[RFC][PATCH 2/2] memcgl: Remove dead code now that all tasks of an mm share a memcg

[Eric W. Biederman]

get_mem_cgroup_from_mm no longer needs to deal with offline memory cgroups.

mem_cgroup_can_attach no longer needs to worry about cgroup leaders
as all tasks of an mm will migrate together.

Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
 mm/memcontrol.c | 43 +++++++++----------------------------------
 1 file changed, 9 insertions(+), 34 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 078ef562bb90..a05c1253ad87 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -666,7 +666,7 @@ static void memcg_check_events(struct mem_cgroup *memcg, struct page *page)
 
 static struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
 {
-	struct mem_cgroup *memcg, *omemcg;
+	struct mem_cgroup *memcg;
 
 	rcu_read_lock();
 	/*
@@ -677,37 +677,18 @@ static struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
 	if (unlikely(!mm))
 		goto root_memcg;
 
-	/* Loop trying to get a reference while mm->memcg is changing */
-	for (omemcg = NULL;; omemcg = memcg) {
+	do {
 		memcg = rcu_dereference(mm->memcg);
 		if (unlikely(!memcg))
 			goto root_memcg;
-		if (likely(css_tryget_online(&memcg->css)))
-			goto found;
-		if ((memcg == omemcg) && css_tryget(&omemcg->css))
-			break;
-	}
+	} while (!css_tryget_online(&memcg->css));
+	rcu_read_unlock();
+	return memcg;
 
-	/* Walk up and find a live memory cgroup */
-	while (memcg != root_mem_cgroup) {
-		memcg = mem_cgroup_from_css(memcg->css.parent);
-		if (css_tryget_online(&memcg->css))
-			goto found_parent;
-	}
-	css_put(&omemcg->css);
 root_memcg:
 	css_get(&root_mem_cgroup->css);
 	rcu_read_unlock();
 	return root_mem_cgroup;
-
-found_parent:
-	css_get(&memcg->css);
-	mm_update_memcg(mm, memcg);
-	css_put(&omemcg->css);
-found:
-	rcu_read_unlock();
-	return memcg;
-
 }
 
 /**
@@ -4847,7 +4828,7 @@ static int mem_cgroup_can_attach(struct cgroup_taskset *tset)
 	struct cgroup_subsys_state *css;
 	struct mem_cgroup *memcg = NULL; /* unneeded init to make gcc happy */
 	struct mem_cgroup *from;
-	struct task_struct *leader, *p;
+	struct task_struct *p;
 	struct mm_struct *mm;
 	unsigned long move_flags;
 	int ret;
@@ -4863,18 +4844,12 @@ static int mem_cgroup_can_attach(struct cgroup_taskset *tset)
 
 	/*
 	 * Multi-process migrations only happen on the default hierarchy
-	 * where charge immigration is not used.  Perform charge
-	 * immigration if @tset contains a leader and whine if there are
-	 * multiple.
+	 * where charge immigration is not used.
 	 */
-	p = NULL;
-	cgroup_taskset_for_each_leader(leader, css, tset) {
-		WARN_ON_ONCE(p);
-		p = leader;
-		memcg = mem_cgroup_from_css(css);
-	}
+	p = cgroup_taskset_first(tset, &css);
 	if (!p)
 		return 0;
+	memcg = mem_cgroup_from_css(css);
 
 	/*
 	 * We are now commited to this value whatever it is. Changes in this
-- 
2.14.1