mm/page_alloc.c: pull out init code from build_all_zonelists

Pulling the code protected by if (system_state == SYSTEM_BOOTING) into
its own helper allows us to shrink .text a little. This relies on
build_all_zonelists already having a __ref annotation. Add a comment
explaining why so one doesn't have to track it down through git log.

The real saving comes in 3/5, ("mm/mm_init.c: Mark mminit_verify_zonelist
as __init"), where we save about 400 bytes
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Rik van Riel <riel@redhat.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Vishnu Pratap Singh <vishnu.ps@samsung.com>
Cc: Pintu Kumar <pintu.k@samsung.com>
Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -3871,18 +3871,29 @@ static int __build_all_zonelists(void *data)
 	return 0;
 }
 
+static noinline void __init
+build_all_zonelists_init(void)
+{
+	__build_all_zonelists(NULL);
+	mminit_verify_zonelist();
+	cpuset_init_current_mems_allowed();
+}
+
 /*
  * Called with zonelists_mutex held always
  * unless system_state == SYSTEM_BOOTING.
+ *
+ * __ref due to (1) call of __meminit annotated setup_zone_pageset
+ * [we're only called with non-NULL zone through __meminit paths] and
+ * (2) call of __init annotated helper build_all_zonelists_init
+ * [protected by SYSTEM_BOOTING].
  */
 void __ref build_all_zonelists(pg_data_t *pgdat, struct zone *zone)
 {
 	set_zonelist_order();
 
 	if (system_state == SYSTEM_BOOTING) {
-		__build_all_zonelists(NULL);
-		mminit_verify_zonelist();
-		cpuset_init_current_mems_allowed();
+		build_all_zonelists_init();
 	} else {
 #ifdef CONFIG_MEMORY_HOTPLUG
 		if (zone)