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Date: Sat, 3 Oct 2015 09:44:28 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Daniel J Blueman <daniel@numascale.com>,
        Jiang Liu <jiang.liu@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>, Len Brown <len.brown@intel.com>,
        x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] x86/apic: Use smaller array for __apicid_to_node[]
 mapping
Message-ID: <20151003074428.GA25143@gmail.com>
References: <1443813145-29102-1-git-send-email-dvlasenk@redhat.com>
 <1443813145-29102-3-git-send-email-dvlasenk@redhat.com>
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* Denys Vlasenko <dvlasenk@redhat.com> wrote:

> @@ -56,16 +56,34 @@ early_param("numa", numa_setup);
>  /*
>   * apicid, cpu, node mappings
>   */
> -s16 __apicid_to_node[MAX_LOCAL_APICID] = {
> -	[0 ... MAX_LOCAL_APICID-1] = NUMA_NO_NODE
> +
> +struct apicid_to_node __apicid_to_node[NR_CPUS] = {
> +	[0 ... NR_CPUS-1] = {-1, NUMA_NO_NODE}
>  };
>  
> +void set_apicid_to_node(int apicid, s16 node)
> +{
> +	static int ent;

having such statics inside functions is really obscure and makes review harder.
I had to look twice to see it. Please move it outside and also name it 
appropriately.

> +	/* Protect against small kernel on large system */
> +	if (ent >= NR_CPUS)
> +		return;
> +
> +	__apicid_to_node[ent].apicid = apicid;
> +	__apicid_to_node[ent].node = node;
> +	ent++;
> +}

So what happens if we run a small kernel and run out of entries? We just silently 
seem to return, no warning, no nothing - the system will likely fail to boot in 
myserious ways, right?

Thanks,

	Ingo