Re: [PATCH 2/5] memcg: use array and ID for quick look up

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

> > > @@ -2231,7 +2244,7 @@ __mem_cgroup_commit_charge_swapin(struct
> > >  
> > >  		id = swap_cgroup_record(ent, 0);
> > >  		rcu_read_lock();
> > > -		memcg = mem_cgroup_lookup(id);
> > > +		memcg = id_to_memcg(id);
> > >  		if (memcg) {
> > >  			/*
> > >  			 * This recorded memcg can be obsolete one. So, avoid
> > > @@ -2240,9 +2253,10 @@ __mem_cgroup_commit_charge_swapin(struct
> > >  			if (!mem_cgroup_is_root(memcg))
> > >  				res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> > >  			mem_cgroup_swap_statistics(memcg, false);
> > > +			rcu_read_unlock();
> > >  			mem_cgroup_put(memcg);
> > > -		}
> > > -		rcu_read_unlock();
> > > +		} else
> > > +			rcu_read_unlock();
> > >  	}
> > >  	/*
> > >  	 * At swapin, we may charge account against cgroup which has no tasks.
> > > @@ -2495,7 +2509,7 @@ void mem_cgroup_uncharge_swap(swp_entry_
> > >  
> > >  	id = swap_cgroup_record(ent, 0);
> > >  	rcu_read_lock();
> > > -	memcg = mem_cgroup_lookup(id);
> > > +	memcg = id_to_memcg(id);
> > >  	if (memcg) {
> > >  		/*
> > >  		 * We uncharge this because swap is freed.
> > > @@ -2504,9 +2518,10 @@ void mem_cgroup_uncharge_swap(swp_entry_
> > >  		if (!mem_cgroup_is_root(memcg))
> > >  			res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> > >  		mem_cgroup_swap_statistics(memcg, false);
> > > +		rcu_read_unlock();
> > >  		mem_cgroup_put(memcg);
> > > -	}
> > > -	rcu_read_unlock();
> > > +	} else
> > > +		rcu_read_unlock();
> > >  }
> > >  
> > >  /**
> > Could you explain why we need rcu_read_unlock() before mem_cgroup_put() ?
> > I suspect that it's because mem_cgroup_put() can free the memcg, but do we
> > need mem->valid then ?
> > 
> mem_cgroup_put() may call synchronize_rcu(). So, we have to unlock before it.
> 
Ah, I see. Thank you for your explanation.

Daisuke Nishimura.

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