Re: [PATCH 1/2] kho: skip zero-sized lowmem and global bootmem areas

[Pratyush Yadav <pratyush@kernel.org>]

Hi Mike,

On Tue, Nov 03 2026, Mike Rapoport (Microsoft) wrote:

> The lowmem and the global bootmem areas can have zero size, for example
> when none or all of the memory is below ARCH_LOW_ADDRESS_LIMIT, or when
> either of those areas is explicitly set to zero with kho_bootmem=.
>
> For a zero-sized allocation memblock returns an address without reserving
> anything, and that empty area is recorded in kho_bootmem. If there is no
> memory below ARCH_LOW_ADDRESS_LIMIT, allocation of the empty lowmem area
> fails and KHO gets disabled.
>
> Extract a helper that checks for zero size and allocates memory from
> memblock, and use it to skip empty areas the same way as empty per-node
> areas are skipped.
>
> Fixes: 3dc92c311498 ("kexec: add Kexec HandOver (KHO) generation helpers")
> Reported-by: Chen Pei <cp0613@linux.alibaba.com>
> Closes: https://lore.kernel.org/all/20261026131851.9942-1-cp0613@linux.alibaba.com
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Thanks for the patch! LGTM for the most part, one minor nitpick below.

> ---
>  kernel/liveupdate/kexec_handover.c | 61 ++++++++++++++++++--------------------
>  1 file changed, 29 insertions(+), 32 deletions(-)
>
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 6f1f55fe31819..f268c57413a7e 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -820,6 +820,27 @@ bool kho_bootmem_overlap(phys_addr_t phys, size_t size)
>  	return false;
>  }
>  
> +static int __init kho_reserve_bootmem_area(int *idx, phys_addr_t size,
> +					   phys_addr_t end, int nid)
> +{
> +	phys_addr_t addr;
> +	int i = *idx;
> +
> +	if (!size)
> +		return 0;
> +
> +	addr = memblock_alloc_range_nid(size, BOOTMEM_ALIGNMENT_BYTES, 0, end,
> +					nid, true);
> +	if (!addr)
> +		return -ENOMEM;
> +
> +	kho_bootmem[i].addr = addr;
> +	kho_bootmem[i].size = size;
> +	*idx = i + 1;
> +
> +	return 0;

I reckon it will be cleaner if you update kho_bootmem_cnt here directly.
So you directly do:

	kho_bootmem[kho_bootmem_cnt].addr = addr;
	kho_bootmem[kho_bootmem_cnt].size = size;
	kho_bootmem_cnt++;

You won't have to pass around *idx and you won't need the final
kho_bootmem_cnt assignment.

You might also want to have a kho_bootmem_max_cnt and do some bounds
checking here.

What do you think?

> +}
> +
>  /**
>   * kho_reserve_bootmem - Reserve a contiguous chunk of memory for kexec
>   *
> @@ -832,7 +853,7 @@ bool kho_bootmem_overlap(phys_addr_t phys, size_t size)
>  static void __init kho_reserve_bootmem(void)
>  {
>  	int nid, i = 0, max_nr_bootmem;
> -	phys_addr_t addr, size;
> +	phys_addr_t size;
>  
>  	if (!kho_enable)
>  		return;
> @@ -852,56 +873,32 @@ static void __init kho_reserve_bootmem(void)
>  	 * reserve bootmem area in low memory for lowmem allocations in the
>  	 * next kernel
>  	 */
> -	size = bootmem_size_lowmem;
> -	addr = memblock_phys_alloc_range(size, BOOTMEM_ALIGNMENT_BYTES, 0,
> -					 ARCH_LOW_ADDRESS_LIMIT);
> -	if (!addr) {
> +	if (kho_reserve_bootmem_area(&i, bootmem_size_lowmem,
> +				     ARCH_LOW_ADDRESS_LIMIT, NUMA_NO_NODE)) {
>  		pr_err("Failed to reserve lowmem bootmem\n");
>  		goto err_free_bootmem_desc;
>  	}
>  
> -	kho_bootmem[i].addr = addr;
> -	kho_bootmem[i].size = size;
> -	i++;
> -
>  	/* reserve large contiguous area for allocations without nid */
> -	size = bootmem_size_global;
> -	addr = memblock_phys_alloc(size, BOOTMEM_ALIGNMENT_BYTES);
> -	if (!addr) {
> +	if (kho_reserve_bootmem_area(&i, bootmem_size_global,
> +				     MEMBLOCK_ALLOC_ACCESSIBLE, NUMA_NO_NODE)) {
>  		pr_err("Failed to reserve global bootmem\n");
>  		goto err_free_bootmem_areas;
>  	}
>  
> -	kho_bootmem[i].addr = addr;
> -	kho_bootmem[i].size = size;
> -	i++;
> -
>  	/*
>  	 * Loop over nodes that have both memory and are online. Skip
>  	 * memoryless nodes, as we can not allocate bootmem areas there.
>  	 */
>  	for_each_node_state(nid, N_MEMORY) {
> -		size = bootmem_size_node(nid);
> -		if (!size)
> -			continue;
> -
> -		addr = memblock_alloc_range_nid(size, BOOTMEM_ALIGNMENT_BYTES,
> -						0, MEMBLOCK_ALLOC_ACCESSIBLE,
> -						nid, true);
> -		if (!addr) {
> +		if (kho_reserve_bootmem_area(&i, bootmem_size_node(nid),
> +					     MEMBLOCK_ALLOC_ACCESSIBLE, nid)) {
>  			pr_err("Failed to reserve nid %d bootmem\n", nid);
>  			goto err_free_bootmem_areas;
>  		}
> -
> -		kho_bootmem[i].addr = addr;
> -		kho_bootmem[i].size = size;
> -		i++;
>  	}
>  
> -	/*
> -	 * Nodes that needed no bootmem were skipped, so fewer areas than
> -	 * initially estimated may have been reserved.
> -	 */
> +	/* Areas with zero size were skipped */
>  	kho_bootmem_cnt = i;
>  
>  	return;

-- 
Regards,
Pratyush Yadav