Similar to commit 243cf1f67b36 ("mm: memory: fix truncation removing mapped folios"), the '!pmd_present()' check in page_vma_mapped_walk() can also race with concurrent page table reclamation via zap_empty_pte_table(). page_vma_mapped_walk() is used by many functions including try_to_unmap(), try_to_migrate(), folio_mkclean(), pfn_mkclean_range(), remove_migration_ptes(), page_mapped_in_vma(), and mapping_wrprotect_range(). Except for try_to_unmap() and try_to_migrate(), the other callers do not perform post-walk accounting checks, so the race has no observable impact on them. For migration, the race can cause unnecessary retries. For reclaim via try_to_unmap(), the race can cause failure to reclaim a folio that is in fact already unmapped (as shown below), which may lead to unexpected OOM or increased refaults. CPU 0 (reclaim) CPU 1 (MADV_DONTNEED) -------------------------------- -------------------------------- folio_lock(F) try_to_unmap(F, TTU_SYNC) zap_pte_range(full PMD range) spin_lock(ptl) clear PTEs zap_empty_pte_table() spin_trylock(pml) pmd_clear() spin_unlock(pml) page_vma_mapped_walk() observe !pmd_present(pmde) nr_pages < HPAGE_PMD_NR skip synchronization return false try_to_unmap() returns folio_mapped(F) == true nr_unmap_fail += folio_nr_pages(F) tlb_flush_mmu_tlbonly() tlb_flush_rmaps() remove F's rmaps spin_unlock(ptl) Commit 243cf1f67b36 ("mm: memory: fix truncation removing mapped folios") has already moved the PMD clear operation (via zap_empty_pte_table()) to after tlb_flush_rmaps(). Similarly, page_vma_mapped_walk() also needs to use sync_with_folio_pmd_zap() to synchronize and avoid memory ordering issues on weakly ordered CPUs. Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in madvise(MADV_DONTNEED)") Cc: stable@vger.kernel.org Suggested-by: David Hildenbrand (Arm) Signed-off-by: Baolin Wang --- mm/page_vma_mapped.c | 5 +---- 1 file changed, 1 insertion(+), 4 deletions(-) diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c index e6b6170c2f4f..8797467b6f0c 100644 --- a/mm/page_vma_mapped.c +++ b/mm/page_vma_mapped.c @@ -293,10 +293,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) spin_unlock(pvmw->ptl); pvmw->ptl = NULL; } else if (!pmd_present(pmde)) { - if ((pvmw->flags & PVMW_SYNC) && - thp_vma_suitable_order(vma, pvmw->address, - PMD_ORDER) && - (pvmw->nr_pages >= HPAGE_PMD_NR)) + if (pvmw->flags & PVMW_SYNC) sync_with_folio_pmd_zap(mm, pvmw->pmd); goto next_pmd; -- 2.47.3