On Thu, Jun 4, 2026 at 4:22 AM Lorenzo Stoakes <[email protected]> wrote: > > On Fri, May 22, 2026 at 09:00:01AM -0600, 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 > > access/changes to the page tables. This can happen if the rmap walkers hit > > a pmd_none while the PMD entry is currently unavailable due to being > > temporarily removed during the collapse phase. > > > > Acked-by: Usama Arif <[email protected]> > > Signed-off-by: Nico Pache <[email protected]> > > --- > > mm/khugepaged.c | 93 +++++++++++++++++++++++++++++-------------------- > > 1 file changed, 55 insertions(+), 38 deletions(-) > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > index fab35d318641..d64f42f66236 100644 > > --- a/mm/khugepaged.c > > +++ b/mm/khugepaged.c > > @@ -1214,34 +1214,36 @@ static enum scan_result alloc_charge_folio(struct > > folio **foliop, struct mm_stru > > * while allocating a THP, as that could trigger direct reclaim/compaction. > > * Note that the VMA must be rechecked after grabbing the mmap_lock again. > > */ > > -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, > > + unsigned int order) > > { > > + const unsigned long pmd_addr = start_addr & HPAGE_PMD_MASK; > > + const unsigned long end_addr = start_addr + (PAGE_SIZE << order); > > LIST_HEAD(compound_pagelist); > > pmd_t *pmd, _pmd; > > - pte_t *pte; > > + pte_t *pte = NULL; > > Hmm, this part of the patch wasn't taken, and now we have uninitialised state > being dereferenced (see [0])
Good catch, I was just looking at your report and wondering what happened there. Hopefully, with the v19 we apply this correctly :) -- Nico > > [0]:https://lore.kernel.org/all/aiFO1RlpZ7Ki44y1@lucifer/ > > Did a review comment here somehow cause this to be changed in the patch? > > Andrew - was there an error in applying the patch somehow? > > Thanks, Lorenzo > > > 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; > > > > - VM_BUG_ON(address & ~HPAGE_PMD_MASK); > > - > > - 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); > > + result = hugepage_vma_revalidate(mm, pmd_addr, /*expect_anon=*/ true, > > + &vma, cc, order); > > if (result != SCAN_SUCCEED) { > > mmap_read_unlock(mm); > > goto out_nolock; > > } > > > > - result = find_pmd_or_thp_or_none(mm, address, &pmd); > > + result = find_pmd_or_thp_or_none(mm, pmd_addr, &pmd); > > if (result != SCAN_SUCCEED) { > > mmap_read_unlock(mm); > > goto out_nolock; > > @@ -1253,8 +1255,8 @@ 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); > > + result = __collapse_huge_page_swapin(mm, vma, start_addr, pmd, > > + referenced, order); > > if (result != SCAN_SUCCEED) > > goto out_nolock; > > } > > @@ -1269,20 +1271,21 @@ 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_addr, /*expect_anon=*/ 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_addr, 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, > > + end_addr); > > mmu_notifier_invalidate_range_start(&range); > > > > pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */ > > @@ -1294,26 +1297,23 @@ 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_addr, 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)); > > + WARN_ON_ONCE(!pmd_none(*pmd)); > > /* > > * We can only use set_pmd_at when establishing > > * hugepmds and never for establishing regular pmds that > > @@ -1321,21 +1321,24 @@ 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 the PMD entry has been > > + * removed and not all pages are isolated and locked, so we must hold > > + * the lock to prevent neighboring folios from attempting to access > > + * this PMD until its reinstalled. > > */ > > - 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; > > > > @@ -1345,18 +1348,32 @@ static enum scan_result collapse_huge_page(struct > > mm_struct *mm, unsigned long a > > * write. > > */ > > __folio_mark_uptodate(folio); > > - 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); > > + WARN_ON_ONCE(!pmd_none(*pmd)); > > + if (is_pmd_order(order)) { > > + pgtable = pmd_pgtable(_pmd); > > + pgtable_trans_huge_deposit(mm, pmd, pgtable); > > + map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr); > > + } else { > > + /* > > + * set_ptes is called in map_anon_folio_pte_nopf with the > > + * pmd_ptl lock still held; this is safe as the PMD is > > expected > > + * to be none. The pmd entry is then repopulated below. > > + */ > > + map_anon_folio_pte_nopf(folio, pte, vma, start_addr, > > /*uffd_wp=*/ false); > > + smp_wmb(); /* make PTEs visible before PMD. See pmd_install() > > */ > > + pmd_populate(mm, pmd, pmd_pgtable(_pmd)); > > + } > > spin_unlock(pmd_ptl); > > > > folio = NULL; > > > > result = SCAN_SUCCEED; > > out_up_write: > > + if (anon_vma_locked) > > + anon_vma_unlock_write(vma->anon_vma); > > + if (pte) > > + pte_unmap(pte); > > mmap_write_unlock(mm); > > out_nolock: > > if (folio) > > @@ -1536,7 +1553,7 @@ static enum scan_result collapse_scan_pmd(struct > > mm_struct *mm, > > /* collapse_huge_page expects the lock to be dropped before > > calling */ > > mmap_read_unlock(mm); > > result = collapse_huge_page(mm, start_addr, referenced, > > - unmapped, cc); > > + unmapped, cc, HPAGE_PMD_ORDER); > > /* collapse_huge_page will return with the mmap_lock released > > */ > > *lock_dropped = true; > > } > > -- > > 2.54.0 > > >
