Re: [PATCH] mm/memcg: bail out early when !memcg in mem_cgroup_lruvec

[Alex Shi]

在 2020/11/28 下午12:02, Andrew Morton 写道:
> On Fri, 27 Nov 2020 11:08:35 +0800 Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx> wrote:
> 
>> Sometime, we use NULL memcg in mem_cgroup_lruvec(memcg, pgdat)
>> so we could get out early in the situation to avoid useless checking.
>>
>> Also warning if both parameter are NULL.
> 
> Why do you think a warning is needed here?

Uh, Consider there are no problem for long time, it could be saved.

> 
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -613,14 +613,13 @@ static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
>>  	struct mem_cgroup_per_node *mz;
>>  	struct lruvec *lruvec;
>>  
>> -	if (mem_cgroup_disabled()) {
>> +	VM_WARN_ON_ONCE(!memcg && !pgdat);
>> +
>> +	if (mem_cgroup_disabled() || !memcg) {
>>  		lruvec = &pgdat->__lruvec;
>>  		goto out;
>>  	}
>>  
>> -	if (!memcg)
>> -		memcg = root_mem_cgroup;
>> -
> 
> This change isn't obviously equivalent, is it?

If !memcg, the root_mem_cgroup will still lead the lruvec to a pgdat
same as parameter.

> 
>>  	mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
>>  	lruvec = &mz->lruvec;
>>  out:
> 
> And the resulting code is awkward:
> 
> 	if (mem_cgroup_disabled() || !memcg) {
> 		lruvec = &pgdat->__lruvec;
> 		goto out;
> 	}
> 
> 	mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
> 	lruvec = &mz->lruvec;
> out:
> 
> 
> could be
> 
> 	if (mem_cgroup_disabled() || !memcg) {
> 		lruvec = &pgdat->__lruvec;
> 	} else {
> 		mem_cgroup_per_node mz;
> 
> 		mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
> 		lruvec = &mz->lruvec;
> 	}
> 

Right. remove 'goto' is better for understander.

So, is the following patch ok?