linux-kernel - Re: [PATCH] mm/memory_hotplug: just build zonelist for new added node

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Date:   Wed, 28 Jun 2017 11:23:29 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     akpm@...ux-foundation.org, vbabka@...e.cz, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/memory_hotplug: just build zonelist for new added node

On Mon 26-06-17 11:58:22, Wei Yang wrote:
> In commit (9adb62a5df9c0fbef7) "mm/hotplug: correctly setup fallback
> zonelists when creating new pgdat" tries to build the correct zonelist for
> a new added node, while it is not necessary to rebuild it for already exist
> nodes.
> 
> In build_zonelists(), it will iterate on nodes with memory. For a new added
> node, it will have memory until node_states_set_node() is called in
> online_pages().
> 
> This patch will avoid to rebuild the zonelists for already exist nodes.

It is not very clear from the changelog why that actually matters. The
only effect I can see is that other zonelists on other online nodes will
not learn about the currently memory less node. This is a good think
because we do not pointlessly try to allocate from that node.

> Signed-off-by: Wei Yang <richard.weiyang@...il.com>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/page_alloc.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 560eafe8234d..fc8181b44fd8 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5200,15 +5200,17 @@ static int __build_all_zonelists(void *data)
>  	memset(node_load, 0, sizeof(node_load));
>  #endif
>  
> -	if (self && !node_online(self->node_id)) {
> +	/* This node is hotadded and no memory preset yet.
> +	 * So just build zonelists is fine, no need to touch other nodes.
> +	 */

This comment doesn't make much sense to me. What about
	/*
	 * Do not rebuild zonelists on all online nodes if the current
	 * node is not online yet (it doesn't have any memory) and
	 * allocating from it is pointless. Still build zonelist for
	 * self because we need to handle memoryless nodes.
	 */
> +	if (self && !node_online(self->node_id))
>  		build_zonelists(self);
> -	}
> -
> -	for_each_online_node(nid) {
> -		pg_data_t *pgdat = NODE_DATA(nid);
> +	else
> +		for_each_online_node(nid) {
> +			pg_data_t *pgdat = NODE_DATA(nid);
>  
> -		build_zonelists(pgdat);
> -	}
> +			build_zonelists(pgdat);
> +		}
>  
>  	/*
>  	 * Initialize the boot_pagesets that are going to be used
> -- 
> 2.11.0
> 
> --
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-- 
Michal Hocko
SUSE Labs