khugepaged: fix wrong result value for trace_mm_collapse_huge_page_isolate()

In writable and !referenced case, the result value should be
SCAN_LACK_REFERENCED_PAGE for trace_mm_collapse_huge_page_isolate()
instead of default 0 (SCAN_FAIL) here.

Link: https://lkml.kernel.org/r/20210306032947.35921-5-linmiaohe@huawei.com
Fixes: 7d2eba05 ("mm: add tracepoint for scanning pages")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Ebru Akagunduz <ebru.akagunduz@gmail.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/khugepaged.c

@@ -716,17 +716,17 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		if (pte_write(pteval))
 			writable = true;
 	}
-	if (likely(writable)) {
-		if (likely(referenced)) {
+
+	if (unlikely(!writable)) {
+		result = SCAN_PAGE_RO;
+	} else if (unlikely(!referenced)) {
+		result = SCAN_LACK_REFERENCED_PAGE;
+	} else {
 		result = SCAN_SUCCEED;
 		trace_mm_collapse_huge_page_isolate(page, none_or_zero,
 						    referenced, writable, result);
 		return 1;
 	}
-	} else {
-		result = SCAN_PAGE_RO;
-	}
-
 out:
 	release_pte_pages(pte, _pte, compound_pagelist);
 	trace_mm_collapse_huge_page_isolate(page, none_or_zero,