Re: [PATCH v2] arm64: kexec: Add support for crashkernel CMA reservation

[Mike Rapoport <rppt@kernel.org>]

On Mon, Jan 26, 2026 at 04:13:34PM +0800, Jinjie Ruan wrote:
> Commit 35c18f2933c5 ("Add a new optional ",cma" suffix to the
> crashkernel= command line option") and commit ab475510e042 ("kdump:
> implement reserve_crashkernel_cma") added CMA support for kdump
> crashkernel reservation.
> 
> Crash kernel memory reservation wastes production resources if too
> large, risks kdump failure if too small, and faces allocation difficulties
> on fragmented systems due to contiguous block constraints. The new
> CMA-based crashkernel reservation scheme splits the "large fixed
> reservation" into a "small fixed region + large CMA dynamic region": the
> CMA memory is available to userspace during normal operation to avoid
> waste, and is reclaimed for kdump upon crash—saving memory while
> improving reliability.
> 
> So extend crashkernel CMA reservation support to arm64. The following
> changes are made to enable CMA reservation:
> 
> - Parse and obtain the CMA reservation size along with other crashkernel
>   parameters.
> - Call reserve_crashkernel_cma() to allocate the CMA region for kdump.
> - Include the CMA-reserved ranges for kdump kernel to use.
> - Exclude the CMA-reserved ranges from the crash kernel memory to
>   prevent them from being exported through /proc/vmcore.
> 
> Update kernel-parameters.txt to document CMA support for crashkernel on
> arm64 architecture.

I'm looking at this and at almost identical patch for riscv 
https://lore.kernel.org/all/20260126080738.696723-1-ruanjinjie@huawei.com
and it feels wrong that we have duplicate the code that excludes cma
ranges.
CMA ranges are known to the crash_core and I don't see why we cannot
exclude them there.
 
> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
> ---
> v2:
> - Free cmem in prepare_elf_headers()
> - Add the mtivation.
> ---
>  Documentation/admin-guide/kernel-parameters.txt |  2 +-
>  arch/arm64/kernel/machine_kexec_file.c          | 15 ++++++++++++++-
>  arch/arm64/mm/init.c                            |  5 +++--
>  3 files changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 1058f2a6d6a8..36bb642a7edd 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1119,7 +1119,7 @@ Kernel parameters
>  			It will be ignored when crashkernel=X,high is not used
>  			or memory reserved is below 4G.
>  	crashkernel=size[KMG],cma
> -			[KNL, X86, ppc] Reserve additional crash kernel memory from
> +			[KNL, X86, ARM64, ppc] Reserve additional crash kernel memory from
>  			CMA. This reservation is usable by the first system's
>  			userspace memory and kernel movable allocations (memory
>  			balloon, zswap). Pages allocated from this memory range
> diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_kexec_file.c
> index 410060ebd86d..ef6ce9aaba80 100644
> --- a/arch/arm64/kernel/machine_kexec_file.c
> +++ b/arch/arm64/kernel/machine_kexec_file.c
> @@ -48,7 +48,7 @@ static int prepare_elf_headers(void **addr, unsigned long *sz)
>  	u64 i;
>  	phys_addr_t start, end;
>  
> -	nr_ranges = 2; /* for exclusion of crashkernel region */
> +	nr_ranges = 2 + crashk_cma_cnt; /* for exclusion of crashkernel region */
>  	for_each_mem_range(i, &start, &end)
>  		nr_ranges++;
>  
> @@ -64,6 +64,12 @@ static int prepare_elf_headers(void **addr, unsigned long *sz)
>  		cmem->nr_ranges++;
>  	}
>  
> +	for (i = 0; i < crashk_cma_cnt; i++) {
> +		cmem->ranges[cmem->nr_ranges].start = crashk_cma_ranges[i].start;
> +		cmem->ranges[cmem->nr_ranges].end = crashk_cma_ranges[i].end;
> +		cmem->nr_ranges++;
> +	}
> +
>  	/* Exclude crashkernel region */
>  	ret = crash_exclude_mem_range(cmem, crashk_res.start, crashk_res.end);
>  	if (ret)
> @@ -75,6 +81,13 @@ static int prepare_elf_headers(void **addr, unsigned long *sz)
>  			goto out;
>  	}
>  
> +	for (i = 0; i < crashk_cma_cnt; ++i) {
> +		ret = crash_exclude_mem_range(cmem, crashk_cma_ranges[i].start,
> +					      crashk_cma_ranges[i].end);
> +		if (ret)
> +			goto out;
> +	}
> +
>  	ret = crash_prepare_elf64_headers(cmem, true, addr, sz);
>  
>  out:
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 524d34a0e921..28165d94af08 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -96,8 +96,8 @@ phys_addr_t __ro_after_init arm64_dma_phys_limit;
>  
>  static void __init arch_reserve_crashkernel(void)
>  {
> +	unsigned long long crash_base, crash_size, cma_size = 0;
>  	unsigned long long low_size = 0;
> -	unsigned long long crash_base, crash_size;
>  	bool high = false;
>  	int ret;
>  
> @@ -106,11 +106,12 @@ static void __init arch_reserve_crashkernel(void)
>  
>  	ret = parse_crashkernel(boot_command_line, memblock_phys_mem_size(),
>  				&crash_size, &crash_base,
> -				&low_size, NULL, &high);
> +				&low_size, &cma_size, &high);
>  	if (ret)
>  		return;
>  
>  	reserve_crashkernel_generic(crash_size, crash_base, low_size, high);
> +	reserve_crashkernel_cma(cma_size);
>  }
>  
>  static phys_addr_t __init max_zone_phys(phys_addr_t zone_limit)
> -- 
> 2.34.1
> 

-- 
Sincerely yours,
Mike.