Re: [PATCH v3 2/2] mm/memblock: only mark/clear KHO scratch memory when needed

[Mike Rapoport <rppt@kernel.org>]

On Fri, Nov 28, 2025 at 05:29:34PM +0000, Usama Arif wrote:
> The scratch memory for kexec handover is used to bootstrap the
> kexec'ed kernel. Only the 1st 1MB is used as scratch, and its a
> hack to get around limitations with KHO. It is only needed when
> CONFIG_KEXEC_HANDOVER is enabled and only if it is a KHO boot
> (both checked by is_kho_boot). Add check to prevent marking a KHO
> scratch region unless needed.

I'm going to rewrite the changelog and queue this for upstream:

The scratch memory for kexec handover is used to bootstrap the kexec'ed
kernel and it is only needed when it is a KHO boot, i.e. a kexec boot with
handover data passed from the previous kernel.

Currently x86 marks the first megabyte of memory as KHO scratch even for
non-KHO boots if CONFIG_KEXEC_HANDOVER is enabled.

Add check to prevent marking a KHO scratch regions unless they are actually
needed.

> Fixes: a2daf83e10378 ("x86/e820: temporarily enable KHO scratch for memory below 1M")
> Reported-by: Vlad Poenaru <thevlad@meta.com>
> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
> ---
>  mm/memblock.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 8b13d5c28922a..913cf322eb89a 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -20,8 +20,8 @@
>  
>  #ifdef CONFIG_KEXEC_HANDOVER
>  #include <linux/libfdt.h>
> -#include <linux/kexec_handover.h>
>  #endif /* CONFIG_KEXEC_HANDOVER */
> +#include <linux/kexec_handover.h>
>  
>  #include <asm/sections.h>
>  #include <linux/io.h>
> @@ -1126,8 +1126,10 @@ int __init_memblock memblock_reserved_mark_noinit(phys_addr_t base, phys_addr_t
>   */
>  __init int memblock_mark_kho_scratch(phys_addr_t base, phys_addr_t size)
>  {
> -	return memblock_setclr_flag(&memblock.memory, base, size, 1,
> -				    MEMBLOCK_KHO_SCRATCH);
> +	if (is_kho_boot())
> +		return memblock_setclr_flag(&memblock.memory, base, size, 1,
> +					    MEMBLOCK_KHO_SCRATCH);
> +	return 0;
>  }
>  
>  /**
> @@ -1140,8 +1142,10 @@ __init int memblock_mark_kho_scratch(phys_addr_t base, phys_addr_t size)
>   */
>  __init int memblock_clear_kho_scratch(phys_addr_t base, phys_addr_t size)
>  {
> -	return memblock_setclr_flag(&memblock.memory, base, size, 0,
> -				    MEMBLOCK_KHO_SCRATCH);
> +	if (is_kho_boot())
> +		return memblock_setclr_flag(&memblock.memory, base, size, 0,
> +					    MEMBLOCK_KHO_SCRATCH);
> +	return 0;
>  }
>  
>  static bool should_skip_region(struct memblock_type *type,
> -- 
> 2.47.3
> 

-- 
Sincerely yours,
Mike.