Re: [PATCH] kho: do not reserve scratch memory in the kdump kernel

[Mike Rapoport <rppt@kernel.org>]

Hi,

On Mon, Aug 03, 2026 at 03:39:38PM -0400, Maxi Saparov wrote:
> From: Maxi Saparov <masaparov@coreweave.com>
> 
> If a kernel with CONFIG_KEXEC_HANDOVER_ENABLE_DEFAULT=y (or kho=on)
> does not receive a handover FDT, it reserves KHO scratch memory in
> kho_reserve_scratch(). The function computes the sizes from the early
> memblock reservations of the kernel. The default scratch_scale is
> 200%. The lowmem and global scratch areas each get twice the size of
> the early reservations. Each NUMA node gets one more scratch area at
> the same scale.
> 
> The kdump kernel always takes this path. Since commit a6715d7ec472
> ("kho: skip KHO for crash kernel"), the kernel does not add KHO
> metadata to the crash kimage. As a result, the kdump kernel never
> receives a handover FDT and always reserves scratch memory.
> 
> The kdump kernel boots in the small crashkernel= memory reservation.
> On our x86 hosts, the kernel and initramfs make approximately 176 MB
> of early reservations. The scratch areas then use approximately
> 650 MB more of the crashkernel memory. When a large early allocation
> fails, the kdump kernel panics:
> 
>   bio: can't create integrity buf pool
> 
> A kdump kernel has nothing to hand over: its only task is to dump the
> memory of the old kernel and reboot. Disable KHO in the kdump kernel
> so that it does not reserve scratch memory.
> 
> Fixes: 3dc92c311498 ("kexec: add Kexec HandOver (KHO) generation helpers")
> Cc: stable@vger.kernel.org
> Signed-off-by: Maxi Saparov <masaparov@coreweave.com>
> ---
>  kernel/liveupdate/kexec_handover.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 6fad9152387a..53b59edb5b36 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -12,6 +12,7 @@
>  
>  #include <linux/cleanup.h>
>  #include <linux/cma.h>
> +#include <linux/crash_dump.h>
>  #include <linux/kmemleak.h>
>  #include <linux/count_zeros.h>
>  #include <linux/kasan.h>
> @@ -1636,6 +1637,9 @@ void __init kho_memory_init(void)
>  
>  		if (kho_mem_retrieve(kho_get_fdt()))
>  			kho_in.fdt_phys = 0;
> +	} else if (is_kdump_kernel()) {
> +		kho_enable = false;
> +		pr_info("disabled in the kdump kernel\n");

Can we put this on top as a separate if, like

	if (is_kdump_kernel()) {
		/* disable KHO */
		return;
	}

>  	} else {
>  		kho_reserve_scratch();
>  	}
> 
> base-commit: 3a0b8fa2eb36afc88b62a95f33f0c77c71fa5ded
> -- 
> 2.55.0
> 

-- 
Sincerely yours,
Mike.