Re: [PATCH RESEND -mm 01/12] memcg: flush cache creation works before memcg cache destruction

[Michal Hocko <mhocko@suse.cz>]

On Thu 13-03-14 19:06:39, Vladimir Davydov wrote:
> When we get to memcg cache destruction, either from the root cache
> destruction path or when turning memcg offline, there still might be
> memcg cache creation works pending that was scheduled before we
> initiated destruction. We need to flush them before starting to destroy
> memcg caches, otherwise we can get a leaked kmem cache or, even worse,
> an attempt to use after free.

How can we use-after-free? Even if there is a pending work item to
create a new cache then we keep the css reference for the memcg and
release it from the worker (memcg_create_cache_work_func). So although
this can race with memcg offlining the memcg itself will be still alive.

> Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.cz>
> Cc: Glauber Costa <glommer@gmail.com>
> ---
>  mm/memcontrol.c |   32 +++++++++++++++++++++++++++++++-
>  1 file changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 9d489a9e7701..b183aaf1b616 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2904,6 +2904,7 @@ static DEFINE_MUTEX(set_limit_mutex);
>  
>  #ifdef CONFIG_MEMCG_KMEM
>  static DEFINE_MUTEX(activate_kmem_mutex);
> +static struct workqueue_struct *memcg_cache_create_wq;
>  
>  static inline bool memcg_can_account_kmem(struct mem_cgroup *memcg)
>  {
> @@ -3327,6 +3328,15 @@ int __kmem_cache_destroy_memcg_children(struct kmem_cache *s)
>  	int i, failed = 0;
>  
>  	/*
> +	 * Since the cache is being destroyed, it shouldn't be allocated from
> +	 * any more, and therefore no new memcg cache creation works could be
> +	 * scheduled. However, there still might be pending works scheduled
> +	 * before the cache destruction was initiated. Flush them before
> +	 * destroying child caches to avoid nasty races.
> +	 */
> +	flush_workqueue(memcg_cache_create_wq);
> +
> +	/*
>  	 * If the cache is being destroyed, we trust that there is no one else
>  	 * requesting objects from it. Even if there are, the sanity checks in
>  	 * kmem_cache_destroy should caught this ill-case.
> @@ -3374,6 +3384,15 @@ static void mem_cgroup_destroy_all_caches(struct mem_cgroup *memcg)
>  	if (!memcg_kmem_is_active(memcg))
>  		return;
>  
> +	/*
> +	 * By the time we get here, the cgroup must be empty. That said no new
> +	 * allocations can happen from its caches, and therefore no new memcg
> +	 * cache creation works can be scheduled. However, there still might be
> +	 * pending works scheduled before the cgroup was turned offline. Flush
> +	 * them before destroying memcg caches to avoid nasty races.
> +	 */
> +	flush_workqueue(memcg_cache_create_wq);
> +
>  	mutex_lock(&memcg->slab_caches_mutex);
>  	list_for_each_entry(params, &memcg->memcg_slab_caches, list) {
>  		cachep = memcg_params_to_cache(params);
> @@ -3418,7 +3437,7 @@ static void __memcg_create_cache_enqueue(struct mem_cgroup *memcg,
>  	cw->cachep = cachep;
>  
>  	INIT_WORK(&cw->work, memcg_create_cache_work_func);
> -	schedule_work(&cw->work);
> +	queue_work(memcg_cache_create_wq, &cw->work);
>  }
>  
>  static void memcg_create_cache_enqueue(struct mem_cgroup *memcg,
> @@ -3621,10 +3640,20 @@ void __memcg_kmem_uncharge_pages(struct page *page, int order)
>  	VM_BUG_ON_PAGE(mem_cgroup_is_root(memcg), page);
>  	memcg_uncharge_kmem(memcg, PAGE_SIZE << order);
>  }
> +
> +static void __init memcg_kmem_init(void)
> +{
> +	memcg_cache_create_wq = alloc_workqueue("memcg_cache_create", 0, 1);
> +	BUG_ON(!memcg_cache_create_wq);
> +}
>  #else
>  static inline void mem_cgroup_destroy_all_caches(struct mem_cgroup *memcg)
>  {
>  }
> +
> +static void __init memcg_kmem_init(void)
> +{
> +}
>  #endif /* CONFIG_MEMCG_KMEM */
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> @@ -7181,6 +7210,7 @@ static int __init mem_cgroup_init(void)
>  	enable_swap_cgroup();
>  	mem_cgroup_soft_limit_tree_init();
>  	memcg_stock_init();
> +	memcg_kmem_init();
>  	return 0;
>  }
>  subsys_initcall(mem_cgroup_init);
> -- 
> 1.7.10.4
> 

-- 
Michal Hocko
SUSE Labs

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