Re: [PATCH 2/4] arm64: kdump: support reserving crashkernel above 4G

[James Morse <james.morse@arm.com>]

Hello,

On 07/05/2019 04:50, Chen Zhou wrote:
> When crashkernel is reserved above 4G in memory, kernel should
> reserve some amount of low memory for swiotlb and some DMA buffers.

> Meanwhile, support crashkernel=X,[high,low] in arm64. When use
> crashkernel=X parameter, try low memory first and fall back to high
> memory unless "crashkernel=X,high" is specified.

What is the 'unless crashkernel=...,high' for? I think it would be simpler to relax the
ARCH_LOW_ADDRESS_LIMIT if reserve_crashkernel_low() allocated something.

This way "crashkernel=1G" tries to allocate 1G below 4G, but fails if there isn't enough
memory. "crashkernel=1G crashkernel=16M,low" allocates 16M below 4G, which is more likely
to succeed, if it does it can then place the 1G block anywhere.


> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> index 413d566..82cd9a0 100644
> --- a/arch/arm64/kernel/setup.c
> +++ b/arch/arm64/kernel/setup.c
> @@ -243,6 +243,9 @@ static void __init request_standard_resources(void)
>  			request_resource(res, &kernel_data);
>  #ifdef CONFIG_KEXEC_CORE
>  		/* Userspace will find "Crash kernel" region in /proc/iomem. */
> +		if (crashk_low_res.end && crashk_low_res.start >= res->start &&
> +		    crashk_low_res.end <= res->end)
> +			request_resource(res, &crashk_low_res);
>  		if (crashk_res.end && crashk_res.start >= res->start &&
>  		    crashk_res.end <= res->end)
>  			request_resource(res, &crashk_res);

With both crashk_low_res and crashk_res, we end up with two entries in /proc/iomem called
"Crash kernel". Because its sorted by address, and kexec-tools stops searching when it
find "Crash kernel", you are always going to get the kernel placed in the lower portion.

I suspect this isn't what you want, can we rename crashk_low_res for arm64 so that
existing kexec-tools doesn't use it?


> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index d2adffb..3fcd739 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -74,20 +74,37 @@ phys_addr_t arm64_dma_phys_limit __ro_after_init;
>  static void __init reserve_crashkernel(void)
>  {
>  	unsigned long long crash_base, crash_size;
> +	bool high = false;
>  	int ret;
>  
>  	ret = parse_crashkernel(boot_command_line, memblock_phys_mem_size(),
>  				&crash_size, &crash_base);
>  	/* no crashkernel= or invalid value specified */
> -	if (ret || !crash_size)
> -		return;
> +	if (ret || !crash_size) {
> +		/* crashkernel=X,high */
> +		ret = parse_crashkernel_high(boot_command_line,
> +				memblock_phys_mem_size(),
> +				&crash_size, &crash_base);
> +		if (ret || !crash_size)
> +			return;
> +		high = true;
> +	}
>  
>  	crash_size = PAGE_ALIGN(crash_size);
>  
>  	if (crash_base == 0) {
> -		/* Current arm64 boot protocol requires 2MB alignment */
> -		crash_base = memblock_find_in_range(0, ARCH_LOW_ADDRESS_LIMIT,
> -				crash_size, SZ_2M);
> +		/*
> +		 * Try low memory first and fall back to high memory
> +		 * unless "crashkernel=size[KMG],high" is specified.
> +		 */
> +		if (!high)
> +			crash_base = memblock_find_in_range(0,
> +					ARCH_LOW_ADDRESS_LIMIT,
> +					crash_size, CRASH_ALIGN);
> +		if (!crash_base)
> +			crash_base = memblock_find_in_range(0,
> +					memblock_end_of_DRAM(),
> +					crash_size, CRASH_ALIGN);
>  		if (crash_base == 0) {
>  			pr_warn("cannot allocate crashkernel (size:0x%llx)\n",
>  				crash_size);
> @@ -105,13 +122,18 @@ static void __init reserve_crashkernel(void)
>  			return;
>  		}
>  
> -		if (!IS_ALIGNED(crash_base, SZ_2M)) {
> +		if (!IS_ALIGNED(crash_base, CRASH_ALIGN)) {
>  			pr_warn("cannot reserve crashkernel: base address is not 2MB aligned\n");
>  			return;
>  		}
>  	}
>  	memblock_reserve(crash_base, crash_size);
>  
> +	if (crash_base >= SZ_4G && reserve_crashkernel_low()) {
> +		memblock_free(crash_base, crash_size);
> +		return;

This is going to be annoying on platforms that don't have, and don't need memory below 4G.
A "crashkernel=...,low" on these system will break crashdump. I don't think we should
expect users to know the memory layout. (I'm assuming distro's are going to add a low
reservation everywhere, just in case)

I think the 'low' region should be a small optional/best-effort extra, that kexec-tools
can't touch.


I'm afraid you've missed the ugly bit of the crashkernel reservation...

arch/arm64/mm/mmu.c::map_mem() marks the crashkernel as 'nomap' during the first pass of
page-table generation. This means it isn't mapped in the linear map. It then maps it with
page-size mappings, and removes the nomap flag.

This is done so that arch_kexec_protect_crashkres() and
arch_kexec_unprotect_crashkres() can remove the valid bits of the crashkernel mapping.
This way the old-kernel can't accidentally overwrite the crashkernel. It also saves us if
the old-kernel and the crashkernel use different memory attributes for the mapping.

As your low-memory reservation is intended to be used for devices, having it mapped by the
old-kernel as cacheable memory is going to cause problems if those CPUs aren't taken
offline and go corrupting this memory. (we did crash for a reason after all)


I think the simplest thing to do is mark the low region as 'nomap' in
reserve_crashkernel() and always leave it unmapped. We can then describe it via a
different string in /proc/iomem, something like "Crash kernel (low)". Older kexec-tools
shouldn't use it, (I assume its not using strncmp() in a way that would do this by
accident), and newer kexec-tools can know to describe it in the DT, but it can't write to it.


Thanks,

James

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