Re: [PATCH] mm: synchronize with PTE table reclaim for all folio sizes

[Lance Yang]

On Thu, Nov 05, 2026 at 10:56:28AM +0800, Baolin Wang wrote:
>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().

Good catch!

>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@xxxxxxxxxxxxxxx
>Suggested-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>---
> 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)

Right. Whether the folio can be PMD-mapped doesn't tell us whether its
PTE table can be reclaimed, so those extra checks can skip synchronization
that the caller explicitly asked for ;)

LGTM, thanks!

Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>

> 				sync_with_folio_pmd_zap(mm, pvmw->pmd);
> 
> 			goto next_pmd;
>-- 
>2.47.3
>
>