memcg: restructure mem_cgroup_can_attach()

Restructure it to lower nesting level and help the planned threadgroup
leader iteration changes.

This is pure reorganization.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Reviewed-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -4834,10 +4834,12 @@ static void mem_cgroup_clear_mc(void)
 static int mem_cgroup_can_attach(struct cgroup_subsys_state *css,
 				 struct cgroup_taskset *tset)
 {
-	struct task_struct *p = cgroup_taskset_first(tset);
-	int ret = 0;
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
+	struct mem_cgroup *from;
+	struct task_struct *p;
+	struct mm_struct *mm;
 	unsigned long move_flags;
+	int ret = 0;
 
 	/*
 	 * We are now commited to this value whatever it is. Changes in this
@@ -4845,9 +4847,11 @@ static int mem_cgroup_can_attach(struct cgroup_subsys_state *css,
 	 * So we need to save it, and keep it going.
 	 */
 	move_flags = READ_ONCE(memcg->move_charge_at_immigrate);
-	if (move_flags) {
-		struct mm_struct *mm;
-		struct mem_cgroup *from = mem_cgroup_from_task(p);
+	if (!move_flags)
+		return 0;
+
+	p = cgroup_taskset_first(tset);
+	from = mem_cgroup_from_task(p);
 
 	VM_BUG_ON(from == memcg);
 
@@ -4874,7 +4878,6 @@ static int mem_cgroup_can_attach(struct cgroup_subsys_state *css,
 			mem_cgroup_clear_mc();
 	}
 	mmput(mm);
-	}
 	return ret;
 }