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

[Balbir Singh]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2009-06-26 09:57:45]:

> On Tue, 23 Jun 2009 09:01:16 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> 
> Add comments for the reason of smp_wmb() in mem_cgroup_commit_charge().
> 
> Cc: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> Cc: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> ---
>  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.
 

-- 
	Balbir
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