Re: [PATCH v2 3/3] kho: fix bootmem sizing

[Pratyush Yadav <pratyush@kernel.org>]

On Wed, Nov 04 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.
>
> If ARCH_LOW_ADDRESS_LIMIT covers all memory, as may happen on arm64, any
> area can serve lowmem allocations, so don't reserve the lowmem area.
>
> 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>

Reviewed-by: Pratyush Yadav <pratyush@kernel.org>

[...]

-- 
Regards,
Pratyush Yadav