linux-kernel - Re: [PATCH] memcg: fix kmem_account_flags check in memcg_can_account

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Date:	Fri, 29 Nov 2013 10:45:02 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	cgroups@...r.kernel.org, devel@...nvz.org,
	Johannes Weiner <hannes@...xchg.org>,
	Balbir Singh <bsingharora@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] memcg: fix kmem_account_flags check in
 memcg_can_account_kmem()

On Wed 27-11-13 19:46:01, Vladimir Davydov wrote:
> We should start kmem accounting for a memory cgroup only after both its
> kmem limit is set (KMEM_ACCOUNTED_ACTIVE) and related call sites are
> patched (KMEM_ACCOUNTED_ACTIVATED).

This should be vice-versa, no? ACTIVE is set after
static_key_slow_inc(&memcg_kmem_enabled_key) AFAICS.

> Currently memcg_can_account_kmem() allows kmem accounting even if only
> one of the conditions is true.
> Fix it.

It would be nice to describe, what is the actual problem here. I assume
this is a charge vs. enable race. Let me try

So we have KMEM_ACCOUNTED_ACTIVATED (set by memcg_update_cache_sizes)
but the static key is not enabled yet (so KMEM_ACCOUNTED_ACTIVE is not
set yet). memcg_can_account_kmem is called from 2 contexts during charge
	- memcg_kmem_get_cache via __memcg_kmem_get_cache
	- memcg_kmem_newpage_charge via __memcg_kmem_newpage_charge

both of them start by checking memcg_kmem_enabled which is our
static key before memcg_can_account_kmem. This would suggest that
static_key+ACTIVE check memcg_can_account_kmem is sufficient. No?

That being said the proposed change is not incorrect it just doesn't
seem to _fix_ anything. I would much rather see a comprehensive
documentation of the whole enabling workflow. E.g. why do we need
ACTIVATED at all? Nobody seem to care in the code...
 
> Signed-off-by: Vladimir Davydov <vdavydov@...allels.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: Balbir Singh <bsingharora@...il.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
>  mm/memcontrol.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index f1a0ae6..40efb9d 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2956,7 +2956,8 @@ static DEFINE_MUTEX(set_limit_mutex);
>  static inline bool memcg_can_account_kmem(struct mem_cgroup *memcg)
>  {
>  	return !mem_cgroup_disabled() && !mem_cgroup_is_root(memcg) &&
> -		(memcg->kmem_account_flags & KMEM_ACCOUNTED_MASK);
> +		(memcg->kmem_account_flags & KMEM_ACCOUNTED_MASK) ==
> +							KMEM_ACCOUNTED_MASK;
>  }
>  
>  /*
> -- 
> 1.7.10.4
> 

-- 
Michal Hocko
SUSE Labs
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