linux-kernel - Re: [PATCH v3 2/2] arm64: kdump: Support crashkernel=X fall back to reserve region above DMA zones

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Message-ID: <Y2k8yuUghrbQE6da@arm.com>
Date:   Mon, 7 Nov 2022 17:13:46 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Zhen Lei <thunder.leizhen@...wei.com>
Cc:     Dave Young <dyoung@...hat.com>, Baoquan He <bhe@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Feng Zhou <zhoufeng.zf@...edance.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Chen Zhou <dingguo.cz@...group.com>,
        John Donnelly <John.p.donnelly@...cle.com>,
        Dave Kleikamp <dave.kleikamp@...cle.com>
Subject: Re: [PATCH v3 2/2] arm64: kdump: Support crashkernel=X fall back to
 reserve region above DMA zones

On Mon, Jul 11, 2022 at 05:03:19PM +0800, Zhen Lei wrote:
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 5390f361208ccf7..8539598f9e58b4d 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -138,6 +138,7 @@ static void __init reserve_crashkernel(void)
>  	unsigned long long crash_max = CRASH_ADDR_LOW_MAX;
>  	char *cmdline = boot_command_line;
>  	int ret;
> +	bool fixed_base;
>  
>  	if (!IS_ENABLED(CONFIG_KEXEC_CORE))
>  		return;
> @@ -166,15 +167,28 @@ static void __init reserve_crashkernel(void)
>  		return;
>  	}
>  
> +	fixed_base = !!crash_base;
>  	crash_size = PAGE_ALIGN(crash_size);
>  
>  	/* User specifies base address explicitly. */
> -	if (crash_base)
> +	if (fixed_base)
>  		crash_max = crash_base + crash_size;

Not a fan of '!!', it is converted automatically. If you don't like the
conversion, just initialise fixed_base to false and here:

	if (crash_base) {
		fixed_base = true;
		crash_max = crash_base + crash_size;
	}

> +retry:
>  	crash_base = memblock_phys_alloc_range(crash_size, CRASH_ALIGN,
>  					       crash_base, crash_max);
>  	if (!crash_base) {
> +		/*
> +		 * Attempt to fully allocate low memory failed, fall back
> +		 * to high memory, the minimum required low memory will be
> +		 * reserved later.
> +		 */

I'm not sure this comment makes sense. If !crash_base, it doesn't mean
the kernel failed to fully allocate low memory. crash_max here could be
CRASH_ADDR_HIGH_MAX if crashkerne=X,high was specified. Maybe says
something like "If the first attempt was for low memory, fall back to
high ..."

> +		if (!fixed_base && (crash_max == CRASH_ADDR_LOW_MAX)) {
> +			crash_max = CRASH_ADDR_HIGH_MAX;
> +			crash_low_size = DEFAULT_CRASH_KERNEL_LOW_SIZE;
> +			goto retry;
> +		}

The retry logic looks fine, it only happens once as crash_max is
updated.

-- 
Catalin