[PATCH 8/8] memcg: get rid of mem_cgroup_from_task

Michal Hocko
2015-07-08T12:28:32Z

From: Michal Hocko <mhocko@suse.cz>

mem_cgroup_from_task has always been a tricky API. It was added
by 78fb74669e80 ("Memory controller: accounting setup") for
mm_struct::mem_cgroup initialization. Later on it gained new callers
mostly due to mm_struct::mem_cgroup -> mem_cgroup::owner transition and
most users had to do mem_cgroup_from_task(mm->owner) to get the
resulting memcg. Now that mm_struct::owner is gone this is not
necessary, yet the API is still confusing.

One tricky part has always been that the API sounds generic but it is
not really. mem_cgroup_from_task(current) doesn't necessarily mean the
same thing as current->mm->memcg (resp.
mem_cgroup_from_task(current->mm->owner) previously) because mm might be
associated with a different cgroup than the process.

Another tricky part is that p->mm->memcg is unsafe if p!=current
as pointed by Oleg because nobody is holding a reference on that
mm. This is not a problem right now because we have only 2 callers in
the tree. sock_update_memcg operates on current and task_in_mem_cgroup
is providing non-NULL task so it is always using task_css.

Let's ditch this function and use current->mm->memcg for
sock_update_memcg and use task_css for task_in_mem_cgroup. This doesn't
have any functional effect.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
---
mm/memcontrol.c | 24 +++++++-----------------
1 file changed, 7 insertions(+), 17 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 4069ec8f52be..fb8e9bd04a29 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -294,18 +294,6 @@ static inline struct mem_cgroup *mem_cgroup_from_id(unsigned short id)
return mem_cgroup_from_css(css);
}

-static struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
-{
- if (p->mm)
- return rcu_dereference(p->mm->memcg);
-
- /*
- * If the process doesn't have mm struct anymore we have to fallback
- * to the task_css.
- */
- return mem_cgroup_from_css(task_css(p, memory_cgrp_id));
-}
-
/* Writing them here to avoid exposing memcg's inner layout */
#if defined(CONFIG_INET) && defined(CONFIG_MEMCG_KMEM)

@@ -332,7 +320,7 @@ void sock_update_memcg(struct sock *sk)
}

rcu_read_lock();
- memcg = mem_cgroup_from_task(current);
+ memcg = rcu_dereference(current->mm->memcg);
cg_proto = sk->sk_prot->proto_cgroup(memcg);
if (cg_proto && memcg_proto_active(cg_proto) &&
css_tryget_online(&memcg->css)) {
@@ -1091,12 +1079,14 @@ bool task_in_mem_cgroup(struct task_struct *task, struct mem_cgroup *memcg)
task_unlock(p);
} else {
/*
- * All threads may have already detached their mm's, but the oom
- * killer still needs to detect if they have already been oom
- * killed to prevent needlessly killing additional tasks.
+ * All threads have already detached their mm's but we should
+ * still be able to at least guess the original memcg from the
+ * task_css. These two will match most of the time but there are
+ * corner cases where task->mm and task_css refer to a different
+ * cgroups.
*/
rcu_read_lock();
- task_memcg = mem_cgroup_from_task(task);
+ task_memcg = mem_cgroup_from_css(task_css(task, memory_cgrp_id));
css_get(&task_memcg->css);
rcu_read_unlock();
}
--
2.1.4