On Sep 24, 2024, at 14:11, Qi Zheng [off-list ref] wrote:
In collapse_pte_mapped_thp(), we may modify the pte and pmd entry after
acquring the ptl, so convert it to using pte_offset_map_rw_nolock(). At
this time, the pte_same() check is not performed after the PTL held. So we
should get pgt_pmd and do pmd_same() check after the ptl held.

Signed-off-by: Qi Zheng <redacted>
---
mm/khugepaged.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 6498721d4783a..8ab79c13d077f 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c

@@ -1605,7 +1605,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
   if (userfaultfd_armed(vma) && !(vma->vm_flags & VM_SHARED))
       pml = pmd_lock(mm, pmd);

-    start_pte = pte_offset_map_nolock(mm, pmd, haddr, &ptl);
+    start_pte = pte_offset_map_rw_nolock(mm, pmd, haddr, &pgt_pmd, &ptl);
   if (!start_pte)        /* mmap_lock + page lock should prevent this */
       goto abort;
   if (!pml)

@@ -1613,6 +1613,9 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
   else if (ptl != pml)
       spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);

+    if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd))))
+        goto abort;
+
   /* step 2: clear page table and adjust rmap */
   for (i = 0, addr = haddr, pte = start_pte;
        i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE, pte++) {

@@ -1645,7 +1648,6 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
       nr_ptes++;
   }

-    pte_unmap(start_pte);
   if (!pml)
       spin_unlock(ptl);

@@ -1658,13 +1660,19 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
   /* step 4: remove empty page table */
   if (!pml) {
       pml = pmd_lock(mm, pmd);
-        if (ptl != pml)
+        if (ptl != pml) {
           spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
+            if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) {
+                spin_unlock(pml);
+                goto abort;