linux-kernel - Re: [BUGFIX][PATCH] memcg: get/put parents at create/free

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Date:	Fri, 16 Jan 2009 11:17:02 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	"LKML" <linux-kernel@...r.kernel.org>,
	"linux-mm" <linux-mm@...ck.org>,
	"Balbir Singh" <balbir@...ux.vnet.ibm.com>,
	"Pavel Emelyanov" <xemul@...nvz.org>,
	"Li Zefan" <lizf@...fujitsu.com>, "Paul Menage" <menage@...gle.com>
Subject: Re: [BUGFIX][PATCH] memcg: get/put parents at create/free

On Thu, 15 Jan 2009 18:12:43 -0800
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Fri, 16 Jan 2009 10:50:09 +0900 Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:
> 
> > This version works well in my test.
> > 
> > Andrew, please pick up this one.
> > 
> > ===
> > From: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> > 
> > The lifetime of struct cgroup and struct mem_cgroup is different and
> > mem_cgroup has its own reference count for handling references from swap_cgroup.
> > 
> > This causes strange problem that the parent mem_cgroup dies while
> > child mem_cgroup alive, and this problem causes a bug in case of use_hierarchy==1
> > because res_counter_uncharge climbs up the tree.
> > 
> > This patch is for avoiding it by getting the parent at create, and
> > putting it at freeing.
> > 
> > Signed-off-by: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> > Reviewed-by; KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > ---
> >  mm/memcontrol.c |   23 ++++++++++++++++++++++-
> >  1 files changed, 22 insertions(+), 1 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index fb62b43..45e1b51 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -202,6 +202,8 @@ pcg_default_flags[NR_CHARGE_TYPE] = {
> >  
> >  static void mem_cgroup_get(struct mem_cgroup *mem);
> >  static void mem_cgroup_put(struct mem_cgroup *mem);
> > +static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem);
> > +static void mem_cgroup_get_parent(struct mem_cgroup *mem);
> >  
> >  static void mem_cgroup_charge_statistics(struct mem_cgroup *mem,
> >  					 struct page_cgroup *pc,
> > @@ -2185,10 +2187,28 @@ static void mem_cgroup_get(struct mem_cgroup *mem)
> >  
> >  static void mem_cgroup_put(struct mem_cgroup *mem)
> >  {
> > -	if (atomic_dec_and_test(&mem->refcnt))
> > +	if (atomic_dec_and_test(&mem->refcnt)) {
> > +		struct mem_cgroup *parent = parent_mem_cgroup(mem);
> >  		__mem_cgroup_free(mem);
> > +		if (parent)
> > +			mem_cgroup_put(parent);
> > +	}
> > +}
> > +
> > +static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem)
> > +{
> > +	if (!mem->res.parent)
> > +		return NULL;
> > +	return mem_cgroup_from_res_counter(mem->res.parent, res);
> >  }
> >  
> > +static void mem_cgroup_get_parent(struct mem_cgroup *mem)
> > +{
> > +	struct mem_cgroup *parent = parent_mem_cgroup(mem);
> > +
> > +	if (parent)
> > +		mem_cgroup_get(parent);
> > +}
> >  
> >  #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
> >  static void __init enable_swap_cgroup(void)
> > @@ -2237,6 +2257,7 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
> >  	if (parent)
> >  		mem->swappiness = get_swappiness(parent);
> >  	atomic_set(&mem->refcnt, 1);
> > +	mem_cgroup_get_parent(mem);
> >  	return &mem->css;
> >  free_out:
> >  	__mem_cgroup_free(mem);
> 
> It seems strange that we add a little helper function for the get(),
> but open-code the put()?
> 
Maybe I don't feel this as strange because I saw update history of this patch ;(
As you pointed out, I like open-code rather than helper here. Nishimura-san,
could you update ?

Thanks,
-Kame




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