Re: [PATCH 1/1] liveupdate: kho: calculate per-node scratch sizes before allocation

[Sourabh Jain]

Hello George,

On 04/09/26 08:21, George Guo wrote:
> From: George Guo <guodongtai@xxxxxxxxxx>
>
> The default percentage-based policy sizes scratch areas from the current
> kernel's MEMBLOCK_RSRV_KERN footprint. This is a reasonable heuristic for
> predicting the early memory demand of the next kernel.
>
> scratch_size_update() calculates the lowmem and global sizes before either
> area is allocated. However, kho_reserve_scratch() calculates each per-node
> size only after allocating the lowmem and global areas. Since memblock
> allocations are marked MEMBLOCK_RSRV_KERN, the per-node calculation
> includes those newly allocated scratch areas and scales them again.

I may be missing something here, but doesn't memblock_reserved_kern_size()
check the node ID of the region before checking the region flag 
(MEMBLOCK_RSRV_KERN)?

Code snippet from memblock_reserved_kern_size()
```
if (nid == memblock_get_region_node(r) || !numa_valid_node(nid))
    if (r->flags & MEMBLOCK_RSRV_KERN)
        total += size;
```

For a valid nid, my understanding is that the global and lowmem scratch
areas should not be counted because they are allocated with NUMA_NO_NODE
(-1). So, ideally, these regions should be excluded when calculating the
reserved memory for a specific node ID.

Based on this, I am not sure that marking the lowmem and global areas as
MEMBLOCK_RSRV_KERN is what causes the per-node size calculation to be
inflated. I am looking into the code further to better understand the
actual cause of the issue that this patch is trying to address.

With that said, I wonder if this fix might be more of a stop-gap solution.
As mentioned above, since NUMA_NO_NODE (-1) is used for the lowmem and
global allocations, my understanding is that these areas ideally should
not be included when calculating the size for a specific node ID.

I could be missing something in my understanding, so I would appreciate
your thoughts on these observations.

- Sourabh Jain

> Fixes: 3dc92c311498 ("kexec: add Kexec HandOver (KHO) generation helpers")
> Reported-by: Kexin Liu <liukexin@xxxxxxxxxx>
> Co-developed-by: Kexin Liu <liukexin@xxxxxxxxxx>
> Signed-off-by: Kexin Liu <liukexin@xxxxxxxxxx>
> Signed-off-by: George Guo <guodongtai@xxxxxxxxxx>
> ---
>   kernel/liveupdate/kexec_handover.c | 13 ++++++++++++-
>   1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 7c4d86daf86d..39f489a258d9 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -847,6 +847,17 @@ static void __init kho_reserve_scratch(void)
>   		goto err_disable_kho;
>   	}
>   
> +	/*
> +	 * Calculate the per-node sizes before reserving any scratch areas.
> +	 * memblock allocations are marked MEMBLOCK_RSRV_KERN, so calculating
> +	 * them later would count the lowmem and global scratch areas as kernel
> +	 * allocations and scale them again.
> +	 */
> +	i = 2;
> +	for_each_node_state(nid, N_MEMORY)
> +		kho_scratch[i++].size = scratch_size_node(nid);
> +	i = 0;
> +
>   	/*
>   	 * reserve scratch area in low memory for lowmem allocations in the
>   	 * next kernel
> @@ -880,7 +891,7 @@ static void __init kho_reserve_scratch(void)
>   	 * memoryless nodes, as we can not allocate scratch areas there.
>   	 */
>   	for_each_node_state(nid, N_MEMORY) {
> -		size = scratch_size_node(nid);
> +		size = kho_scratch[i].size;
>   		addr = memblock_alloc_range_nid(size, SCRATCH_ALIGNMENT_BYTES,
>   						0, MEMBLOCK_ALLOC_ACCESSIBLE,
>   						nid, true);