Re: [PATCH] memcg: add commens for expaing memory barrier (Was Re: Low overhead patches for the memory cgroup controller (v5)

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Fri, 26 Jun 2009 10:18:03 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-06-26 09:57:45]:
> 
> > On Tue, 23 Jun 2009 09:01:16 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > > > Do we still need the smp_wmb()?
> > > > 
> > > > It's hard to say, because we forgot to document it :(
> > > > 
> > > Sorry for lack of documentation.
> > > 
> > > pc->mem_cgroup should be visible before SetPageCgroupUsed(). Othrewise,
> > > A routine believes USED bit will see bad pc->mem_cgroup.
> > > 
> > > I'd like to  add a comment later (againt new mmotm.)
> > > 
> > 
> > Ok, it's now.
> > ==
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > 
> > Add comments for the reason of smp_wmb() in mem_cgroup_commit_charge().
> > 
> > Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> > Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > ---
> >  mm/memcontrol.c |    7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > Index: mmotm-2.6.31-Jun25/mm/memcontrol.c
> > ===================================================================
> > --- mmotm-2.6.31-Jun25.orig/mm/memcontrol.c
> > +++ mmotm-2.6.31-Jun25/mm/memcontrol.c
> > @@ -1134,6 +1134,13 @@ static void __mem_cgroup_commit_charge(s
> >  	}
> > 
> >  	pc->mem_cgroup = mem;
> > +	/*
> > + 	 * We access a page_cgroup asynchronously without lock_page_cgroup().
> > + 	 * Especially when a page_cgroup is taken from a page, pc->mem_cgroup
> > + 	 * is accessed after testing USED bit. To make pc->mem_cgroup visible
> > + 	 * before USED bit, we need memory barrier here.
> > + 	 * See mem_cgroup_add_lru_list(), etc.
> > + 	 */
> 
> 
> I don't think this is sufficient, since in
> mem_cgroup_get_reclaim_stat_from_page() we say we need this since we
> set used bit without atomic operation. The used bit is now atomically
> set. I think we need to reword other comments as well.
>  
ok, plz.

Maybe we need total review. 

Thanks,
-Kame


> 
> -- 
> 	Balbir
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