[Devel] Re: [PATCH v4 05/25] memcg: Always free struct memcg through schedule_work()

[Michal Hocko]

On Mon 18-06-12 14:27:58, Glauber Costa wrote:
> Right now we free struct memcg with kfree right after a
> rcu grace period, but defer it if we need to use vfree() to get
> rid of that memory area. We do that by need, because we need vfree
> to be called in a process context.
> 
> This patch unifies this behavior, by ensuring that even kfree will
> happen in a separate thread. The goal is to have a stable place to
> call the upcoming jump label destruction function outside the realm
> of the complicated and quite far-reaching cgroup lock (that can't be
> held when calling neither the cpu_hotplug.lock nor the jump_label_mutex)

This one is in memcg-devel (mmotm) tree for quite some time with acks
from me and Kamezawa.

> Signed-off-by: Glauber Costa <glommer at parallels.com>
> CC: Tejun Heo <tj at kernel.org>
> CC: Li Zefan <lizefan at huawei.com>
> CC: Kamezawa Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
> CC: Johannes Weiner <hannes at cmpxchg.org>
> CC: Michal Hocko <mhocko at suse.cz>
> ---
>  mm/memcontrol.c |   24 +++++++++++++-----------
>  1 file changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e3b528e..ce15be4 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -245,8 +245,8 @@ struct mem_cgroup {
>  		 */
>  		struct rcu_head rcu_freeing;
>  		/*
> -		 * But when using vfree(), that cannot be done at
> -		 * interrupt time, so we must then queue the work.
> +		 * We also need some space for a worker in deferred freeing.
> +		 * By the time we call it, rcu_freeing is not longer in use.
>  		 */
>  		struct work_struct work_freeing;
>  	};
> @@ -4826,23 +4826,28 @@ out_free:
>  }
>  
>  /*
> - * Helpers for freeing a vzalloc()ed mem_cgroup by RCU,
> + * Helpers for freeing a kmalloc()ed/vzalloc()ed mem_cgroup by RCU,
>   * but in process context.  The work_freeing structure is overlaid
>   * on the rcu_freeing structure, which itself is overlaid on memsw.
>   */
> -static void vfree_work(struct work_struct *work)
> +static void free_work(struct work_struct *work)
>  {
>  	struct mem_cgroup *memcg;
> +	int size = sizeof(struct mem_cgroup);
>  
>  	memcg = container_of(work, struct mem_cgroup, work_freeing);
> -	vfree(memcg);
> +	if (size < PAGE_SIZE)
> +		kfree(memcg);
> +	else
> +		vfree(memcg);
>  }
> -static void vfree_rcu(struct rcu_head *rcu_head)
> +
> +static void free_rcu(struct rcu_head *rcu_head)
>  {
>  	struct mem_cgroup *memcg;
>  
>  	memcg = container_of(rcu_head, struct mem_cgroup, rcu_freeing);
> -	INIT_WORK(&memcg->work_freeing, vfree_work);
> +	INIT_WORK(&memcg->work_freeing, free_work);
>  	schedule_work(&memcg->work_freeing);
>  }
>  
> @@ -4868,10 +4873,7 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
>  		free_mem_cgroup_per_zone_info(memcg, node);
>  
>  	free_percpu(memcg->stat);
> -	if (sizeof(struct mem_cgroup) < PAGE_SIZE)
> -		kfree_rcu(memcg, rcu_freeing);
> -	else
> -		call_rcu(&memcg->rcu_freeing, vfree_rcu);
> +	call_rcu(&memcg->rcu_freeing, free_rcu);
>  }
>  
>  static void mem_cgroup_get(struct mem_cgroup *memcg)
> -- 
> 1.7.10.2
> 
> --
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic