Re: [PATCH 2/2 update] X86/kdump: fall back to reserve high crashkernel memory

[Baoquan He <bhe@redhat.com>]

On 04/22/19 at 11:19am, Dave Young wrote:
> crashkernel=xM tries to reserve crashkernel memory under 4G, which
> is enough for usual cases.  But this could fail sometimes, for example
> one tries to reserve a big chunk like 2G, it is possible to fail.
> 
> So let the crashkernel=xM just fall back to use high memory in case it
> fails to find a suitable low range.  Do not set the ,high as default
> because it allocates extra low memory for DMA buffers and swiotlb, this is
> not always necessary for all machines. Typically like crashkernel=128M
> usually work with low reservation under 4G, so still keep <4G as default.
> 
> Signed-off-by: Dave Young <dyoung@redhat.com>
> Reviewed-by: Ingo Molnar <mingo@kernel.org>
> ---

Ack the whole series, thanks for the effort.

Acked-by: Baoquan He <bhe@redhat.com>

>  Documentation/admin-guide/kernel-parameters.txt |    7 +++++--
>  arch/x86/kernel/setup.c                         |   22 ++++++++++++++--------
>  2 files changed, 19 insertions(+), 10 deletions(-)
> 
> --- linux-x86.orig/arch/x86/kernel/setup.c
> +++ linux-x86/arch/x86/kernel/setup.c
> @@ -541,21 +541,27 @@ static void __init reserve_crashkernel(v
>  	}
>  
>  	/* 0 means: find the address automatically */
> -	if (crash_base <= 0) {
> +	if (!crash_base) {
>  		/*
>  		 * Set CRASH_ADDR_LOW_MAX upper bound for crash memory,
> -		 * as old kexec-tools loads bzImage below that, unless
> -		 * "crashkernel=size[KMG],high" is specified.
> +		 * crashkernel=x,high reserves memory over 4G, also allocates
> +		 * 256M extra low memory for DMA buffers and swiotlb.
> +		 * but the extra memory is not required for all machines.
> +		 * So prefer low memory first, and fall back to high memory
> +		 * unless "crashkernel=size[KMG],high" is specified.
>  		 */
> -		crash_base = memblock_find_in_range(CRASH_ALIGN,
> -						    high ? CRASH_ADDR_HIGH_MAX
> -							 : CRASH_ADDR_LOW_MAX,
> -						    crash_size, CRASH_ALIGN);
> +		if (!high)
> +			crash_base = memblock_find_in_range(CRASH_ALIGN,
> +						CRASH_ADDR_LOW_MAX,
> +						crash_size, CRASH_ALIGN);
> +		if (!crash_base)
> +			crash_base = memblock_find_in_range(CRASH_ALIGN,
> +						CRASH_ADDR_HIGH_MAX,
> +						crash_size, CRASH_ALIGN);
>  		if (!crash_base) {
>  			pr_info("crashkernel reservation failed - No suitable area found.\n");
>  			return;
>  		}
> -
>  	} else {
>  		unsigned long long start;
>  
> --- linux-x86.orig/Documentation/admin-guide/kernel-parameters.txt
> +++ linux-x86/Documentation/admin-guide/kernel-parameters.txt
> @@ -704,8 +704,11 @@
>  			upon panic. This parameter reserves the physical
>  			memory region [offset, offset + size] for that kernel
>  			image. If '@offset' is omitted, then a suitable offset
> -			is selected automatically. Check
> -			Documentation/kdump/kdump.txt for further details.
> +			is selected automatically.
> +			[KNL, x86_64] select a region under 4G first, and
> +			fall back to reserve region above 4G in case without
> +			'@offset'.
> +			See Documentation/kdump/kdump.txt for further details.
>  
>  	crashkernel=range1:size1[,range2:size2,...][@offset]
>  			[KNL] Same as above, but depends on the memory