[Devel] [PATCH -mm 03/12] memcg: fix root vs memcg cache destruction race
Vladimir Davydov
vdavydov at parallels.com
Wed Feb 26 07:05:08 PST 2014
When a root cache is being destroyed and we are about to initiate
destruction of its child caches (see kmem_cache_destroy_memcg_children),
we should handle races with pending memcg cache destruction works
somehow. Currently, we simply cancel the memcg_params::destroy work
before calling kmem_cache_destroy for a memcg cache, but that's totally
wrong, because the work handler may destroy and free the cache resulting
in a use-after-free in cancel_work_sync. Furthermore, we do not handle
the case when memcg cache destruction is scheduled after we start to
destroy the root cache - that's possible, because nothing prevents
memcgs from going offline while we are destroying a root cache. In that
case we are likely to get a use-after-free in the work handler.
This patch resolves the race as follows:
1) It makes kmem_cache_destroy silently exit if it is called for a memcg
cache while the corresponding root cache is being destroyed, leaving
the destruction to kmem_cache_destroy_memcg_children. That makes call
to cancel_work_sync safe if it is called from the root cache
destruction path.
2) It moves cancel_work_sync to be called after we unregistered a child
cache from its memcg. That prevents the work from being rescheduled
from memcg offline path.
Signed-off-by: Vladimir Davydov <vdavydov at parallels.com>
Cc: Johannes Weiner <hannes at cmpxchg.org>
Cc: Michal Hocko <mhocko at suse.cz>
Cc: Glauber Costa <glommer at gmail.com>
---
include/linux/memcontrol.h | 2 +-
include/linux/slab.h | 1 +
mm/memcontrol.c | 22 ++-----------
mm/slab_common.c | 77 ++++++++++++++++++++++++++++++++++++--------
4 files changed, 69 insertions(+), 33 deletions(-)
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index e54fb469a908..6128323ea453 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -512,7 +512,7 @@ void memcg_update_array_size(int num_groups);
struct kmem_cache *
__memcg_kmem_get_cache(struct kmem_cache *cachep, gfp_t gfp);
-int __kmem_cache_destroy_memcg_children(struct kmem_cache *s);
+int kmem_cache_destroy_memcg_children(struct kmem_cache *s);
/**
* memcg_kmem_newpage_charge: verify if a new kmem allocation is allowed.
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 3ed53de256ea..ee9f1b0382ac 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -117,6 +117,7 @@ struct kmem_cache *kmem_cache_create(const char *, size_t, size_t,
void (*)(void *));
#ifdef CONFIG_MEMCG_KMEM
void kmem_cache_create_memcg(struct mem_cgroup *, struct kmem_cache *);
+void kmem_cache_destroy_memcg(struct kmem_cache *, bool);
#endif
void kmem_cache_destroy(struct kmem_cache *);
int kmem_cache_shrink(struct kmem_cache *);
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 416da5dc3d2a..1b9634090454 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3345,10 +3345,10 @@ static void kmem_cache_destroy_work_func(struct work_struct *w)
return;
}
- kmem_cache_destroy(cachep);
+ kmem_cache_destroy_memcg(cachep, false);
}
-int __kmem_cache_destroy_memcg_children(struct kmem_cache *s)
+int kmem_cache_destroy_memcg_children(struct kmem_cache *s)
{
struct kmem_cache *c;
int i, failed = 0;
@@ -3378,23 +3378,7 @@ int __kmem_cache_destroy_memcg_children(struct kmem_cache *s)
if (!c)
continue;
- /*
- * We will now manually delete the caches, so to avoid races
- * we need to cancel all pending destruction workers and
- * proceed with destruction ourselves.
- *
- * kmem_cache_destroy() will call kmem_cache_shrink internally,
- * and that could spawn the workers again: it is likely that
- * the cache still have active pages until this very moment.
- * This would lead us back to memcg_release_pages().
- *
- * But that will not execute at all if the refcount is > 0, so
- * increment it to guarantee we are in control.
