[PATCH v4 08/13] mm: copy_pte_range() use pte_offset_map_rw_nolock()

[Qi Zheng]

In copy_pte_range(), we may modify the src_pte entry after holding the
src_ptl, so convert it to using pte_offset_map_rw_nolock(). Since we
already hold the exclusive mmap_lock, and the copy_pte_range() and
retract_page_tables() are using vma->anon_vma to be exclusive, so the PTE
page is stable, there is no need to get pmdval and do pmd_same() check.

Signed-off-by: Qi Zheng <zhengqi.a...@bytedance.com>
---
 mm/memory.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 6432b636d1ba7..c19cf14e1c565 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1086,6 +1086,7 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct 
vm_area_struct *src_vma,
        struct mm_struct *src_mm = src_vma->vm_mm;
        pte_t *orig_src_pte, *orig_dst_pte;
        pte_t *src_pte, *dst_pte;
+       pmd_t dummy_pmdval;
        pte_t ptent;
        spinlock_t *src_ptl, *dst_ptl;
        int progress, max_nr, ret = 0;
@@ -1111,7 +1112,15 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct 
vm_area_struct *src_vma,
                ret = -ENOMEM;
                goto out;
        }
-       src_pte = pte_offset_map_nolock(src_mm, src_pmd, addr, &src_ptl);
+
+       /*
+        * We already hold the exclusive mmap_lock, the copy_pte_range() and
+        * retract_page_tables() are using vma->anon_vma to be exclusive, so
+        * the PTE page is stable, and there is no need to get pmdval and do
+        * pmd_same() check.
+        */
+       src_pte = pte_offset_map_rw_nolock(src_mm, src_pmd, addr, &dummy_pmdval,
+                                          &src_ptl);
        if (!src_pte) {
                pte_unmap_unlock(dst_pte, dst_ptl);
                /* ret == 0 */
-- 
2.20.1