linux-kernel - Re: [PATCH RFC 02/14] mm: memcg: introduce mem_cgroup

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Date:   Thu, 5 Sep 2019 22:34:37 +0000
From:   Roman Gushchin <guro@...com>
To:     "linux-mm@...ck.org" <linux-mm@...ck.org>
CC:     Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH RFC 02/14] mm: memcg: introduce mem_cgroup_ptr

On Thu, Sep 05, 2019 at 02:45:46PM -0700, Roman Gushchin wrote:
> This commit introduces mem_cgroup_ptr structure and corresponding API.
> It implements a pointer to a memory cgroup with a built-in reference
> counter. The main goal of it is to implement reparenting efficiently.
> 
> If a number of objects (e.g. slab pages) have to keep a pointer and
> a reference to a memory cgroup, they can use mem_cgroup_ptr instead.
> On reparenting, only one mem_cgroup_ptr->memcg pointer has to be
> changed, instead of walking over all accounted objects.
> 
> mem_cgroup_ptr holds a single reference to the corresponding memory
> cgroup. Because it's initialized before the css reference counter,
> css's refcounter can't be bumped at allocation time. Instead, it's
> bumped on reparenting which happens during offlining. A cgroup is
> never released online, so it's fine.
> 
> mem_cgroup_ptr is released using rcu, so memcg->kmem_memcg_ptr can
> be accessed in a rcu read section. On reparenting it's atomically
> switched to NULL. If the reader gets NULL, it can just read parent's
> kmem_memcg_ptr instead.
> 
> Each memory cgroup contains a list of kmem_memcg_ptrs. On reparenting
> the list is spliced into the parent's list. The list is protected
> using the css set lock.
> 
> Signed-off-by: Roman Gushchin <guro@...com>
> ---
>  include/linux/memcontrol.h | 50 ++++++++++++++++++++++
>  mm/memcontrol.c            | 87 ++++++++++++++++++++++++++++++++++++--
>  2 files changed, 133 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 120d39066148..dd5ebfe5a86c 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -23,6 +23,7 @@
>  #include <linux/page-flags.h>
>  
>  struct mem_cgroup;
> +struct mem_cgroup_ptr;
>  struct page;
>  struct mm_struct;
>  struct kmem_cache;
> @@ -197,6 +198,22 @@ struct memcg_cgwb_frn {
>  	int memcg_id;			/* memcg->css.id of foreign inode */
>  	u64 at;				/* jiffies_64 at the time of dirtying */
>  	struct wb_completion done;	/* tracks in-flight foreign writebacks */
> +}

Oops, a semicolon has been lost during the final rebase.
I'll send a correct version of this patch separately.

Sorry for the mess.