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Date: Tue, 28 Apr 2020 16:59:12 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Subject: Re: [PATCH v2 3/3] mm/page_alloc: Keep memoryless cpuless node 0
 offline
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Cc: Michal Hocko <mhocko@suse.com>,
 Linus Torvalds <torvalds@linux-foundation.org>, linux-kernel@vger.kernel.org,
 linux-mm@kvack.org, Mel Gorman <mgorman@suse.de>,
 "Kirill A. Shutemov" <kirill@shutemov.name>,
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 Vlastimil Babka <vbabka@suse.cz>
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On Tue, 28 Apr 2020 15:08:36 +0530 Srikar Dronamraju <srikar@linux.vnet.ibm.com> wrote:

> Currently Linux kernel with CONFIG_NUMA on a system with multiple
> possible nodes, marks node 0 as online at boot.  However in practice,
> there are systems which have node 0 as memoryless and cpuless.
> 
> This can cause numa_balancing to be enabled on systems with only one node
> with memory and CPUs. The existence of this dummy node which is cpuless and
> memoryless node can confuse users/scripts looking at output of lscpu /
> numactl.
> 
> By marking, N_ONLINE as NODE_MASK_NONE, lets stop assuming that Node 0 is
> always online.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -116,8 +116,10 @@ EXPORT_SYMBOL(latent_entropy);
>   */
>  nodemask_t node_states[NR_NODE_STATES] __read_mostly = {
>  	[N_POSSIBLE] = NODE_MASK_ALL,
> +#ifdef CONFIG_NUMA
> +	[N_ONLINE] = NODE_MASK_NONE,
> +#else
>  	[N_ONLINE] = { { [0] = 1UL } },
> -#ifndef CONFIG_NUMA
>  	[N_NORMAL_MEMORY] = { { [0] = 1UL } },
>  #ifdef CONFIG_HIGHMEM
>  	[N_HIGH_MEMORY] = { { [0] = 1UL } },

So on all other NUMA machines, when does node 0 get marked online?

This change means that for some time during boot, such machines will
now be running with node 0 marked as offline.  What are the
implications of this?  Will something break?