Re: [PATCH v2 1/7] mm/thp: fix __split_huge_pmd_locked() for migration PMD

On Wed, Sep 2, 2020 at 9:58 AM Ralph Campbell rcampbell@nvidia.com 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.

Thanks for catching this. The fix looks good to me. Reviewed-by: Yang Shi shy828301@gmail.com

I think this fix can go separately with the series.

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

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