linux-kernel - Re: [Patch v2 2/2] mm/page_alloc.c: define node

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Message-ID: <4c9d8138-d379-810f-64e7-0d018ed019df@nvidia.com>
Date:   Fri, 27 Mar 2020 15:37:57 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Wei Yang <richard.weiyang@...il.com>, <akpm@...ux-foundation.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <jgg@...pe.ca>, <david@...hat.com>
Subject: Re: [Patch v2 2/2] mm/page_alloc.c: define node_order with all zero

On 3/27/20 3:01 PM, Wei Yang wrote:
> Since we always clear node_order before getting it, we can leverage
> compiler to do this instead of at run time.
> 
> Signed-off-by: Wei Yang <richard.weiyang@...il.com>
> ---
>   mm/page_alloc.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index dfcf2682ed40..49dd1f25c000 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5585,7 +5585,7 @@ static void build_thisnode_zonelists(pg_data_t *pgdat)
>   
>   static void build_zonelists(pg_data_t *pgdat)
>   {
> -	static int node_order[MAX_NUMNODES];
> +	static int node_order[MAX_NUMNODES] = {0};


Looks wrong: now the single instance of node_order is initialized just once by
the compiler. And that means that only the first caller of this function
gets a zeroed node_order array...


>   	int node, load, nr_nodes = 0;
>   	nodemask_t used_mask = NODE_MASK_NONE;
>   	int local_node, prev_node;
> @@ -5595,7 +5595,6 @@ static void build_zonelists(pg_data_t *pgdat)
>   	load = nr_online_nodes;
>   	prev_node = local_node;
>   
> -	memset(node_order, 0, sizeof(node_order));

...and all subsequent callers are left with whatever debris is remaining in
node_order. So this is not good.

The reason that memset() was used here, is that there aren't many other ways
to get node_order zeroed, given that it is a statically defined variable.


>   	while ((node = find_next_best_node(local_node, &used_mask)) >= 0) {
>   		/*
>   		 * We don't want to pressure a particular node.
> 



thanks,
-- 
John Hubbard
NVIDIA