->anon_vma);
>  		goto out_up_write;
>  	}
>
>  	/*
> -	 * All pages are isolated and locked so anon_vma rmap
> -	 * can't run anymore.
> +	 * For PMD collapse all pages are isolated and locked so anon_vma
> +	 * rmap can't run anymore. For mTHP collapse we must hold the lock
>  	 */
> -	anon_vma_unlock_write(vma->anon_vma);
> +	if (is_pmd_order(order)) {
> +		anon_vma_unlock_write(vma->anon_vma);
> +		anon_vma_locked = false;
> +	}
>
>  	result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
> -					   vma, address, pte_ptl,
> -					   HPAGE_PMD_ORDER,
> -					   &compound_pagelist);
> -	pte_unmap(pte);
> +					   vma, start_addr, pte_ptl,
> +					   order, &compound_pagelist);
>  	if (unlikely(result != SCAN_SUCCEED))
>  		goto out_up_write;
>
> @@ -1290,20 +1300,42 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
>  	 * write.
>  	 */
>  	__folio_mark_uptodate(folio);
> -	pgtable = pmd_pgtable(_pmd);
> +	if (is_pmd_order(order)) { /* PMD collapse */
> +		pgtable = pmd_pgtable(_pmd);
>
> -	spin_lock(pmd_ptl);
> -	BUG_ON(!pmd_none(*pmd));
> -	pgtable_trans_huge_deposit(mm, pmd, pgtable);
> -	map_anon_folio_pmd_nopf(folio, pmd, vma, address);
> +		spin_lock(pmd_ptl);
> +		WARN_ON_ONCE(!pmd_none(*pmd));
> +		pgtable_trans_huge_deposit(mm, pmd, pgtable);
> +		map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_address);
> +	} else { /* mTHP collapse */
> +		pte_t mthp_pte = mk_pte(folio_page(folio, 0), vma->vm_page_prot);
> +
> +		mthp_pte = maybe_mkwrite(pte_mkdirty(mthp_pte), vma);
> +		spin_lock(pmd_ptl);
> +		WARN_ON_ONCE(!pmd_none(*pmd));
> +		folio_ref_add(folio, nr_pages - 1);
> +		folio_add_new_anon_rmap(folio, vma, start_addr, RMAP_EXCLUSIVE);
> +		folio_add_lru_vma(folio, vma);
> +		set_ptes(vma->vm_mm, start_addr, pte, mthp_pte, nr_pages);
> +		update_mmu_cache_range(NULL, vma, start_addr, pte, nr_pages);
> +
> +		smp_wmb(); /* make PTEs visible before PMD. See pmd_install() */
> +		pmd_populate(mm, pmd, pmd_pgtable(_pmd));

I seriously hate this being open-coded, can we separate it out into another
function?

> +	}
>  	spin_unlock(pmd_ptl);
>
>  	folio = NULL;
>
>  	result = SCAN_SUCCEED;
>  out_up_write:
> +	if (anon_vma_locked)
> +		anon_vma_unlock_write(vma->anon_vma);

Thanks it's much better tracking this specifically.

The whole damn thing needs refactoring (by this I mean - khugepaged and really
THP in general to be clear :) but it's not your fault.

Could I ask though whether you might help out with some cleanups after this
lands :)

I feel like we all need to do our bit to pay down some technical debt!

> +	if (pte)
> +		pte_unmap(pte);
>  	mmap_write_unlock(mm);
> +	*mmap_locked = false;
>  out_nolock:
> +	WARN_ON_ONCE(*mmap_locked);
>  	if (folio)
>  		folio_put(folio);
>  	trace_mm_collapse_huge_page(mm, result == SCAN_SUCCEED, result);
> @@ -1471,9 +1503,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
>  	pte_unmap_unlock(pte, ptl);
>  	if (result == SCAN_SUCCEED) {
>  		result = collapse_huge_page(mm, start_addr, referenced,
> -					    unmapped, cc);
> -		/* collapse_huge_page will return with the mmap_lock released */
> -		*mmap_locked = false;
> +					    unmapped, cc, mmap_locked,
> +					    HPAGE_PMD_ORDER);
>  	}
>  out:
>  	trace_mm_khugepaged_scan_pmd(mm, folio, referenced,
> --
> 2.52.0
>

Cheers, Lorenzo

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