[patch 2/3] mm: memcontrol: remove obsolete kmemcg pinning tricks

Johannes Weiner
2014-09-20T20:00:34Z

As charges now pin the css explicitely, there is no more need for
kmemcg to acquire a proxy reference for outstanding pages during
offlining, or maintain state to identify such "dead" groups.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
mm/memcontrol.c | 74 +--------------------------------------------------------
1 file changed, 1 insertion(+), 73 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b832c87ec43b..019a44ac25d6 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -471,7 +471,6 @@ struct mem_cgroup {
/* internal only representation about the status of kmem accounting. */
enum {
KMEM_ACCOUNTED_ACTIVE, /* accounted by this cgroup itself */
- KMEM_ACCOUNTED_DEAD, /* dead memcg with pending kmem charges */
};

#ifdef CONFIG_MEMCG_KMEM
@@ -485,22 +484,6 @@ static bool memcg_kmem_is_active(struct mem_cgroup *memcg)
return test_bit(KMEM_ACCOUNTED_ACTIVE, &memcg->kmem_account_flags);
}

-static void memcg_kmem_mark_dead(struct mem_cgroup *memcg)
-{
- /*
- * Our caller must use css_get() first, because memcg_uncharge_kmem()
- * will call css_put() if it sees the memcg is dead.
- */
- smp_wmb();
- if (test_bit(KMEM_ACCOUNTED_ACTIVE, &memcg->kmem_account_flags))
- set_bit(KMEM_ACCOUNTED_DEAD, &memcg->kmem_account_flags);
-}
-
-static bool memcg_kmem_test_and_clear_dead(struct mem_cgroup *memcg)
-{
- return test_and_clear_bit(KMEM_ACCOUNTED_DEAD,
- &memcg->kmem_account_flags);
-}
#endif

/* Stuffs for move charges at task migration. */
@@ -2807,22 +2790,7 @@ static void memcg_uncharge_kmem(struct mem_cgroup *memcg,
if (do_swap_account)
page_counter_uncharge(&memcg->memsw, nr_pages);

- /* Not down to 0 */
- if (page_counter_uncharge(&memcg->kmem, nr_pages)) {
- css_put_many(&memcg->css, nr_pages);
- return;
- }
-
- /*
- * Releases a reference taken in kmem_cgroup_css_offline in case
- * this last uncharge is racing with the offlining code or it is
- * outliving the memcg existence.
- *
- * The memory barrier imposed by test&clear is paired with the
- * explicit one in memcg_kmem_mark_dead().
- */
- if (memcg_kmem_test_and_clear_dead(memcg))
- css_put(&memcg->css);
+ page_counter_uncharge(&memcg->kmem, nr_pages);

css_put_many(&memcg->css, nr_pages);
}
@@ -4805,40 +4773,6 @@ static void memcg_destroy_kmem(struct mem_cgroup *memcg)
{
mem_cgroup_sockets_destroy(memcg);
}
-
-static void kmem_cgroup_css_offline(struct mem_cgroup *memcg)
-{
- if (!memcg_kmem_is_active(memcg))
- return;
-
- /*
- * kmem charges can outlive the cgroup. In the case of slab
- * pages, for instance, a page contain objects from various
- * processes. As we prevent from taking a reference for every
- * such allocation we have to be careful when doing uncharge
- * (see memcg_uncharge_kmem) and here during offlining.
- *
- * The idea is that that only the _last_ uncharge which sees
- * the dead memcg will drop the last reference. An additional
- * reference is taken here before the group is marked dead
- * which is then paired with css_put during uncharge resp. here.
- *
- * Although this might sound strange as this path is called from
- * css_offline() when the referencemight have dropped down to 0 and
- * shouldn't be incremented anymore (css_tryget_online() would
- * fail) we do not have other options because of the kmem
- * allocations lifetime.
- */
- css_get(&memcg->css);
-
- memcg_kmem_mark_dead(memcg);
-
- if (atomic_long_read(&memcg->kmem.count))
- return;
-
- if (memcg_kmem_test_and_clear_dead(memcg))
- css_put(&memcg->css);
-}
#else
static int memcg_init_kmem(struct mem_cgroup *memcg, struct cgroup_subsys *ss)
{
@@ -4848,10 +4782,6 @@ static int memcg_init_kmem(struct mem_cgroup *memcg, struct cgroup_subsys *ss)
static void memcg_destroy_kmem(struct mem_cgroup *memcg)
{
}
-
-static void kmem_cgroup_css_offline(struct mem_cgroup *memcg)
-{
-}
#endif

/*
@@ -5443,8 +5373,6 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
}
spin_unlock(&memcg->event_list_lock);

- kmem_cgroup_css_offline(memcg);
-
/*
* This requires that offlining is serialized. Right now that is
* guaranteed because css_killed_work_fn() holds the cgroup_mutex.
--
2.1.0