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

[Qi Zheng]

On 11/5/26 10:56 AM, 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().
>
> 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)
>   				sync_with_folio_pmd_zap(mm, pvmw->pmd);
>   
>   			goto next_pmd;

Acked-by: Qi Zheng <qi.zheng@xxxxxxxxx>

Thanks,
Qi