memcg: remove redundant code

- try_get_mem_cgroup_from_mm() calls rcu_read_lock/unlock by itself, so we
  don't have to call them in task_in_mem_cgroup().
- *mz is not used in __mem_cgroup_uncharge_common().
- we don't have to call lookup_page_cgroup() in mem_cgroup_end_migration()
  after we've cleared PCG_MIGRATION of @oldpage.
- remove empty comment.
- remove redundant empty line in mem_cgroup_cache_charge().
Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -840,9 +840,7 @@ int task_in_mem_cgroup(struct task_struct *task, const struct mem_cgroup *mem)
 	struct mem_cgroup *curr = NULL;
 
 	task_lock(task);
-	rcu_read_lock();
 	curr = try_get_mem_cgroup_from_mm(task->mm);
-	rcu_read_unlock();
 	task_unlock(task);
 	if (!curr)
 		return 0;
@@ -2092,7 +2090,6 @@ int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
 	if (!(gfp_mask & __GFP_WAIT)) {
 		struct page_cgroup *pc;
 
-
 		pc = lookup_page_cgroup(page);
 		if (!pc)
 			return 0;
@@ -2286,7 +2283,6 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype)
 {
 	struct page_cgroup *pc;
 	struct mem_cgroup *mem = NULL;
-	struct mem_cgroup_per_zone *mz;
 
 	if (mem_cgroup_disabled())
 		return NULL;
@@ -2340,7 +2336,6 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype)
 	 * special functions.
 	 */
 
-	mz = page_cgroup_zoneinfo(pc);
 	unlock_page_cgroup(pc);
 
 	memcg_check_events(mem, page);
@@ -2652,11 +2647,8 @@ void mem_cgroup_end_migration(struct mem_cgroup *mem,
 	ClearPageCgroupMigration(pc);
 	unlock_page_cgroup(pc);
 
-	if (unused != oldpage)
-		pc = lookup_page_cgroup(unused);
 	__mem_cgroup_uncharge_common(unused, MEM_CGROUP_CHARGE_TYPE_FORCE);
 
-	pc = lookup_page_cgroup(used);
 	/*
 	 * If a page is a file cache, radix-tree replacement is very atomic
 	 * and we can skip this check. When it was an Anon page, its mapcount
@@ -3800,8 +3792,6 @@ static int mem_cgroup_oom_control_read(struct cgroup *cgrp,
 	return 0;
 }
 
-/*
- */
 static int mem_cgroup_oom_control_write(struct cgroup *cgrp,
 	struct cftype *cft, u64 val)
 {