linux-kernel - [PATCH v2] kho: skip memoryless NUMA nodes when reserving scratch areas

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Message-ID: <20260120175913.34368-1-epetron@amazon.de>
Date: Tue, 20 Jan 2026 17:59:11 +0000
From: Evangelos Petrongonas <epetron@...zon.de>
To: Mike Rapoport <rppt@...nel.org>
CC: Evangelos Petrongonas <epetron@...zon.de>, 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: [PATCH v2] kho: skip memoryless NUMA nodes when reserving scratch areas

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 only counting 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>
---
v2:
- Removed kho_mem_nodes_count in favour of nodes_weight(nodes_state[N_MEMORY])
- Use for_each_node_state(nid, N_MEMORY) to loop over nodes that are both
online and have memory.
TIL: Nodes in N_MEMORY are a subset of those that are online. Thanks Mike :)

 kernel/liveupdate/kexec_handover.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
index 9dc51fab604f..979ebaf015bf 100644
--- a/kernel/liveupdate/kexec_handover.c
+++ b/kernel/liveupdate/kexec_handover.c
@@ -643,7 +643,7 @@ 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 = nodes_weight(node_states[N_MEMORY]) + 2;
 	size = kho_scratch_cnt * sizeof(*kho_scratch);
 	kho_scratch = memblock_alloc(size, PAGE_SIZE);
 	if (!kho_scratch)
@@ -673,7 +673,11 @@ static void __init kho_reserve_scratch(void)
 	kho_scratch[i].size = size;
 	i++;

-	for_each_online_node(nid) {
+	/*
+	 * Loop over nodes that have both memory and are online. Skip
+	 * memoryless nodes, as we can not allocate scratch areas there.
+	 */
+	for_each_node_state(nid, N_MEMORY) {
 		size = scratch_size_node(nid);
 		addr = memblock_alloc_range_nid(size, CMA_MIN_ALIGNMENT_BYTES,
 						0, MEMBLOCK_ALLOC_ACCESSIBLE,
-- 
2.43.0

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