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

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

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