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

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Date:   Sun, 21 Apr 2019 20:26:08 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Dave Young <dyoung@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, vgoyal@...hat.com, bhe@...hat.com,
        piliu@...hat.com, Yinghai Lu <yinghai@...nel.org>,
        Eric Biederman <ebiederm@...ssion.com>
Subject: Re: [PATCH 2/2] X86/kdump: fall back to reserve high crashkernel
 memory


* Dave Young <dyoung@...hat.com> 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 allocs 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@...hat.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.
> +		 * as crashkernel=x,high allocs memory over 4G, also allocs

s/allocs
 /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 fallback to high memory

s/fallback
 /fall back

> +		 * 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 '@...set' 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
> +			fallback to reserve region above 4G in case without

s/fallback
 /fall back

> +			'@...set'.
> +			See Documentation/kdump/kdump.txt for further details.
>  
>  	crashkernel=range1:size1[,range2:size2,...][@offset]
>  			[KNL] Same as above, but depends on the memory

With the nits fixed:

Reviewed-by: Ingo Molnar <mingo@...nel.org>

Thanks,

	Ingo