Re: [PATCH 2/2] kho: fix bootmem sizing

[Pratyush Yadav <pratyush@kernel.org>]

Hi Mike,

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

> Bootmem areas are sized from overlapping views of the memory reserved in
> memblock:
>
> * the global area is sized from all reserved memory, including memory
>   allocated on specific nodes, so on NUMA systems everything allocated
>   on a node is accounted in both the global area and the area of that
>   node.
> * per-node sizes are calculated after the lowmem and the global areas
>   are reserved. With CONFIG_NUMA=n all memblock regions belong to node 0,
>   so the lowmem and the global areas are accounted in the size of node 0
>   area and scaled once again.
>
> As a result, bootmem is about twice bigger than it should be on NUMA
> systems and about four times bigger with CONFIG_NUMA=n. On small systems
> reservation of the bootmem may fail and then KHO gets disabled.
>
> Make the areas cover separate parts of the reserved memory: the lowmem
> area covers allocations below ARCH_LOW_ADDRESS_LIMIT, per-node areas
> cover allocations above it on each node and the global area covers
> allocations that are not bound to any node. Calculate all sizes before
> reserving any of the areas.
>
> With CONFIG_NUMA=n the global area becomes empty and it is not reserved.
>
> Bootmem sizes in MiB in x86 QEMU VMs, before -> after:
>
>                      CONFIG_NUMA=y     CONFIG_NUMA=n
>   1G,  1 node         176 ->   92       320 ->   80
>   4G,  1 node         508 ->  332      1264 ->  316
>   16G, 2 nodes       1268 ->  704      2944 ->  736
>   32G, 4 nodes       2296 -> 1224      5196 -> 1300
>   64G, 4 nodes       4344 -> 2248      9664 -> 2420
>
> Fixes: 3dc92c311498 ("kexec: add Kexec HandOver (KHO) generation helpers")
> Reported-by: Kexin Liu <liukexin@kylinos.cn>
> Reported-by: George Guo <guodongtai@kylinos.cn>
> Closes: https://lore.kernel.org/all/20260904025101.9959-1-dongtai.guo@linux.dev
> Reported-by: Sourabh Jain <sourabhjain@linux.ibm.com>
> Closes: https://lore.kernel.org/all/20260922131217.698809-1-sourabhjain@linux.ibm.com
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Thanks for the patch! My LLM caught an interesting problems that is
worth considering.

On arm64, ARCH_LOW_ADDRESS_LIMIT is arm64_dma_phys_limit - 1, and
dma_limits_init() sets the default value to PHYS_MASK + 1, and it can
also be memblock_end_of_dram() in some cases. So on arm64, it is
possible for lowmem to span the entire RAM.

Currently, that results in over-allocation of bootmem, and it ends up
being 4x what it should be. While obviously problematic, it should still
work. With this patch, all memory is in lowmem, and nothing ever goes to
per-node bootmem (except for the node that hosts the lowmem area).

All this is from reading the code, I haven't tried anything out yet.

Not sure how to fix this though. Should we perhaps use
min(ARCH_LOW_ADDRESS_LIMIT, U32_MAX) for the lowmem limit?

Also, a couple nitpicks below.

