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

[Mike Rapoport <rppt@kernel.org>]

On Fri, Jan 16, 2026 at 11:57:14AM +0000, Pratyush Yadav wrote:
> 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@amazon.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;
> > +

And here you can use 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,
> 
> -- 
> Regards,
> Pratyush Yadav

-- 
Sincerely yours,
Mike.