x86/kdump: crashkernel=X try to reserve below 896M first then below 4G and MAXMEM

[Dave Young]

On 10/20/17 at 01:52pm, Dave Young wrote:
> Now crashkernel=X will fail if there's not enough memory at low region
> (below 896M) when trying to reserve large memory size.  One can use
> crashkernel=xM,high to reserve it at high region (>4G) but it is more
> convinient to improve crashkernel=X to: 
> 
>  - First try to reserve X below 896M (for being compatible with old
>    kexec-tools).
>  - If fails, try to reserve X below 4G (swiotlb need to stay below 4G).
>  - If fails, try to reserve X from MAXMEM top down.
> 
> It's more transparent and user-friendly.
> 
> If crashkernel is large and the reserved is beyond 896M, old kexec-tools
> is not compatible with new kernel because old kexec-tools can not load
> kernel at high memory region, there was an old discussion below
> (previously posted by Chao Wang):
> https://lkml.org/lkml/2013/10/15/601
> 
> But actually the behavior is consistent during my test. Suppose
> old kernel fail to reserve memory at low areas, kdump does not
> work because no memory reserved. With this patch, suppose new kernel
> successfully reserved memory at high areas, old kexec-tools still fail
> to load kdump kernel (tested 2.0.2), so it is acceptable, no need to
> worry about the compatibility.
> 
> Here is the test result (kexec-tools 2.0.2, no high memory load
> support):
> Crashkernel over 4G:
> # cat /proc/iomem|grep Crash
>   be000000-cdffffff : Crash kernel
>   213000000-21effffff : Crash kernel
> # ./kexec  -p /boot/vmlinuz-`uname -r`
> Memory for crashkernel is not reserved
> Please reserve memory by passing "crashkernel=X at Y" parameter to the kernel
> Then try loading kdump kernel
> 
> crashkernel: 896M-4G:
> # cat /proc/iomem|grep Crash
>   96000000-cdefffff : Crash kernel
> # ./kexec -p /boot/vmlinuz-4.14.0-rc4+
> ELF core (kcore) parse failed
> Cannot load /boot/vmlinuz-4.14.0-rc4+
> 
> Signed-off-by: Dave Young <dyoung at redhat.com>
> ---
>  arch/x86/kernel/setup.c |   16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> --- linux-x86.orig/arch/x86/kernel/setup.c
> +++ linux-x86/arch/x86/kernel/setup.c
> @@ -568,6 +568,22 @@ static void __init reserve_crashkernel(v
>  						    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;


Hmm, forgot to add [PATCH] prefix, will resend this along with other two
crashkernel patches.

Thanks
Dave