[PATCH 11/11] mm/thp: another PVMW_SYNC fix in page_vma_mapped_walk()

[Hugh Dickins <hughd@google.com>]

Aha!  Shouldn't that quick scan over pte_none()s make sure that it holds
ptlock in the PVMW_SYNC case?  That too might have been responsible for
BUGs or WARNs in split_huge_page_to_list() or its unmap_page(), though
I've never seen any.

Fixes: ace71a19cec5 ("mm: introduce page_vma_mapped_walk()")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
---
 mm/page_vma_mapped.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index 6eb2f1863506..7ae4a016304b 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -277,6 +277,10 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 				goto restart;
 			}
 			pvmw->pte++;
+			if ((pvmw->flags & PVMW_SYNC) && !pvmw->ptl) {
+				pvmw->ptl = pte_lockptr(mm, pvmw->pmd);
+				spin_lock(pvmw->ptl);
+			}
 		} while (pte_none(*pvmw->pte));
 
 		if (!pvmw->ptl) {
-- 
2.26.2