linux-kernel - Re: [PATCH] kho: skip memoryless NUMA nodes when reserving scratch areas

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Message-ID: <2vxzsec57oth.fsf@kernel.org>
Date: Fri, 16 Jan 2026 11:57:14 +0000
From: Pratyush Yadav <pratyush@...nel.org>
To: Evangelos Petrongonas <epetron@...zon.de>
Cc: Mike Rapoport <rppt@...nel.org>,  Pasha Tatashin
 <pasha.tatashin@...een.com>,  Pratyush Yadav <pratyush@...nel.org>,
  "Alexander Graf" <graf@...zon.com>,  Jason Miu <jasonmiu@...gle.com>,
  <linux-kernel@...r.kernel.org>,  <kexec@...ts.infradead.org>,
  <linux-mm@...ck.org>,  <nh-open-source@...zon.com>
Subject: Re: [PATCH] kho: skip memoryless NUMA nodes when reserving scratch
 areas

Hi Evangelos,

On Fri, Jan 16 2026, Evangelos Petrongonas wrote:

> kho_reserve_scratch() iterates over all online NUMA nodes to allocate
> per-node scratch memory. On systems with memoryless NUMA nodes (nodes
> that have CPUs but no memory), memblock_alloc_range_nid() fails because
> there is no memory available on that node. This causes KHO initialization
> to fail and kho_enable to be set to false.
>
> Some ARM64 systems have NUMA topologies where certain nodes contain only
> CPUs without any associated memory. These configurations are valid and
> should not prevent KHO from functioning.
>
> Fix this by introducing kho_mem_nodes_count() which counts only nodes
> that have memory (N_MEMORY state), and skip memoryless nodes in the
> per-node scratch allocation loop.
>
> Signed-off-by: Evangelos Petrongonas <epetron@...zon.de>
> ---
>  kernel/liveupdate/kexec_handover.c | 23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 9dc51fab604f..c970ed08b477 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -623,6 +623,23 @@ static phys_addr_t __init scratch_size_node(int nid)
>  	return round_up(size, CMA_MIN_ALIGNMENT_BYTES);
>  }
>  
> +/*
> + * Count online NUMA nodes that have memory. Memoryless nodes cannot have
> + * scratch memory and should be excluded.
> + */
> +static unsigned int __init kho_mem_nodes_count(void)
> +{
> +	unsigned int cnt = 0;
> +	int nid;
> +
> +	for_each_online_node(nid) {
> +		if (node_state(nid, N_MEMORY))
> +			cnt++;
> +	}
> +
> +	return cnt;
> +}
> +

You don't need this. You can use nodes_weight(nodes_state[N_MEMORY])
directly. Other than this, LGTM.

>  /**
>   * kho_reserve_scratch - Reserve a contiguous chunk of memory for kexec
>   *
> @@ -643,7 +660,7 @@ static void __init kho_reserve_scratch(void)
>  	scratch_size_update();
>  
>  	/* FIXME: deal with node hot-plug/remove */
> -	kho_scratch_cnt = num_online_nodes() + 2;
> +	kho_scratch_cnt = kho_mem_nodes_count() + 2;
>  	size = kho_scratch_cnt * sizeof(*kho_scratch);
>  	kho_scratch = memblock_alloc(size, PAGE_SIZE);
>  	if (!kho_scratch)
> @@ -674,6 +691,10 @@ static void __init kho_reserve_scratch(void)
>  	i++;
>  
>  	for_each_online_node(nid) {
> +		/* Skip memoryless nodes - we cannot allocate scratch memory there */
> +		if (!node_state(nid, N_MEMORY))
> +			continue;
> +
>  		size = scratch_size_node(nid);
>  		addr = memblock_alloc_range_nid(size, CMA_MIN_ALIGNMENT_BYTES,
>  						0, MEMBLOCK_ALLOC_ACCESSIBLE,

-- 
Regards,
Pratyush Yadav