[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" <ebied...@xmission.com>
---
 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