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Date:   Tue, 16 Oct 2018 14:49:39 +0200
From:   Andrea Parri <andrea.parri@amarulasolutions.com>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@kernel.org,
        vdavydov.dev@gmail.com, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH] mm: Convert mem_cgroup_id::ref to refcount_t type
Message-ID: <20181016124939.GA13278@andrea>
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Hi Kirill,

On Tue, Oct 09, 2018 at 08:46:56PM +0300, Kirill Tkhai wrote:
> This will allow to use generic refcount_t interfaces
> to check counters overflow instead of currently existing
> VM_BUG_ON(). The only difference after the patch is
> VM_BUG_ON() may cause BUG(), while refcount_t fires
> with WARN().

refcount_{sub_and_test,inc_not_zero}() are documented to provide
"slightly" more relaxed ordering than their atomic_* counterpart,
c.f.,

  Documentation/core-api/refcount-vs-atomic.rst
  lib/refcount.c (inline comments)

IIUC, this semantic change won't cause problems here (but please
double-check? ;D ).

  Andrea


>But this seems not to be significant here,
> since such the problems are usually caught by syzbot
> with panic-on-warn enabled.
> 
> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> ---
>  include/linux/memcontrol.h |    2 +-
>  mm/memcontrol.c            |   10 ++++------
>  2 files changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 4399cc3f00e4..7ab2120155a4 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -78,7 +78,7 @@ struct mem_cgroup_reclaim_cookie {
>  
>  struct mem_cgroup_id {
>  	int id;
> -	atomic_t ref;
> +	refcount_t ref;
>  };
>  
>  /*
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 7bebe2ddec05..aa728d5b3d72 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4299,14 +4299,12 @@ static void mem_cgroup_id_remove(struct mem_cgroup *memcg)
>  
>  static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n)
>  {
> -	VM_BUG_ON(atomic_read(&memcg->id.ref) <= 0);
> -	atomic_add(n, &memcg->id.ref);
> +	refcount_add(n, &memcg->id.ref);
>  }
>  
>  static void mem_cgroup_id_put_many(struct mem_cgroup *memcg, unsigned int n)
>  {
> -	VM_BUG_ON(atomic_read(&memcg->id.ref) < n);
> -	if (atomic_sub_and_test(n, &memcg->id.ref)) {
> +	if (refcount_sub_and_test(n, &memcg->id.ref)) {
>  		mem_cgroup_id_remove(memcg);
>  
>  		/* Memcg ID pins CSS */
> @@ -4523,7 +4521,7 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
>  	}
>  
>  	/* Online state pins memcg ID, memcg ID pins CSS */
> -	atomic_set(&memcg->id.ref, 1);
> +	refcount_set(&memcg->id.ref, 1);
>  	css_get(css);
>  	return 0;
>  }
> @@ -6357,7 +6355,7 @@ subsys_initcall(mem_cgroup_init);
>  #ifdef CONFIG_MEMCG_SWAP
>  static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
>  {
> -	while (!atomic_inc_not_zero(&memcg->id.ref)) {
> +	while (!refcount_inc_not_zero(&memcg->id.ref)) {
>  		/*
>  		 * The root cgroup cannot be destroyed, so it's refcount must
>  		 * always be >= 1.
>