> ---
>  kernel/liveupdate/kexec_handover.c | 83 ++++++++++++++++++++------------------
>  1 file changed, 44 insertions(+), 39 deletions(-)
>
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index f268c57413a7e..eaeb895bb7595 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -684,6 +684,7 @@ static unsigned int bootmem_scale __initdata = 200;
>  static phys_addr_t bootmem_size_global __initdata;
>  static phys_addr_t bootmem_size_pernode __initdata;
>  static phys_addr_t bootmem_size_lowmem __initdata;
> +static phys_addr_t bootmem_size_node[MAX_NUMNODES] __initdata;
>  
>  static int __init kho_parse_bootmem_size(char *p)
>  {
> @@ -753,29 +754,48 @@ static int __init kho_parse_bootmem_size(char *p)
>  }
>  early_param("kho_bootmem", kho_parse_bootmem_size);
>  
> +static phys_addr_t __init reserved_mem_size(phys_addr_t limit, int nid)
> +{
> +	return memblock_reserved_kern_size(limit, nid) -
> +	       memblock_reserved_hugetlb_size(limit, nid);
> +}
> +
> +static phys_addr_t __init bootmem_scale_size(phys_addr_t size)
> +{
> +	return size * bootmem_scale / 100;
> +}
> +
> +/*
> + * The lowmem area covers allocations below ARCH_LOW_ADDRESS_LIMIT, per-node
> + * areas cover allocations above it on each node and the global area covers
> + * allocations that are not bound to any node.
> + *
> + * All sizes must be calculated before any area is reserved, otherwise the
> + * areas reserved first will be accounted in the sizes of the others.
> + */
>  static void __init bootmem_size_update(void)
>  {
> -	/*
> -	 * If fixed sizes are not provided via command line, calculate them now.
> -	 * Remove HugeTLB allocations from it because they never get allocated
> -	 * from bootmem.
> -	 */
> +	int nid;
> +
>  	if (bootmem_scale) {
> -		phys_addr_t size;
> -
> -		size = memblock_reserved_kern_size(ARCH_LOW_ADDRESS_LIMIT,
> -						   NUMA_NO_NODE);
> -		size -= memblock_reserved_hugetlb_size(ARCH_LOW_ADDRESS_LIMIT,
> -						       NUMA_NO_NODE);
> -		size = size * bootmem_scale / 100;
> -		bootmem_size_lowmem = size;
> -
> -		size = memblock_reserved_kern_size(MEMBLOCK_ALLOC_ANYWHERE,
> -						   NUMA_NO_NODE);
> -		size -= memblock_reserved_hugetlb_size(MEMBLOCK_ALLOC_ANYWHERE,
> -						       NUMA_NO_NODE);
> -		size = size * bootmem_scale / 100 - bootmem_size_lowmem;
> -		bootmem_size_global = size;
> +		phys_addr_t lowmem, global, size;
> +
> +		lowmem = reserved_mem_size(ARCH_LOW_ADDRESS_LIMIT, NUMA_NO_NODE);
> +		global = reserved_mem_size(MEMBLOCK_ALLOC_ANYWHERE, NUMA_NO_NODE);
> +		global -= lowmem;

Nit: Please add a short comment above this to explain why you remove
lowmem from global.

> +
> +		for_each_node_state(nid, N_MEMORY) {
> +			size = reserved_mem_size(MEMBLOCK_ALLOC_ANYWHERE, nid) -
> +			       reserved_mem_size(ARCH_LOW_ADDRESS_LIMIT, nid);
> +			bootmem_size_node[nid] = bootmem_scale_size(size);
> +			global -= size;

Nit: same here, a comment here would make it easier to understand.

> +		}
> +
> +		bootmem_size_lowmem = bootmem_scale_size(lowmem);
> +		bootmem_size_global = bootmem_scale_size(global);
> +	} else {
> +		for_each_node_state(nid, N_MEMORY)
> +			bootmem_size_node[nid] = bootmem_size_pernode;
>  	}
>  
>  	/*
> @@ -784,24 +804,9 @@ static void __init bootmem_size_update(void)
>  	 */
>  	bootmem_size_lowmem = round_up(bootmem_size_lowmem, BOOTMEM_ALIGNMENT_BYTES);
>  	bootmem_size_global = round_up(bootmem_size_global, BOOTMEM_ALIGNMENT_BYTES);
> -}
> -
> -static phys_addr_t __init bootmem_size_node(int nid)
> -{
> -	phys_addr_t size;
> -
> -	if (bootmem_scale) {
> -		size = memblock_reserved_kern_size(MEMBLOCK_ALLOC_ANYWHERE,
> -						   nid);
> -		/* Do not count HugeTLB pages. */
> -		size -= memblock_reserved_hugetlb_size(MEMBLOCK_ALLOC_ANYWHERE,
> -						       nid);
> -		size = size * bootmem_scale / 100;
> -	} else {
> -		size = bootmem_size_pernode;
> -	}
> -
> -	return round_up(size, BOOTMEM_ALIGNMENT_BYTES);
> +	for_each_node_state(nid, N_MEMORY)
> +		bootmem_size_node[nid] = round_up(bootmem_size_node[nid],
> +						  BOOTMEM_ALIGNMENT_BYTES);
>  }
>  
>  bool kho_bootmem_overlap(phys_addr_t phys, size_t size)
> @@ -891,7 +896,7 @@ static void __init kho_reserve_bootmem(void)
>  	 * memoryless nodes, as we can not allocate bootmem areas there.
>  	 */
>  	for_each_node_state(nid, N_MEMORY) {
> -		if (kho_reserve_bootmem_area(&i, bootmem_size_node(nid),
> +		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;

-- 
Regards,
Pratyush Yadav