Subject: Re: [PATCH 1/2] mm: Replace nr_node_ids for loop with for_each_node in list lru

From: Raghavendra K T <[email-masked]>
Date: Mon, 14 Sep 2015 17:09:31 +0530

On 09/14/2015 02:30 PM, Vladimir Davydov wrote:
> Hi,
>
> On Wed, Sep 09, 2015 at 12:01:46AM +0530, Raghavendra K T wrote:
>> The functions used in the patch are in slowpath, which gets called
>> whenever alloc_super is called during mounts.
>>
>> Though this should not make difference for the architectures with
>> sequential numa node ids, for the powerpc which can potentially have
>> sparse node ids (for e.g., 4 node system having numa ids, 0,1,16,17
>> is common), this patch saves some unnecessary allocations for
>> non existing numa nodes.
>>
>> Even without that saving, perhaps patch makes code more readable.
>
> Do I understand correctly that node 0 must always be in
> node_possible_map? I ask, because we currently test
> lru->node[0].memcg_lrus to determine if the list is memcg aware.
>

Yes, node 0 is always there. So it should not be a problem.

>>
>> Signed-off-by: Raghavendra K T <[email-masked]>
>> ---
>>   mm/list_lru.c | 23 +++++++++++++++--------
>>   1 file changed, 15 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/list_lru.c b/mm/list_lru.c
>> index 909eca2..5a97f83 100644
>> --- a/mm/list_lru.c
>> +++ b/mm/list_lru.c
>> @@ -377,7 +377,7 @@ static int memcg_init_list_lru(struct list_lru *lru, bool memcg_aware)
>>   {
>>   	int i;
>>
>> -	for (i = 0; i < nr_node_ids; i++) {
>> +	for_each_node(i) {
>>   		if (!memcg_aware)
>>   			lru->node[i].memcg_lrus = NULL;
>
> So, we don't explicitly initialize memcg_lrus for nodes that are not in
> node_possible_map. That's OK, because we allocate lru->node using
> kzalloc. However, this partial nullifying in case !memcg_aware looks
> confusing IMO. Let's drop it, I mean something like this:

Yes, you are right. and we do not have to have memcg_aware check inside
for loop too.
Will change as per your suggestion and send V2.
Thanks for the review.

>
> static int memcg_init_list_lru(struct list_lru *lru, bool memcg_aware)
> {
> 	int i;
>
> 	if (!memcg_aware)
> 		return 0;
>
> 	for_each_node(i) {
> 		if (memcg_init_list_lru_node(&lru->node[i]))
> 			goto fail;
> 	}
>