Date:   Fri, 20 Jan 2023 10:04:49 +0100
From:   Simon Horman <horms@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
        will@...nel.org, thunder.leizhen@...wei.com,
        John.p.donnelly@...cle.com, wangkefeng.wang@...wei.com
Subject: Re: [PATCH 1/2] arm64: kdump: simplify the reservation behaviour of
 crashkernel=,high

On Tue, Jan 17, 2023 at 11:49:20AM +0800, Baoquan He wrote:
> On arm64, reservation for 'crashkernel=xM,high' is taken by searching for
> suitable memory region up down. If the 'xM' of crashkernel high memory
> is reserved from high memory successfully, it will try to reserve
> crashkernel low memory later accoringly. Otherwise, it will try to search
> low memory area for the 'xM' suitable region.
> 
> While we observed an unexpected case where a reserved region crosses the
> high and low meomry boundary. E.g on a system with 4G as low memory end,
> user added the kernel parameters like: 'crashkernel=512M,high', it could
> finally have [4G-126M, 4G+386M], [1G, 1G+128M] regions in running kernel.
> This looks very strange because we have two low memory regions
> [4G-126M, 4G] and [1G, 1G+128M]. Much explanation need be given to tell
> why that happened.
> 
> Here, for crashkernel=xM,high, search the high memory for the suitable
> region above the high and low memory boundary. If failed, try reserving
> the suitable region below the boundary. Like this, the crashkernel high
> region will only exist in high memory, and crashkernel low region only
> exists in low memory. The reservation behaviour for crashkernel=,high is
> clearer and simpler.
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
>  arch/arm64/mm/init.c | 30 +++++++++++++++++++++++-------
>  1 file changed, 23 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 58a0bb2c17f1..26a05af2bfa8 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -127,12 +127,13 @@ 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;
> @@ -155,7 +156,9 @@ static void __init reserve_crashkernel(void)
>  		else if (ret)
>  			return;
>  
> +		search_base = CRASH_ADDR_LOW_MAX;
>  		crash_max = CRASH_ADDR_HIGH_MAX;
> +		high = true;
>  	} else if (ret || !crash_size) {
>  		/* The specified value is invalid */
>  		return;
> @@ -166,31 +169,44 @@ 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 (fixed_base) {
> +			pr_warn("cannot reserve crashkernel region [0x%llx-0x%llx]\n",
> +				search_base, crash_max);
> +			return;
> +		}
> +
>  		/*
>  		 * If the first attempt was for low memory, fall back to
>  		 * high memory, the minimum required low memory will be
>  		 * reserved later.
>  		 */
> -		if (!fixed_base && (crash_max == CRASH_ADDR_LOW_MAX)) {
> +		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;
>  		}
>  
> +		if (high && (crash_max == CRASH_ADDR_HIGH_MAX)) {

nit: unnecessary (and inconsistent with code just above) parentheses.

> +			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
> 
> 
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