[PATCH 11/14] mm: userfaultfd: move_pages_pte() use pte_offset_map_maywrite_nolock()

[Qi Zheng]

In move_pages_pte(), we may modify the dst_pte and src_pte after acquiring
the ptl, so convert it to using pte_offset_map_maywrite_nolock(). But
since we already do the pte_same() check, there is no need to get pmdval
to do pmd_same() check, just pass NULL to pmdvalp parameter.

Signed-off-by: Qi Zheng <zhengqi.arch at bytedance.com>
---
 mm/userfaultfd.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 6ef42d9cd482e..310289fad2b32 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1146,7 +1146,13 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
 				src_addr, src_addr + PAGE_SIZE);
 	mmu_notifier_invalidate_range_start(&range);
 retry:
-	dst_pte = pte_offset_map_nolock(mm, dst_pmd, dst_addr, &dst_ptl);
+	/*
+	 * Use the maywrite version to indicate that dst_pte will be modified,
+	 * but since we will use pte_same() to detect the change of the pte
+	 * entry, there is no need to get pmdval.
+	 */
+	dst_pte = pte_offset_map_maywrite_nolock(mm, dst_pmd, dst_addr, NULL,
+						 &dst_ptl);
 
 	/* Retry if a huge pmd materialized from under us */
 	if (unlikely(!dst_pte)) {
@@ -1154,7 +1160,9 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
 		goto out;
 	}
 
-	src_pte = pte_offset_map_nolock(mm, src_pmd, src_addr, &src_ptl);
+	/* same as dst_pte */
+	src_pte = pte_offset_map_maywrite_nolock(mm, src_pmd, src_addr, NULL,
+						 &src_ptl);
 
 	/*
 	 * We held the mmap_lock for reading so MADV_DONTNEED
-- 
2.20.1