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: cd62734 ("mm/rmap: split try_to_munlock from try_to_unmap")
Signed-off-by: Hugh Dickins <[email protected]>
Reviewed-by: Alistair Popple <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Reported-by: kernel test robot <[email protected]>
Link: https://lore.kernel.org/lkml/20210711151446.GB4070@xsang-OptiPlex-9020/
Link: https://lore.kernel.org/lkml/[email protected]/
Cc: Andrew Morton <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Ralph Campbell <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Hugh Dickins

mm/rmap.c

@@ -1990,14 +1990,13 @@ static bool page_mlock_one(struct page *page, struct vm_area_struct *vma,
 			 * this function is never called when PageDoubleMap().
 			 */
 			mlock_vma_page(page);
+			/*
+			 * No need to scan further once the page is marked
+			 * as mlocked.
+			 */
 			page_vma_mapped_walk_done(&pvmw);
+			return false;
 		}
-
-		/*
-		 * no need to continue scanning other vma's if the page has
-		 * been locked.
-		 */
-		return false;
 	}
 
 	return true;