mm: PageBuddy and mapcount robustness

Change the _mapcount value indicating PageBuddy from -2 to -128 for
more robusteness against page_mapcount() undeflows.

Use reset_page_mapcount instead of __ClearPageBuddy in bad_page to
ignore the previous retval of PageBuddy().

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index bd7625676a6..7945247b1e5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -286,7 +286,7 @@ static void bad_page(struct page *page)
 
 	/* Don't complain about poisoned pages */
 	if (PageHWPoison(page)) {
-		__ClearPageBuddy(page);
+		reset_page_mapcount(page); /* remove PageBuddy */
 		return;
 	}
 
@@ -317,7 +317,7 @@ static void bad_page(struct page *page)
 	dump_stack();
 out:
 	/* Leave bad fields for debug, except PageBuddy could make trouble */
-	__ClearPageBuddy(page);
+	reset_page_mapcount(page); /* remove PageBuddy */
 	add_taint(TAINT_BAD_PAGE);
 }