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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

CC Barry, who added the TTU_SYNC in shrink_folio_list().

On 11/5/26 1:48 PM, Lance Yang wrote:
> 
> 
> 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?

Thanks Lance. But this matches the purpose of the TTU_SYNC flag, which 
is used to avoid racy checks with PVMW_SYNC. If no TTU_SYNC flag is set, 
it means the caller can tolerate some racy checks.

For this case, I think we should set TTU_SYNC unconditionally in 
shrink_folio_list(). Barry, what do you think?