linux-kernel - Re: [PATCH v3 3/5] mm: kmem: make memcg keep a reference to the original objcg

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Message-ID: <802261a7-553d-348f-7dd4-109eac704727@suse.cz>
Date:   Wed, 18 Oct 2023 16:06:35 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Roman Gushchin <roman.gushchin@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, shakeelb@...gle.com,
        Muchun Song <muchun.song@...ux.dev>,
        Dennis Zhou <dennis@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>
Subject: Re: [PATCH v3 3/5] mm: kmem: make memcg keep a reference to the
 original objcg

On 10/17/23 00:18, Roman Gushchin wrote:
> Keep a reference to the original objcg object for the entire life
> of a memcg structure.
> 
> This allows to simplify the synchronization on the kernel memory
> allocation paths: pinning a (live) memcg will also pin the
> corresponding objcg.
> 
> The memory overhead of this change is minimal because object cgroups
> usually outlive their corresponding memory cgroups even without this
> change, so it's only an additional pointer per memcg.
> 
> Signed-off-by: Roman Gushchin (Cruise) <roman.gushchin@...ux.dev>
> Tested-by: Naresh Kamboju <naresh.kamboju@...aro.org>
> Acked-by: Shakeel Butt <shakeelb@...gle.com>

Forgot to point out typo:

> ---
>  include/linux/memcontrol.h | 8 +++++++-
>  mm/memcontrol.c            | 5 +++++
>  2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index ab94ad4597d0..277690af383d 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -299,7 +299,13 @@ struct mem_cgroup {
>  
>  #ifdef CONFIG_MEMCG_KMEM
>  	int kmemcg_id;
> -	struct obj_cgroup __rcu *objcg;
> +	/*
> +	 * memcg->objcg is wiped out as a part of the objcg repaprenting

						reparenting ^

> +	 * process. memcg->orig_objcg preserves a pointer (and a reference)
> +	 * to the original objcg until the end of live of memcg.
> +	 */
> +	struct obj_cgroup __rcu	*objcg;
> +	struct obj_cgroup	*orig_objcg;
>  	/* list of inherited objcgs, protected by objcg_lock */
>  	struct list_head objcg_list;
>  #endif
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 0605e45bd4a2..d90cc19e4113 100644