mm/rmap: fix new bug: premature return from page_mlock_one()

In the unlikely race case that page_mlock_one() finds VM_LOCKED has been
cleared by the time it got page table lock, page_vma_mapped_walk_done()
must be called before returning, either explicitly, or by a final call
to page_vma_mapped_walk() - otherwise the page table remains locked.

Fixes: cd62734c ("mm/rmap: split try_to_munlock from try_to_unmap")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
Link: https://lore.kernel.org/lkml/20210711151446.GB4070@xsang-OptiPlex-9020/
Link: https://lore.kernel.org/lkml/f71f8523-cba7-3342-40a7-114abc5d1f51@google.com/
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/rmap.c

@@ -1990,15 +1990,14 @@ static bool page_mlock_one(struct page *page, struct vm_area_struct *vma,
 			 * this function is never called when PageDoubleMap().
 			 */
 			mlock_vma_page(page);
-			page_vma_mapped_walk_done(&pvmw);
-		}
-
 			/*
-		 * no need to continue scanning other vma's if the page has
-		 * been locked.
+			 * No need to scan further once the page is marked
+			 * as mlocked.
 			 */
+			page_vma_mapped_walk_done(&pvmw);
 			return false;
 		}
+	}
 
 	return true;
 }