Re: [PATCH] kho: fix global scratch size calculation

[Mike Rapoport <rppt@kernel.org>]

Hi Sourabh,

On Tue, Sep 22, 2026 at 06:42:16PM +0530, Sourabh Jain wrote:
> KHO calculates the global scratch size based on memblock-reserved kernel
> memory. It passes NUMA_NO_NODE to memblock_reserved_kern_size() for
> this calculation.
> 
> When memblock_reserved_kern_size() is called with NUMA_NO_NODE, it
> counts both:
> 
> - memory reserved for a specific NUMA node
> - memory reserved with NUMA_NO_NODE
> 
> KHO needs to distinguish between these two types of reservations.
> When calculating the size of global scratch memory, KHO only needs to
> account for reservations made with NUMA_NO_NODE. Reservations made for
> a specific NUMA node must not be included in the global scratch size.
> 
> Add memblock_reserved_size_nid() to calculate reserved memory for a
> given reservation type and NUMA node. When NUMA_NO_NODE is passed, it
> counts only memory reserved with NUMA_NO_NODE.
> 
> Use the new API for lowmem, global, and per-node KHO scratch size
> calculations. For lowmem and global scratch, count only memory
> reservations that were made with NUMA_NO_NODE. For per-node scratch,
> count only memory reservations that were made with the corresponding
> NUMA node ID.
> 
> Remove memblock_reserved_hugetlb_size() since it has the same
> implementation as the new API and differs only in the memblock
> reservation flag being checked. The new API handles both kernel and
> HugeTLB reservations through its reservation type argument.
> 
> Define the new helper as a static function in the KHO implementation,
> since it is only used by KHO and has no users outside
> kernel/liveupdate/kexec_handover.c.
> 
> On powerpc, the difference can be seen in the scratch_len values
> reported by:
> 
> cat /sys/kernel/debug/kho/out/scratch_len
> 
> Before this change, the global scratch allocation was 0x12000000
> (288 MB):
> 
> 0x1000000   (16 MB)
> 0x12000000  (288 MB)  <- global allocation
> 0x5000000   (80 MB)
> 
> After this change, the global scratch allocation is 0xd000000
> (208 MB):
> 
> 0x1000000   (16 MB)
> 0xd000000   (208 MB)  <- global allocation
> 0x5000000   (80 MB)
> 
> The 80 MB difference is the per-node reservation that was previously
> being included in the global allocation.
> 
> The same issue also affects lowmem scratch memory, but its impact is
> limited because the lowmem scratch memory calculation is restricted to
> the first 4G of memory. The changes also cover the lowmem scratch
> memory case.
> 
> Cc: Alexander Graf <graf@amazon.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: George Guo <guodongtai@kylinos.cn>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
> Cc: Pratyush Yadav <pratyush@kernel.org>
> Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-mm@kvack.org
> Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
> ---
>  include/linux/memblock.h           |  1 -
>  kernel/liveupdate/kexec_handover.c | 49 ++++++++++++++++++++++--------
>  mm/memblock.c                      | 22 --------------
>  3 files changed, 37 insertions(+), 35 deletions(-)
> 
> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> index d62db9e776cf..678fe466529a 100644
> --- a/include/linux/memblock.h
> +++ b/include/linux/memblock.h
> @@ -487,7 +487,6 @@ static inline __init_memblock bool memblock_bottom_up(void)
>  phys_addr_t memblock_phys_mem_size(void);
>  phys_addr_t memblock_reserved_size(void);
>  phys_addr_t memblock_reserved_kern_size(phys_addr_t limit, int nid);
> -phys_addr_t memblock_reserved_hugetlb_size(phys_addr_t limit, int nid);
>  unsigned long memblock_estimated_nr_free_pages(void);
>  phys_addr_t memblock_start_of_DRAM(void);
>  phys_addr_t memblock_end_of_DRAM(void);
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 7c4d86daf86d..dc809e1e768c 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -752,6 +752,31 @@ static int __init kho_parse_scratch_size(char *p)
>  }
>  early_param("kho_scratch", kho_parse_scratch_size);
>  
> +static phys_addr_t __init_memblock memblock_reserved_size_nid(phys_addr_t limit, int nid,
> +							      enum memblock_flags region_type)
> +{

Why is this in kexec_handover.c and not in memblock.c?

> +	struct memblock_region *r;
> +	phys_addr_t total = 0;
> +
> +	for_each_reserved_mem_region(r) {
> +		phys_addr_t size = r->size;
> +
> +		if (r->base > limit)
> +			break;
> +
> +		if (r->base + r->size > limit)
> +			size = limit - r->base;
> +
> +#ifdef CONFIG_NUMA
> +		if (nid == memblock_get_region_node(r))
> +#endif

Do we really need the #ifdef here?

> +			if (r->flags & region_type)
> +				total += size;
> +	}
> +
> +	return total;
> +}
> +
>  static void __init scratch_size_update(void)
>  {
>  	/*

-- 
Sincerely yours,
Mike.