- */
- atomic_inc(&c->memcg_params->refcount);
- cancel_work_sync(&c->memcg_params->destroy);
- kmem_cache_destroy(c);
-
+ kmem_cache_destroy_memcg(c, true);
if (cache_from_memcg_idx(s, i))
failed++;
}
diff --git a/mm/slab_common.c b/mm/slab_common.c
index f3cfccf76dda..05ba3cd1b507 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -302,28 +302,70 @@ out_unlock:
put_online_cpus();
}
-static int kmem_cache_destroy_memcg_children(struct kmem_cache *s)
+static void __kmem_cache_destroy(struct kmem_cache *, bool);
+
+void kmem_cache_destroy_memcg(struct kmem_cache *s, bool destroying_root)
+{
+ BUG_ON(is_root_cache(s));
+ __kmem_cache_destroy(s, destroying_root);
+}
+
+static int __kmem_cache_shutdown_memcg(struct kmem_cache *s,
+ bool destroying_root)
{
- int rc;
+ int rc = 0;
- if (!s->memcg_params ||
- !s->memcg_params->is_root_cache)
+ if (!destroying_root) {
+ struct kmem_cache *root;
+
+ root = memcg_root_cache(s);
+ BUG_ON(root == s);
+ /*
+ * If we are racing with the root cache destruction, let
+ * kmem_cache_destroy_memcg_children() finish with this cache.
+ */
+ if (!root->refcount) {
+ s->refcount++;
+ return 1;
+ }
+ }
+
+ if (!s->memcg_params)
return 0;
mutex_unlock(&slab_mutex);
- rc = __kmem_cache_destroy_memcg_children(s);
+ if (s->memcg_params->is_root_cache) {
+ rc = kmem_cache_destroy_memcg_children(s);
+ } else {
+ /*
+ * There might be a destruction work pending, which needs to be
+ * cancelled before we start to destroy the cache.
+ *
+ * This should be done after we deleted all the references to
+ * this cache, otherwise the work could be rescheduled.
+ *
+ * __kmem_cache_shutdown() will call kmem_cache_shrink()
+ * internally, and that could spawn the worker again. We
+ * increment the refcount to avoid that.
+ */
+ if (destroying_root) {
+ cancel_work_sync(&s->memcg_params->destroy);
+ atomic_inc(&s->memcg_params->refcount);
+ }
+ }
mutex_lock(&slab_mutex);
return rc;
}
#else
-static int kmem_cache_destroy_memcg_children(struct kmem_cache *s)
+static int __kmem_cache_shutdown_memcg(struct kmem_cache *s,
+ bool destroying_root)
{
return 0;
}
#endif /* CONFIG_MEMCG_KMEM */
-void kmem_cache_destroy(struct kmem_cache *s)
+static void __kmem_cache_destroy(struct kmem_cache *s, bool destroying_root)
{
get_online_cpus();
mutex_lock(&slab_mutex);
@@ -332,19 +374,17 @@ void kmem_cache_destroy(struct kmem_cache *s)
if (s->refcount)
goto out_unlock;
- if (kmem_cache_destroy_memcg_children(s) != 0)
- goto out_unlock;
-
list_del(&s->list);
memcg_unregister_cache(s);
+ if (__kmem_cache_shutdown_memcg(s, destroying_root) != 0)
+ goto out_undelete;
+
if (__kmem_cache_shutdown(s) != 0) {
- list_add(&s->list, &slab_caches);
- memcg_register_cache(s);
printk(KERN_ERR "kmem_cache_destroy %s: "
"Slab cache still has objects\n", s->name);
dump_stack();
- goto out_unlock;
+ goto out_undelete;
}
mutex_unlock(&slab_mutex);
@@ -360,6 +400,17 @@ out_unlock:
mutex_unlock(&slab_mutex);
out_put_cpus:
put_online_cpus();
+ return;
+out_undelete:
+ list_add(&s->list, &slab_caches);
+ memcg_register_cache(s);
+ goto out_unlock;
+}
+
+void kmem_cache_destroy(struct kmem_cache *s)
+{
+ BUG_ON(!is_root_cache(s));
+ __kmem_cache_destroy(s, true);
}
EXPORT_SYMBOL(kmem_cache_destroy);
--
1.7.10.4
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