Re: [PATCH] mm: synchronize with PTE table reclaim for all folio sizes
From: Baolin Wang
Date: Thu Nov 05 2026 - 04:05:55 EST
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@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 ;)
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?