memcg: add rwsem to synchronize against memcg_caches arrays relocation

We need a stable value of memcg_nr_cache_ids in kmem_cache_create()
(memcg_alloc_cache_params() wants it for root caches), where we only
hold the slab_mutex and no memcg-related locks.  As a result, we have to
update memcg_nr_cache_ids under the slab_mutex, which we can only take
on the slab's side (see memcg_update_array_size).  This looks awkward
and will become even worse when per-memcg list_lru is introduced, which
also wants stable access to memcg_nr_cache_ids.

To get rid of this dependency between the memcg_nr_cache_ids and the
slab_mutex, this patch introduces a special rwsem.  The rwsem is held
for writing during memcg_caches arrays relocation and memcg_nr_cache_ids
updates.  Therefore one can take it for reading to get a stable access
to memcg_caches arrays and/or memcg_nr_cache_ids.

Currently the semaphore is taken for reading only from
kmem_cache_create, right before taking the slab_mutex, so right now
there's no much point in using rwsem instead of mutex.  However, once
list_lru is made per-memcg it will allow list_lru initializations to
proceed concurrently.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Greg Thelen <gthelen@google.com>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 19 insertions, 10 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8608fa543b84..6706e5fa5ac0 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -544,6 +544,19 @@ static void disarm_sock_keys(struct mem_cgroup *memcg)
 static DEFINE_IDA(memcg_cache_ida);
 int memcg_nr_cache_ids;
 
+/* Protects memcg_nr_cache_ids */
+static DECLARE_RWSEM(memcg_cache_ids_sem);
+
+void memcg_get_cache_ids(void)
+{
+        down_read(&memcg_cache_ids_sem);
+}
+
+void memcg_put_cache_ids(void)
+{
+        up_read(&memcg_cache_ids_sem);
+}
+
 /*
  * MIN_SIZE is different than 1, because we would like to avoid going through
  * the alloc/free process all the time. In a small machine, 4 kmem-limited
@@ -2549,6 +2562,7 @@ static int memcg_alloc_cache_id(void)
          * There's no space for the new id in memcg_caches arrays,
          * so we have to grow them.
          */
+        down_write(&memcg_cache_ids_sem);
 
         size = 2 * (id + 1);
         if (size < MEMCG_CACHES_MIN_SIZE)
@@ -2557,6 +2571,11 @@ static int memcg_alloc_cache_id(void)
                 size = MEMCG_CACHES_MAX_SIZE;
 
         err = memcg_update_all_caches(size);
+        if (!err)
+                memcg_nr_cache_ids = size;
+
+        up_write(&memcg_cache_ids_sem);
+
         if (err) {
                 ida_simple_remove(&memcg_cache_ida, id);
                 return err;
@@ -2569,16 +2588,6 @@ static void memcg_free_cache_id(int id)
         ida_simple_remove(&memcg_cache_ida, id);
 }
 
-/*
- * We should update the current array size iff all caches updates succeed. This
- * can only be done from the slab side. The slab mutex needs to be held when
- * calling this.
- */
-void memcg_update_array_size(int num)
-{
-        memcg_nr_cache_ids = num;
-}
-
 struct memcg_kmem_cache_create_work {
         struct mem_cgroup *memcg;
         struct kmem_cache *cachep;