Re: [PATCHv7] x86/kdump: bugfix, make the behavior of crashkernel=X consistent with kaslr

[Jerry Hoemann]

On Tue, Jan 15, 2019 at 04:07:03PM +0800, Pingfan Liu wrote:
> People reported a bug on a high end server with many pcie devices, where
> kernel bootup with crashkernel=384M, and kaslr is enabled. Even
> though we still see much memory under 896 MB, the finding still failed
> intermittently. Because currently we can only find region under 896 MB,
> if without ',high' specified. Then KASLR breaks 896 MB into several parts
> randomly, and crashkernel reservation need be aligned to 128 MB, that's
> why failure is found. It raises confusion to the end user that sometimes
> crashkernel=X works while sometimes fails.
> If want to make it succeed, customer can change kernel option to
> "crashkernel=384M,high". Just this give "crashkernel=xx@yy" a very
> limited space to behave even though its grammar looks more generic.
> And we can't answer questions raised from customer that confidently:
> 1) why it doesn't succeed to reserve 896 MB;
> 2) what's wrong with memory region under 4G;
> 3) why I have to add ',high', I only require 384 MB, not 3840 MB.
> This patch tries to get memory region from 896 MB firstly, then [896MB,4G],
> finally above 4G.

While allocating crashkernel from below 4G seems fine, won't we have
problems if the crash kernel gets allocated above 4G because of the SWIOTLB?

thanks


> Dave Young sent the original post, and I just re-post it with commit log
> improvement as his requirement.
> http://lists.infradead.org/pipermail/kexec/2017-October/019571.html
> There was an old discussion below (previously posted by Chao Wang):
> https://lkml.org/lkml/2013/10/15/601
> 
> Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> Cc: Dave Young <dyoung@xxxxxxxxxx>
> Cc: Baoquan He <bhe@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
> Cc: yinghai@xxxxxxxxxx,
> Cc: vgoyal@xxxxxxxxxx
> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> ---
> v6 -> v7: fix spelling mistake pointed out by Randy
>  arch/x86/kernel/setup.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 3d872a5..fa62c81 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -551,6 +551,22 @@ static void __init reserve_crashkernel(void)
>  						    high ? CRASH_ADDR_HIGH_MAX
>  							 : CRASH_ADDR_LOW_MAX,
>  						    crash_size, CRASH_ALIGN);
> +#ifdef CONFIG_X86_64
> +		/*
> +		 * crashkernel=X reserve below 896M fails? Try below 4G
> +		 */
> +		if (!high && !crash_base)
> +			crash_base = memblock_find_in_range(CRASH_ALIGN,
> +						(1ULL << 32),
> +						crash_size, CRASH_ALIGN);
> +		/*
> +		 * crashkernel=X reserve below 4G fails? Try MAXMEM
> +		 */
> +		if (!high && !crash_base)
> +			crash_base = memblock_find_in_range(CRASH_ALIGN,
> +						CRASH_ADDR_HIGH_MAX,
> +						crash_size, CRASH_ALIGN);
> +#endif
>  		if (!crash_base) {
>  			pr_info("crashkernel reservation failed - No suitable area found.\n");
>  			return;
> -- 
> 2.7.4
> 
> 
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Jerry Hoemann                  Software Engineer   Hewlett Packard Enterprise
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