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Subject[PATCH v5.4.y, v4.19.y] mm: memcg/slab: fix memory leak at non-root kmem_cache destroy
Date
If the kmem_cache refcount is greater than one, we should not
mark the root kmem_cache as dying. If we mark the root kmem_cache
dying incorrectly, the non-root kmem_cache can never be destroyed.
It resulted in memory leak when memcg was destroyed. We can use the
following steps to reproduce.

1) Use kmem_cache_create() to create a new kmem_cache named A.
2) Coincidentally, the kmem_cache A is an alias for kmem_cache B,
so the refcount of B is just increased.
3) Use kmem_cache_destroy() to destroy the kmem_cache A, just
decrease the B's refcount but mark the B as dying.
4) Create a new memory cgroup and alloc memory from the kmem_cache
A. It leads to create a non-root kmem_cache for allocating.
5) When destroy the memory cgroup created in the step 4), the
non-root kmem_cache can never be destroyed.

If we repeat steps 4) and 5), this will cause a lot of memory leak.
So only when refcount reach zero, we mark the root kmem_cache as dying.

Fixes: 92ee383f6daa ("mm: fix race between kmem_cache destroy, create and deactivate")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
mm/slab_common.c | 43 +++++++++++++++++++++++++++++++++++++++++--
1 file changed, 41 insertions(+), 2 deletions(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 8c1ffbf7de45..83ee6211aec7 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -258,6 +258,11 @@ static void memcg_unlink_cache(struct kmem_cache *s)
list_del(&s->memcg_params.kmem_caches_node);
}
}
+
+static inline bool memcg_kmem_cache_dying(struct kmem_cache *s)
+{
+ return is_root_cache(s) && s->memcg_params.dying;
+}
#else
static inline int init_memcg_params(struct kmem_cache *s,
struct kmem_cache *root_cache)
@@ -272,6 +277,11 @@ static inline void destroy_memcg_params(struct kmem_cache *s)
static inline void memcg_unlink_cache(struct kmem_cache *s)
{
}
+
+static inline bool memcg_kmem_cache_dying(struct kmem_cache *s)
+{
+ return false;
+}
#endif /* CONFIG_MEMCG_KMEM */

/*
@@ -326,6 +336,13 @@ int slab_unmergeable(struct kmem_cache *s)
if (s->refcount < 0)
return 1;

+ /*
+ * If the kmem_cache is dying. We should also skip this
+ * kmem_cache.
+ */
+ if (memcg_kmem_cache_dying(s))
+ return 1;
+
return 0;
}

@@ -944,8 +961,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
if (unlikely(!s))
return;

- flush_memcg_workqueue(s);
-
get_online_cpus();
get_online_mems();

@@ -955,6 +970,30 @@ void kmem_cache_destroy(struct kmem_cache *s)
if (s->refcount)
goto out_unlock;

+#ifdef CONFIG_MEMCG_KMEM
+ mutex_unlock(&slab_mutex);
+
+ put_online_mems();
+ put_online_cpus();
+
+ flush_memcg_workqueue(s);
+
+ get_online_cpus();
+ get_online_mems();
+
+ mutex_lock(&slab_mutex);
+
+ if (WARN(s->refcount,
+ "kmem_cache_destroy %s: Slab cache is still referenced\n",
+ s->name)) {
+ /*
+ * Reset the dying flag setted by flush_memcg_workqueue().
+ */
+ s->memcg_params.dying = false;
+ goto out_unlock;
+ }
+#endif
+
err = shutdown_memcg_caches(s);
if (!err)
err = shutdown_cache(s);
--
2.11.0
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