linux-kernel - Re: [PATCH v5] arm64: kdump: simplify the reservation behaviour of crashkernel=,high

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Message-ID: <ZC+Axh1G/+NyIdwg@MiWiFi-R3L-srv>
Date:   Fri, 7 Apr 2023 10:32:38 +0800
From:   Baoquan He <bhe@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     catalin.marinas@....com, horms@...nel.org,
        thunder.leizhen@...wei.com, John.p.donnelly@...cle.com,
        will@...nel.org, kexec@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v5] arm64: kdump: simplify the reservation behaviour of
 crashkernel=,high

On 04/07/23 at 10:24am, Baoquan He wrote:
......
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 66e70ca47680..307263c01292 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -69,6 +69,7 @@ phys_addr_t __ro_after_init arm64_dma_phys_limit;
>  
>  #define CRASH_ADDR_LOW_MAX		arm64_dma_phys_limit
>  #define CRASH_ADDR_HIGH_MAX		(PHYS_MASK + 1)
> +#define CRASH_HIGH_SEARCH_BASE		SZ_4G
>  
>  #define DEFAULT_CRASH_KERNEL_LOW_SIZE	(128UL << 20)
>  
> @@ -101,12 +102,13 @@ static int __init reserve_crashkernel_low(unsigned long long low_size)
>   */
>  static void __init reserve_crashkernel(void)
>  {
> -	unsigned long long crash_base, crash_size;
> -	unsigned long long crash_low_size = 0;
> +	unsigned long long crash_base, crash_size, search_base;
>  	unsigned long long crash_max = CRASH_ADDR_LOW_MAX;
> +	unsigned long long crash_low_size = 0;
>  	char *cmdline = boot_command_line;
> -	int ret;
>  	bool fixed_base = false;
> +	bool high = false;
> +	int ret;
>  
>  	if (!IS_ENABLED(CONFIG_KEXEC_CORE))
>  		return;
> @@ -129,7 +131,9 @@ static void __init reserve_crashkernel(void)
>  		else if (ret)
>  			return;
>  
> +		search_base = CRASH_HIGH_SEARCH_BASE;

Here, I am hesitant if a conditional check is needed as below. On
special system where both CONFIG_ZONE_DMA and CONFIG_ZONE_DMA32 
are disabled, there's only low memory, means its arm64_dma_phys_limit
equals to (PHYS_MASK + 1). In this case, whatever the crashkernel= is,
it can search the whole system memory for available crashkernel region.
Maybe it's fine since it's not big deal, the memory regoin can be found
anyway.

  		crash_max = CRASH_ADDR_HIGH_MAX;
		if (crash_max != CRASH_ADDR_LOW_MAX)
			search_base = CRASH_HIGH_SEARCH_BASE;

> +		high = true;
>  	} else if (ret || !crash_size) {
>  		/* The specified value is invalid */
>  		return;
> @@ -140,31 +144,51 @@ static void __init reserve_crashkernel(void)
>  	/* User specifies base address explicitly. */
>  	if (crash_base) {
>  		fixed_base = true;
> +		search_base = crash_base;
>  		crash_max = crash_base + crash_size;
>  	}
>  
>  retry:
>  	crash_base = memblock_phys_alloc_range(crash_size, CRASH_ALIGN,
> -					       crash_base, crash_max);
> +					       search_base, crash_max);
>  	if (!crash_base) {
>  		/*
> -		 * If the first attempt was for low memory, fall back to
> -		 * high memory, the minimum required low memory will be
> -		 * reserved later.
> +		 * For crashkernel=size[KMG]@offset[KMG], print out failure
> +		 * message if can't reserve the specified region.
>  		 */
> -		if (!fixed_base && (crash_max == CRASH_ADDR_LOW_MAX)) {
> +		if (fixed_base) {
> +			pr_warn("crashkernel reservation failed - memory is in use.\n");
> +			return;
> +		}
> +
> +		/*
> +		 * For crashkernel=size[KMG], if the first attempt was for
> +		 * low memory, fall back to high memory, the minimum required
> +		 * low memory will be reserved later.
> +		 */
> +		if (!high && crash_max == CRASH_ADDR_LOW_MAX) {
>  			crash_max = CRASH_ADDR_HIGH_MAX;
> +			search_base = CRASH_ADDR_LOW_MAX;
>  			crash_low_size = DEFAULT_CRASH_KERNEL_LOW_SIZE;
>  			goto retry;
>  		}
>  
> +		/*
> +		 * For crashkernel=size[KMG],high, if the first attempt was
> +		 * for high memory, fall back to low memory.
> +		 */
> +		if (high && crash_max == CRASH_ADDR_HIGH_MAX) {
> +			crash_max = CRASH_ADDR_LOW_MAX;
> +			search_base = 0;
> +			goto retry;
> +		}
>  		pr_warn("cannot allocate crashkernel (size:0x%llx)\n",
>  			crash_size);
>  		return;
>  	}
>  
> -	if ((crash_base > CRASH_ADDR_LOW_MAX - crash_low_size) &&
> -	     crash_low_size && reserve_crashkernel_low(crash_low_size)) {
> +	if ((crash_base >= CRASH_ADDR_LOW_MAX) && crash_low_size &&
> +	     reserve_crashkernel_low(crash_low_size)) {
>  		memblock_phys_free(crash_base, crash_size);
>  		return;
>  	}
> -- 
> 2.34.1
>