The memcg_cache_id (mem_cgroup->kmemcg_id) is used as the index in root
cache's memcg_cache_params->memcg_caches array. Whenever a new kmem
active cgroup is created we must allocate an id for it. As a result, the
array size must always be greater than or equal to the number of memory
cgroups that have memcg_cache_id assigned to them.
Currently we release the id only on css free. This is bad, because css
can be zombieing around for quite a long time after css offline,
occupying an array slot and making the arrays grow larger and larger.
Although the number of arrays is limited - only root kmem caches have
them - we can still experience problems while creating new kmem active
cgroups, because they might require arrays relocation and each array
relocation will require costly high-order page allocations if there are
a lot of ids allocated. The situation will become even worse when
per-memcg list_lru's are introduced, because each super block has a
list_lru, and the number of super blocks is practically unlimited.
So let's release memcg_cache_id on css offline - there's nothing that
prevents us from doing so.
Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
---
mm/memcontrol.c | 42 ++++++++++++++++++++++++++++++++++++------
1 file changed, 36 insertions(+), 6 deletions(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3ee37189e57e..edd951e1e185 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -648,10 +648,8 @@ EXPORT_SYMBOL(memcg_kmem_enabled_key);
static void disarm_kmem_keys(struct mem_cgroup *memcg)
{
- if (memcg_kmem_is_active(memcg)) {
+ if (memcg_kmem_is_active(memcg))
static_key_slow_dec(&memcg_kmem_enabled_key);
- ida_simple_remove(&kmem_limited_groups, memcg->kmemcg_id);
- }
/*
* This check can't live in kmem destruction function,
* since the charges will outlive the cgroup
@@ -3003,6 +3001,12 @@ static void memcg_register_cache(struct mem_cgroup *memcg,
lockdep_assert_held(&memcg_slab_mutex);
id = memcg_cache_id(memcg);
+ /*
+ * The cgroup was taken offline while the create work was pending,
+ * nothing to do then.
+ */
+ if (id < 0)
+ return;
/*
* Since per-memcg caches are created asynchronously on first
@@ -3057,8 +3061,17 @@ static void memcg_unregister_cache(struct kmem_cache *cachep)
memcg = cachep->memcg_params->memcg;
id = memcg_cache_id(memcg);
- BUG_ON(root_cache->memcg_params->memcg_caches[id] != cachep);
- root_cache->memcg_params->memcg_caches[id] = NULL;
+ /*
+ * This function can be called both after and before css offline. If
+ * it's called before css offline, which happens on the root cache
+ * destruction, we should clear the slot corresponding to the cache in
+ * memcg_caches array. Otherwise the slot must have already been
+ * cleared in memcg_unregister_all_caches.
+ */
+ if (id >= 0) {
+ BUG_ON(root_cache->memcg_params->memcg_caches[id] != cachep);
+ root_cache->memcg_params->memcg_caches[id] = NULL;
+ }
list_del(&cachep->memcg_params->list);
@@ -3110,19 +3123,27 @@ void __memcg_cleanup_cache_params(struct kmem_cache *s)
static void memcg_unregister_all_caches(struct mem_cgroup *memcg)
{
struct memcg_cache_params *params, *tmp;
+ int id = memcg_cache_id(memcg);
if (!memcg_kmem_is_active(memcg))
return;
mutex_lock(&memcg_slab_mutex);
+ memcg->kmemcg_id = -1;
list_for_each_entry_safe(params, tmp, &memcg->memcg_slab_caches, list) {
struct kmem_cache *cachep = params->cachep;
+ struct kmem_cache *root_cache = params->root_cache;
+
+ BUG_ON(root_cache->memcg_params->memcg_caches[id] != cachep);
+ root_cache->memcg_params->memcg_caches[id] = NULL;
kmem_cache_shrink(cachep);
if (atomic_read(&cachep->memcg_params->nr_pages) == 0)
memcg_unregister_cache(cachep);
}
mutex_unlock(&memcg_slab_mutex);
+
+ ida_simple_remove(&kmem_limited_groups, id);
}
struct memcg_register_cache_work {
@@ -3221,6 +3242,7 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
{
struct mem_cgroup *memcg;
struct kmem_cache *memcg_cachep;
+ int id;
VM_BUG_ON(!cachep->memcg_params);
VM_BUG_ON(!cachep->memcg_params->is_root_cache);
@@ -3234,7 +3256,15 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
if (!memcg_can_account_kmem(memcg))
goto out;
- memcg_cachep = cache_from_memcg_idx(cachep, memcg_cache_id(memcg));
+ id = memcg_cache_id(memcg);
+ /*
+ * This can happen if current was migrated to another cgroup and this
+ * cgroup was taken offline after we issued mem_cgroup_from_task above.
+ */
+ if (unlikely(id < 0))
+ goto out;
+
+ memcg_cachep = cache_from_memcg_idx(cachep, id);
if (likely(memcg_cachep)) {
cachep = memcg_cachep;
goto out;
--
1.7.10.4
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