From f79ed973db90cb9c7384f0cfdcf605b2fc6a7e82 Mon Sep 17 00:00:00 2001
From: Andrew Morton <akpm@linux-foundation.org>
Date: Sat, 10 Sep 2016 20:34:11 +1000
Subject: [PATCH] mm: check that we haven't used more than 32 bits in
 address_space.flags

After "mm: don't use radix tree writeback tags for pages in swap cache",
all the flags are now used up on 32-bit builds.

Add a build-time assertion to prevent 64-bit developers from accidentally
breaking things.

Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: "Huang, Ying" <ying.huang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 include/linux/pagemap.h | 2 ++
 init/main.c             | 4 ++++
 2 files changed, 6 insertions(+)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 2f5a65ddbebf..08937bd2c11b 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -27,6 +27,8 @@ enum mapping_flags {
 	AS_EXITING	= __GFP_BITS_SHIFT + 4, /* final truncate in progress */
 	/* writeback related tags are not used */
 	AS_NO_WRITEBACK_TAGS = __GFP_BITS_SHIFT + 5,
+
+	AS_LAST_FLAG,
 };
 
 static inline void mapping_set_error(struct address_space *mapping, int error)
diff --git a/init/main.c b/init/main.c
index a8a58e2794a5..47bdcc39266d 100644
--- a/init/main.c
+++ b/init/main.c
@@ -59,6 +59,7 @@
 #include <linux/pid_namespace.h>
 #include <linux/device.h>
 #include <linux/kthread.h>
+#include <linux/pagemap.h>
 #include <linux/sched.h>
 #include <linux/signal.h>
 #include <linux/idr.h>
@@ -463,6 +464,9 @@ void __init __weak thread_stack_cache_init(void)
  */
 static void __init mm_init(void)
 {
+	/* Does address_space.flags still fit into a 32-bit ulong? */
+	BUILD_BUG_ON(AS_LAST_FLAG > 32);
+
 	/*
 	 * page_ext requires contiguous pages,
 	 * bigger than MAX_ORDER unless SPARSEMEM.
-- 
2.34.1