[PATCH v3 09/14] mm: mremap: move_ptes() use pte_offset_map_rw_nolock()

[Qi Zheng]

In move_ptes(), we may modify the new_pte after acquiring the new_ptl, so
convert it to using pte_offset_map_rw_nolock(). Since we may free the PTE
page in retract_page_tables() without holding the read lock of mmap_lock,
so we still need to do a pmd_same() check after holding the PTL.

Signed-off-by: Qi Zheng <zhengqi.a...@bytedance.com>
---
 mm/mremap.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 24712f8dbb6b5..16e54151395ad 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -143,6 +143,7 @@ static int move_ptes(struct vm_area_struct *vma, pmd_t 
*old_pmd,
        spinlock_t *old_ptl, *new_ptl;
        bool force_flush = false;
        unsigned long len = old_end - old_addr;
+       pmd_t pmdval;
        int err = 0;
 
        /*
@@ -175,14 +176,29 @@ static int move_ptes(struct vm_area_struct *vma, pmd_t 
*old_pmd,
                err = -EAGAIN;
                goto out;
        }
-       new_pte = pte_offset_map_nolock(mm, new_pmd, new_addr, &new_ptl);
+       /*
+        * Since we may free the PTE page in retract_page_tables() without
+        * holding the read lock of mmap_lock, so we still need to do a
+        * pmd_same() check after holding the PTL.
+        */
+       new_pte = pte_offset_map_rw_nolock(mm, new_pmd, new_addr, &pmdval,
+                                          &new_ptl);
        if (!new_pte) {
                pte_unmap_unlock(old_pte, old_ptl);
                err = -EAGAIN;
                goto out;
        }
-       if (new_ptl != old_ptl)
+       if (new_ptl != old_ptl) {
                spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
+
+               if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(new_pmd)))) {
+                       pte_unmap_unlock(new_pte, new_ptl);
+                       pte_unmap_unlock(old_pte, old_ptl);
+                       err = -EAGAIN;
+                       goto out;
+               }
+       }
+
        flush_tlb_batched_pending(vma->vm_mm);
        arch_enter_lazy_mmu_mode();
 
-- 
2.20.1