On 2 Sep 2020, at 12:58, Ralph Campbell wrote:

A migrating transparent huge page has to already be unmapped. Otherwise, the page could be modified while it is being copied to a new page and data could be lost. The function __split_huge_pmd() checks for a PMD migration entry before calling __split_huge_pmd_locked() leading one to think that __split_huge_pmd_locked() can handle splitting a migrating PMD. However, the code always increments the page->_mapcount and adjusts the memory control group accounting assuming the page is mapped. Also, if the PMD entry is a migration PMD entry, the call to is_huge_zero_pmd(*pmd) is incorrect because it calls pmd_pfn(pmd) instead of migration_entry_to_pfn(pmd_to_swp_entry(pmd)). Fix these problems by checking for a PMD migration entry.

Signed-off-by: Ralph Campbell rcampbell@nvidia.com

Thanks for the fix. You can add Reviewed-by: Zi Yan ziy@nvidia.com

I think you also want to add the Fixes tag and cc stable.

Fixes 84c3fc4e9c56 (“mm: thp: check pmd migration entry in common path”) cc: stable@vger.kernel.org # 4.14+

---

mm/huge_memory.c | 42 +++++++++++++++++++++++------------------- 1 file changed, 23 insertions(+), 19 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 2a468a4acb0a..606d712d9505 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2023,7 +2023,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, put_page(page); add_mm_counter(mm, mm_counter_file(page), -HPAGE_PMD_NR); return;

• } else if (is_huge_zero_pmd(*pmd)) {

• } else if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) { /*
  • FIXME: Do we want to invalidate secondary mmu by calling
  • mmu_notifier_invalidate_range() see comments below inside

@@ -2117,30 +2117,34 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, pte = pte_offset_map(&_pmd, addr); BUG_ON(!pte_none(*pte)); set_pte_at(mm, addr, pte, entry);

• atomic_inc(&page[i]._mapcount);
  

• pte_unmap(pte);
  

• }
• /*

• * Set PG_double_map before dropping compound_mapcount to avoid
  

• * false-negative page_mapped().
  

• */
  

• if (compound_mapcount(page) > 1 && !TestSetPageDoubleMap(page)) {

• for (i = 0; i < HPAGE_PMD_NR; i++)
  

• if (!pmd_migration)
  atomic_inc(&page[i]._mapcount);
  

• pte_unmap(pte);
  

  }

• lock_page_memcg(page);
• if (atomic_add_negative(-1, compound_mapcount_ptr(page))) {

• /* Last compound_mapcount is gone. */
  

• __dec_lruvec_page_state(page, NR_ANON_THPS);
  

• if (TestClearPageDoubleMap(page)) {
  

• 	/* No need in mapcount reference anymore */
  

• if (!pmd_migration) {

• /*
  

•  * Set PG_double_map before dropping compound_mapcount to avoid
  

•  * false-negative page_mapped().
  

•  */
  

• if (compound_mapcount(page) > 1 &&
  

•     !TestSetPageDoubleMap(page)) {
  for (i = 0; i < HPAGE_PMD_NR; i++)
  

• 		atomic_dec(&page[i]._mapcount);
  

• 		atomic_inc(&page[i]._mapcount);
  

• }
  

• lock_page_memcg(page);
  

• if (atomic_add_negative(-1, compound_mapcount_ptr(page))) {
  

• 	/* Last compound_mapcount is gone. */
  

• 	__dec_lruvec_page_state(page, NR_ANON_THPS);
  

• 	if (TestClearPageDoubleMap(page)) {
  

• 		/* No need in mapcount reference anymore */
  

• 		for (i = 0; i < HPAGE_PMD_NR; i++)
  

• 			atomic_dec(&page[i]._mapcount);
  

• 	}
  

  }

• unlock_page_memcg(page);
  

  }

• unlock_page_memcg(page);

  smp_wmb(); /* make pte visible before pmd */ pmd_populate(mm, pmd, pgtable);

-- 2.20.1

— Best Regards, Yan Zi