Re: [PATCH 2/2] memcg make move_task_charge check clearer

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

On Thu, 3 Jun 2010 11:51:19 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> 
> Now, for checking a memcg is under task-account-moving, we do css_tryget()
> against mc.to and mc.from. But this is just complicating things. This patch
> makes the check easier.
> 
> This patch adds a spinlock to move_charge_struct and guard modification
> of mc.to and mc.from. By this, we don't have to think about complicated
> races around this not-critical path.
> 
> Changelog:
>  - removed disable/enable irq.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
>  mm/memcontrol.c |   48 ++++++++++++++++++++++++++++--------------------
>  1 file changed, 28 insertions(+), 20 deletions(-)
> 
> Index: mmotm-2.6.34-May21/mm/memcontrol.c
> ===================================================================
> --- mmotm-2.6.34-May21.orig/mm/memcontrol.c
> +++ mmotm-2.6.34-May21/mm/memcontrol.c
> @@ -268,6 +268,7 @@ enum move_type {
>  
>  /* "mc" and its members are protected by cgroup_mutex */
>  static struct move_charge_struct {
> +	spinlock_t	  lock; /* for from, to, moving_task */
>  	struct mem_cgroup *from;
>  	struct mem_cgroup *to;
>  	unsigned long precharge;
> @@ -276,6 +277,7 @@ static struct move_charge_struct {
>  	struct task_struct *moving_task;	/* a task moving charges */
>  	wait_queue_head_t waitq;		/* a waitq for other context */
>  } mc = {
> +	.lock = __SPIN_LOCK_UNLOCKED(mc.lock),
>  	.waitq = __WAIT_QUEUE_HEAD_INITIALIZER(mc.waitq),
>  };
>  
> @@ -1076,26 +1078,25 @@ static unsigned int get_swappiness(struc
>  
>  static bool mem_cgroup_under_move(struct mem_cgroup *mem)
>  {
> -	struct mem_cgroup *from = mc.from;
> -	struct mem_cgroup *to = mc.to;
> +	struct mem_cgroup *from;
> +	struct mem_cgroup *to;
>  	bool ret = false;
> -
> -	if (from == mem || to == mem)
> -		return true;
> -
> -	if (!from || !to || !mem->use_hierarchy)
> -		return false;
> -
> -	rcu_read_lock();
> -	if (css_tryget(&from->css)) {
> -		ret = css_is_ancestor(&from->css, &mem->css);
> -		css_put(&from->css);
> -	}
> -	if (!ret && css_tryget(&to->css)) {
> -		ret = css_is_ancestor(&to->css,	&mem->css);
> +	/*
> +	 * Unlike task_move routines, we access mc.to, mc.from not under
> +	 * mutual exclusion by cgroup_mutex. Here, we take spinlock instead.
> +	 */
> +	spin_lock(&mc.lock);
> +	from = mc.from;
> +	to = mc.to;
> +	if (!from)
> +		goto unlock;
> +	if (from == mem || to == mem
> +	    || (mem->use_hierarchy && css_is_ancestor(&from->css, &mem->css))
> +	    || (mem->use_hierarchy && css_is_ancestor(&to->css,	&mem->css)))
>  		css_put(&to->css);
I think this css_put() must be removed too.
Except for it, this patch looks good to me.

Thank you for cleaning up my dirty code :)

Thanks,
Daisuke Nishimura.

> -	}
> -	rcu_read_unlock();
> +		ret = true;
> +unlock:
> +	spin_unlock(&mc.lock);
>  	return ret;
>  }
>  
> @@ -4447,11 +4448,13 @@ static int mem_cgroup_precharge_mc(struc
>  
>  static void mem_cgroup_clear_mc(void)
>  {
> +	struct mem_cgroup *from = mc.from;
> +	struct mem_cgroup *to = mc.to;
> +
>  	/* we must uncharge all the leftover precharges from mc.to */
>  	if (mc.precharge) {
>  		__mem_cgroup_cancel_charge(mc.to, mc.precharge);
>  		mc.precharge = 0;
> -		memcg_oom_recover(mc.to);
>  	}
>  	/*
>  	 * we didn't uncharge from mc.from at mem_cgroup_move_account(), so
> @@ -4460,7 +4463,6 @@ static void mem_cgroup_clear_mc(void)
>  	if (mc.moved_charge) {
>  		__mem_cgroup_cancel_charge(mc.from, mc.moved_charge);
>  		mc.moved_charge = 0;
> -		memcg_oom_recover(mc.from);
>  	}
>  	/* we must fixup refcnts and charges */
>  	if (mc.moved_swap) {
> @@ -4485,9 +4487,13 @@ static void mem_cgroup_clear_mc(void)
>  
>  		mc.moved_swap = 0;
>  	}
> +	spin_lock(&mc.lock);
>  	mc.from = NULL;
>  	mc.to = NULL;
>  	mc.moving_task = NULL;
> +	spin_unlock(&mc.lock);
> +	memcg_oom_recover(from);
> +	memcg_oom_recover(to);
>  	wake_up_all(&mc.waitq);
>  }
>  
> @@ -4516,12 +4522,14 @@ static int mem_cgroup_can_attach(struct 
>  			VM_BUG_ON(mc.moved_charge);
>  			VM_BUG_ON(mc.moved_swap);
>  			VM_BUG_ON(mc.moving_task);
> +			spin_lock(&mc.lock);
>  			mc.from = from;
>  			mc.to = mem;
>  			mc.precharge = 0;
>  			mc.moved_charge = 0;
>  			mc.moved_swap = 0;
>  			mc.moving_task = current;
> +			spin_unlock(&mc.lock);
>  
>  			ret = mem_cgroup_precharge_mc(mm);
>  			if (ret)
>