[PATCH v3 07/14] mm: khugepaged: collapse_pte_mapped_thp() use pte_offset_map_rw_nolock()

[Qi Zheng]

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.

For the case where the ptl is released first and then the pml is acquired,
the PTE page may have been freed, so we must do pmd_same() check before
reacquiring the ptl.

Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
---
 mm/khugepaged.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 6498721d4783a..a117d35f33aee 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++) {
@@ -1658,6 +1661,16 @@ 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);
+		/*
+		 * We called pte_unmap() and release the ptl before acquiring
+		 * the pml, which means we left the RCU critical section, so the
+		 * PTE page may have been freed, so we must do pmd_same() check
+		 * before reacquiring the ptl.
+		 */
+		if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) {
+			spin_unlock(pml);
+			goto pmd_change;
+		}
 		if (ptl != pml)
 			spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
 	}
@@ -1689,6 +1702,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
 		pte_unmap_unlock(start_pte, ptl);
 	if (pml && pml != ptl)
 		spin_unlock(pml);
+pmd_change:
 	if (notified)
 		mmu_notifier_invalidate_range_end(&range);
 drop_folio:
-- 
2.20.1