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 <[email protected]>
> Tested-by: Baolin Wang <[email protected]>
> Signed-off-by: Nico Pache <[email protected]>
> ---
> 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
