mm/page_vma_mapped.c | 5 +---- 1 file changed, 1 insertion(+), 4 deletions(-)
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@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)
sync_with_folio_pmd_zap(mm, pvmw->pmd);
goto next_pmd;
--
2.47.3
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@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)
> sync_with_folio_pmd_zap(mm, pvmw->pmd);
>
> goto next_pmd;
Acked-by: Qi Zheng <qi.zheng@linux.dev>
Thanks,
Qi
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