[PATCH v2 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(). But since we already hold
the exclusive mmap_lock, there is no need to get pmdval to do pmd_same()
check, just pass a dummy variable to it.

Signed-off-by: Qi Zheng <zhengqi.arch at bytedance.com>
---
 mm/mremap.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 24712f8dbb6b5..f96b025c09079 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 dummy_pmdval;
 	int err = 0;
 
 	/*
@@ -175,7 +176,13 @@ 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);
+	/*
+	 * Use the maywrite version to indicate that new_pte will be modified,
+	 * but since we hold the exclusive mmap_lock, there is no need to
+	 * recheck pmd_same() after acquiring the new_ptl.
+	 */
+	new_pte = pte_offset_map_rw_nolock(mm, new_pmd, new_addr, &dummy_pmdval,
+					   &new_ptl);
 	if (!new_pte) {
 		pte_unmap_unlock(old_pte, old_ptl);
 		err = -EAGAIN;
-- 
2.20.1