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

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Date:   Sat, 28 Mar 2020 00:26:16 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Wei Yang <richard.weiyang@...il.com>, akpm@...ux-foundation.org,
        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 Fri, Mar 27, 2020 at 03:37:57PM -0700, John Hubbard wrote:
>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...
>

What a shame on me. You are right, I miss the static word. 

Well, then I am curious about why we want to define it as static. Each time we
call this function, node_order would be set to 0 and find_next_best_node()
would sort a proper value into it. I don't see the reason to reserve it in a
global area and be used next time.

My suggestion is to remove the static and define it {0} instead of memset
every time. Is my understanding correct here?

>
>>   	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

-- 
Wei Yang
Help you, Help me