Re: [PATCH v3 1/2] arm64: kdump: Provide default size when crashkernel=Y,low is not specified

Catalin Marinas
2022-11-07T17:18:59Z

On Mon, Jul 11, 2022 at 05:03:18PM +0800, Zhen Lei wrote:
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 2522b11e593f239..65a2c3a22a4b57d 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -843,7 +843,7 @@
> available.
> It will be ignored if crashkernel=X is specified.
> crashkernel=size[KMG],low
> - [KNL, X86-64] range under 4G. When crashkernel=X,high
> + [KNL, X86-64, ARM64] range under 4G. When crashkernel=X,high
> is passed, kernel could allocate physical memory region
> above 4G, that cause second kernel crash on system
> that require some amount of low memory, e.g. swiotlb
> @@ -857,12 +857,6 @@
> It will be ignored when crashkernel=X,high is not used
> or memory reserved is below 4G.
>
> - [KNL, ARM64] range in low memory.
> - This one lets the user specify a low range in the
> - DMA zone for the crash dump kernel.
> - It will be ignored when crashkernel=X,high is not used
> - or memory reserved is located in the DMA zones.
> -
> cryptomgr.notests
> [KNL] Disable crypto self-tests
>
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 339ee84e5a61a0b..5390f361208ccf7 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -96,6 +96,14 @@ phys_addr_t __ro_after_init arm64_dma_phys_limit = PHYS_MASK + 1;
> #define CRASH_ADDR_LOW_MAX arm64_dma_phys_limit
> #define CRASH_ADDR_HIGH_MAX (PHYS_MASK + 1)
>
> +/*
> + * This is an empirical value in x86_64 and taken here directly. Please
> + * refer to the code comment in reserve_crashkernel_low() of x86_64 for more
> + * details.
> + */
> +#define DEFAULT_CRASH_KERNEL_LOW_SIZE \
> + max(swiotlb_size_or_default() + (8UL << 20), 256UL << 20)
> +
> static int __init reserve_crashkernel_low(unsigned long long low_size)
> {
> unsigned long long low_base;
> @@ -147,7 +155,9 @@ static void __init reserve_crashkernel(void)
> * is not allowed.
> */
> ret = parse_crashkernel_low(cmdline, 0, &crash_low_size, &crash_base);
> - if (ret && (ret != -ENOENT))
> + if (ret == -ENOENT)
> + crash_low_size = DEFAULT_CRASH_KERNEL_LOW_SIZE;
> + else if (ret)
> return;

BTW, since we want a default low allocation, I think we should change
the checking logic slightly. Currently we have:

if ((crash_base >= CRASH_ADDR_LOW_MAX) &&
crash_low_size && reserve_crashkernel_low(crash_low_size)) {
...

If crash_base is just below CRASH_ADDR_LOW_MAX, we deem it sufficient
but a crashkernel trying to allocate 64MB of swiotlb may fail. So maybe
change this to crash_base >= CRASH_ADDR_LOW_MAX - crash_low_size.

--
Catalin