linux-kernel - Re: [PATCH 1/2] mm: Replace nr_node_ids for loop with for_each

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Date:	Mon, 14 Sep 2015 12:00:10 +0300
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
CC:	<benh@...nel.crashing.org>, <paulus@...ba.org>,
	<mpe@...erman.id.au>, <anton@...ba.org>,
	<akpm@...ux-foundation.org>, <nacc@...ux.vnet.ibm.com>,
	<gkurz@...ux.vnet.ibm.com>, <zhong@...ux.vnet.ibm.com>,
	<grant.likely@...aro.org>, <nikunj@...ux.vnet.ibm.com>,
	<linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>
Subject: Re: [PATCH  1/2] mm: Replace nr_node_ids for loop with for_each_node
 in list lru

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.

> 
> Signed-off-by: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
> ---
>  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:

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;
	}

Thanks,
Vladimir
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