On Sun, 19 Apr 2026 12:57:42 -0600 Nico Pache <[email protected]> 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. > > Signed-off-by: Nico Pache <[email protected]> > --- > mm/khugepaged.c | 103 +++++++++++++++++++++++++++--------------------- > 1 file changed, 57 insertions(+), 46 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 283bb63854a5..ff6f9f1883ed 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -1198,42 +1198,36 @@ 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, > + 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 pmd_addr = start_addr & HPAGE_PMD_MASK; > + const unsigned long end_addr = start_addr + (PAGE_SIZE << order); > > - VM_BUG_ON(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. > - */ > - mmap_read_unlock(mm); > - My understanding now is that the caller will need to drop the mmap_read lock? This needs explicit documentation at the start of the function IMO. I think there are callers in later patches and its very easy to miss this. > - 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; > @@ -1245,8 +1239,8 @@ 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; > } > @@ -1261,20 +1255,21 @@ 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 */ > @@ -1286,26 +1281,23 @@ 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); For an mTHP collapse covering, say, 64KiB of a 2MiB PMD, the patch still flushes the entire PMD via pmdp_collapse_flush and tlb_remove_table_sync_one. That triggers cross-CPU TLB shootdowns for ~1.94MiB of unrelated mappings on every successful sub-PMD collapse. Probably acceptable as a first cut? > 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)); Why was this turned into WARN_ON_ONCE? Would be good to add to commit message what the reason is if it has been discussed earlier. The next line writes a PMD entry over an existing one — that leaks the previous page table or PMD-mapped folio and can corrupt VA mappings. BUG_ON failed loudly and safely; WARN_ON_ONCE continues into the corruption and falls silent after the first hit. > /* > * We can only use set_pmd_at when establishing > * hugepmds and never for establishing regular pmds that > @@ -1313,21 +1305,24 @@ 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; > > @@ -1337,18 +1332,27 @@ 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)) { /* PMD collapse */ > + pgtable = pmd_pgtable(_pmd); > + pgtable_trans_huge_deposit(mm, pmd, pgtable); > + map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr); > + } else { /* mTHP collapse */ > + map_anon_folio_pte_nopf(folio, pte, vma, start_addr, > /*uffd_wp=*/ false); map_anon_folio_pte_nopf calls set_ptes and modifies pagetable, while holding pmd_ptl only. It should be safe as we expect pmd_none. But I think you should put a comment about this? > + 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) > @@ -1525,8 +1529,15 @@ static enum scan_result collapse_scan_pmd(struct > mm_struct *mm, > out_unmap: > pte_unmap_unlock(pte, ptl); > if (result == SCAN_SUCCEED) { > + /* > + * 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. > + */ > + 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; > } > -- > 2.53.0 > >
