linux-kernel - Re: [PATCH] arm64/mm/hotplug: Warn when memory limit has been reduced

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Date:   Thu, 16 Sep 2021 15:34:25 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Will Deacon <will@...nel.org>,
        James Morse <james.morse@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm/hotplug: Warn when memory limit has been reduced

On Tue, Sep 14, 2021 at 12:21:10PM +0530, Anshuman Khandual wrote:
> If the max memory limit has been reduced with 'mem=' kernel command line
> option, there might be UEFI memory map described memory beyond that limit
> which could be hot removed. This might be problematic for subsequent kexec
> kernel which could just access such removed memory.
> 
> Memory offline notifier exists because there is no other way to block the
> removal of boot memory, only the offlining (which isn't actually a problem)
> But with 'mem=', there is no chance to stop such boot memory being offlined
> as it where never in use by the kernel. As 'mem=' is a debug only option on
> arm64 platform, just warn for such a situation and move on.

Just to make sure I understand, is the memory beyond the mem= limit
considered online by the core code and it can be subsequently offlined?
Looking at walk_system_ram_range(), it doesn't seem to care about the
removed memblock ranges. Would such memory beyond the mem= limit need to
have been onlined first?

> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index cfd9deb347c3..7ac39ee876c3 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1627,6 +1627,18 @@ static int __init prevent_bootmem_remove_init(void)
>  	if (!IS_ENABLED(CONFIG_MEMORY_HOTREMOVE))
>  		return ret;
>  
> +	if (has_mem_limit_reduced()) {
> +		/*
> +		 * Physical memory limit has been reduced via the 'mem=' kernel
> +		 * command line option. Memory beyond reduced limit could now be
> +		 * removed and reassigned (guest ?) transparently to the kernel.
> +		 * This might cause subsequent kexec kernel to crash or at least
> +		 * corrupt the memory when accessing UEFI memory map enumerated
> +		 * boot memory which might have been repurposed.
> +		 */
> +		pr_warn("Memory limit reduced, kexec might be problematic\n");
> +	}

I'd actually move the warning to hotplug notifier callback rather than
the init function. I'd also make it pr_warn_once().

-- 
Catalin