linux-kernel - Re: [PATCH v4] arm64: kdump: simplify the reservation behaviour of crashkernel=,high

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Date:   Wed, 15 Mar 2023 14:52:51 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, 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 v4] arm64: kdump: simplify the reservation behaviour of
 crashkernel=,high

On Mon, Mar 06, 2023 at 04:41:24PM +0800, Baoquan He wrote:
> On arm64, reservation for 'crashkernel=xM,high' is taken by searching for
> suitable memory region top 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. Please see the details in
> Documentation/admin-guide/kernel-parameters.txt.
> 
> 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.
> The crashkernel high region crossing low and high memory boudary will bring
> issues:
[...]
> Note: On arm64, the high and low memory boudary could be 1G if it's RPi4
> system, or 4G if other normal systems.

I'm mostly ok with the reworking but I'm not sure about the non-fixed
low memory boundary. As I mentioned on v2, the user doesn't (need to)
know about the ZONE_DMA limit on a specific platform, that's supposed to
be fairly transparent. So on RPi4, specifying 'high' still allows
allocation below 4G which some users may treat as 'low'. The
kernel-parameters.txt doc also only talks about the 4G limit.

> +		/*
> +		 * 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;
>  		}

So I'm more tempted to set the search_base to 4G here rather than
CRASH_ADDR_LOW_MAX. The crashkernel=x,high option on a RPi4 with all
memory below 4G will fall back to low allocation. But RPi4 is the odd
one out, so I think we can live with this.

-- 
Catalin