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

[Lance Yang <lance.yang@linux.dev>]

On 2026/11/5 12:54, Lance Yang wrote:
> 
> 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@vger.kernel.org
>> Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>> 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 ;)

Ouch ... one thing I missed ... what about order-0 reclaim?

shrink_folio_list() only sets TTU_SYNC for large folio, so can't we still
hit the same race there?

Cheers, Lance

> LGTM, thanks!
> 
> Reviewed-by: Lance Yang <lance.yang@linux.dev>
> 
>> 				sync_with_folio_pmd_zap(mm, pvmw->pmd);
>>
>> 			goto next_pmd;
>> -- 
>> 2.47.3
>>
>>