linux-kernel - Re: [PATCH v2 17/28] mm: memcg/slab: save obj

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Date:   Mon, 3 Feb 2020 14:53:31 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Bharata B Rao <bharata@...ux.ibm.com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH v2 17/28] mm: memcg/slab: save obj_cgroup for non-root
 slab objects

On Mon, Jan 27, 2020 at 09:34:42AM -0800, Roman Gushchin wrote:
> Store the obj_cgroup pointer in the corresponding place of
> page->obj_cgroups for each allocated non-root slab object.
> Make sure that each allocated object holds a reference to obj_cgroup.
> 
> Objcg pointer is obtained from the memcg->objcg dereferencing
> in memcg_kmem_get_cache() and passed from pre_alloc_hook to
> post_alloc_hook. Then in case of successful allocation(s) it's
> getting stored in the page->obj_cgroups vector.
> 
> The objcg obtaining part look a bit bulky now, but it will be simplified
> by next commits in the series.
> 
> Signed-off-by: Roman Gushchin <guro@...com>
> ---
>  include/linux/memcontrol.h |  3 +-
>  mm/memcontrol.c            | 14 +++++++--
>  mm/slab.c                  | 18 +++++++-----
>  mm/slab.h                  | 60 ++++++++++++++++++++++++++++++++++----
>  mm/slub.c                  | 14 +++++----
>  5 files changed, 88 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 30bbea3f85e2..54bfb26b5016 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1431,7 +1431,8 @@ static inline void memcg_set_shrinker_bit(struct mem_cgroup *memcg,
>  }
>  #endif
>  
> -struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep);
> +struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep,
> +					struct obj_cgroup **objcgp);
>  void memcg_kmem_put_cache(struct kmem_cache *cachep);
>  
>  #ifdef CONFIG_MEMCG_KMEM
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 94337ab1ebe9..0e9fe272e688 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2896,7 +2896,8 @@ static inline bool memcg_kmem_bypass(void)
>   * done with it, memcg_kmem_put_cache() must be called to release the
>   * reference.
>   */
> -struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep)
> +struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep,
> +					struct obj_cgroup **objcgp)
>  {
>  	struct mem_cgroup *memcg;
>  	struct kmem_cache *memcg_cachep;
> @@ -2952,8 +2953,17 @@ struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep)
>  	 */
>  	if (unlikely(!memcg_cachep))
>  		memcg_schedule_kmem_cache_create(memcg, cachep);
> -	else if (percpu_ref_tryget(&memcg_cachep->memcg_params.refcnt))
> +	else if (percpu_ref_tryget(&memcg_cachep->memcg_params.refcnt)) {
> +		struct obj_cgroup *objcg = rcu_dereference(memcg->objcg);
> +
> +		if (!objcg || !obj_cgroup_tryget(objcg)) {
> +			percpu_ref_put(&memcg_cachep->memcg_params.refcnt);
> +			goto out_unlock;
> +		}

As per the reply to the previous patch: I don't understand why the
objcg requires a pulse check here. As long as the memcg is alive and
can be charged with memory, how can the objcg disappear?