memory controller: make memory resource control aware of boot options

A boot option for the memory controller was discussed on lkml.  It is a good
idea to add it, since it saves memory for people who want to turn off the
memory controller.

By default the option is on for the following two reasons:

1. It provides compatibility with the current scheme where the memory
   controller turns on if the config option is enabled
2. It allows for wider testing of the memory controller, once the config
   option is enabled

We still allow the create, destroy callbacks to succeed, since they are not
aware of boot options.  We do not populate the directory will memory resource
controller specific files.
Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Paul Menage <menage@google.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: Sudhir Kumar <skumar@linux.vnet.ibm.com>
Cc: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -533,6 +533,9 @@ static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm,
 	unsigned long nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
 	struct mem_cgroup_per_zone *mz;
 
+	if (mem_cgroup_subsys.disabled)
+		return 0;
+
 	/*
 	 * Should page_cgroup's go to their own slab?
 	 * One could optimize the performance of the charging routine
@@ -665,6 +668,9 @@ void mem_cgroup_uncharge_page(struct page *page)
 	struct mem_cgroup_per_zone *mz;
 	unsigned long flags;
 
+	if (mem_cgroup_subsys.disabled)
+		return;
+
 	/*
 	 * Check if our page_cgroup is valid
 	 */
@@ -705,6 +711,9 @@ int mem_cgroup_prepare_migration(struct page *page)
 {
 	struct page_cgroup *pc;
 
+	if (mem_cgroup_subsys.disabled)
+		return 0;
+
 	lock_page_cgroup(page);
 	pc = page_get_page_cgroup(page);
 	if (pc)
@@ -803,6 +812,9 @@ static int mem_cgroup_force_empty(struct mem_cgroup *mem)
 	int ret = -EBUSY;
 	int node, zid;
 
+	if (mem_cgroup_subsys.disabled)
+		return 0;
+
 	css_get(&mem->css);
 	/*
 	 * page reclaim code (kswapd etc..) will move pages between
@@ -1053,6 +1065,8 @@ static void mem_cgroup_destroy(struct cgroup_subsys *ss,
 static int mem_cgroup_populate(struct cgroup_subsys *ss,
 				struct cgroup *cont)
 {
+	if (mem_cgroup_subsys.disabled)
+		return 0;
 	return cgroup_add_files(cont, ss, mem_cgroup_files,
 					ARRAY_SIZE(mem_cgroup_files));
 }
@@ -1065,6 +1079,9 @@ static void mem_cgroup_move_task(struct cgroup_subsys *ss,
 	struct mm_struct *mm;
 	struct mem_cgroup *mem, *old_mem;
 
+	if (mem_cgroup_subsys.disabled)
+		return;
+
 	mm = get_task_mm(p);
 	if (mm == NULL)
 		return;