Re: [PATCH 1/5] arm64: kdump: Provide default size when crashkernel=Y,low is not specified

[Baoquan He <bhe@redhat.com>]

On 06/13/22 at 04:09pm, Zhen Lei wrote:
> To be consistent with the implementation of x86 and improve cross-platform
> user experience. Try to allocate at least 256 MiB low memory automatically
> when crashkernel=Y,low is not specified.
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> ---
>  Documentation/admin-guide/kernel-parameters.txt |  8 +-------
>  arch/arm64/mm/init.c                            | 12 +++++++++++-
>  2 files changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 8090130b544b070..61b179232b68001 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -843,7 +843,7 @@
>  			available.
>  			It will be ignored if crashkernel=X is specified.
>  	crashkernel=size[KMG],low
> -			[KNL, X86-64] range under 4G. When crashkernel=X,high
> +			[KNL, X86-64, ARM64] range under 4G. When crashkernel=X,high
>  			is passed, kernel could allocate physical memory region
>  			above 4G, that cause second kernel crash on system
>  			that require some amount of low memory, e.g. swiotlb
> @@ -857,12 +857,6 @@
>  			It will be ignored when crashkernel=X,high is not used
>  			or memory reserved is below 4G.
>  
> -			[KNL, ARM64] range in low memory.
> -			This one lets the user specify a low range in the
> -			DMA zone for the crash dump kernel.
> -			It will be ignored when crashkernel=X,high is not used
> -			or memory reserved is located in the DMA zones.
> -
>  	cryptomgr.notests
>  			[KNL] Disable crypto self-tests
>  
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 339ee84e5a61a0b..5390f361208ccf7 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -96,6 +96,14 @@ phys_addr_t __ro_after_init arm64_dma_phys_limit = PHYS_MASK + 1;
>  #define CRASH_ADDR_LOW_MAX		arm64_dma_phys_limit
>  #define CRASH_ADDR_HIGH_MAX		(PHYS_MASK + 1)
>  
> +/*
> + * This is an empirical value in x86_64 and taken here directly. Please
> + * refer to the code comment in reserve_crashkernel_low() of x86_64 for more
> + * details.
> + */
> +#define DEFAULT_CRASH_KERNEL_LOW_SIZE	\
> +	max(swiotlb_size_or_default() + (8UL << 20), 256UL << 20)

About this default low value, 256M, I am not sure if it can be lowered
down. We have Ampere Mt-Jade systems in Redhat and their biggest
contiguous memory is less than 256M under low 4G when the firmware 32bit
option is disabled. Obviously this will fail the default crashkernel,low
value.

I am not sure how common the 32bit option is disabled. If it's an
important feature and widely set, it need be taken into consideration
when deciding this default crashkernel,low value. Otherwise, the
crashkernel=xM won't work in Ampere Mt-Jade system with 32bit option
disabled, and people need specify crashkernel=xM,high,
crashkernel=yM,low explicitly. The omission of crashkernel,low is also
not allowed in the case.

Hi, Mark and Christopher,

Add you in CC. If you happen to know contact person from Ampere, please
also feel free to add them in this thread.

Thanks
Baoquan

>  static int __init reserve_crashkernel_low(unsigned long long low_size)
>  {
>  	unsigned long long low_base;
> @@ -147,7 +155,9 @@ static void __init reserve_crashkernel(void)
>  		 * is not allowed.
>  		 */
>  		ret = parse_crashkernel_low(cmdline, 0, &crash_low_size, &crash_base);
> -		if (ret && (ret != -ENOENT))
> +		if (ret == -ENOENT)
> +			crash_low_size = DEFAULT_CRASH_KERNEL_LOW_SIZE;
> +		else if (ret)
>  			return;
>  
>  		crash_max = CRASH_ADDR_HIGH_MAX;
> -- 
> 2.25.1
>