linux-kernel - Re: [PATCH V2] arm64/hotplug: Improve memory offline event notifier

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Date:   Fri, 11 Sep 2020 15:06:04 +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>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        Steve Capper <steve.capper@....com>,
        Mark Brown <broonie@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] arm64/hotplug: Improve memory offline event notifier

Hi Anshuman,

On Mon, Aug 24, 2020 at 09:34:29AM +0530, Anshuman Khandual wrote:
> This brings about three different changes to the sole memory event notifier
> for arm64 platform and improves it's robustness while also enhancing debug
> capabilities during potential memory offlining error conditions.
> 
> This moves the memory notifier registration bit earlier in the boot process
> from device_initcall() to setup_arch() which will help in guarding against
> potential early boot memory offline requests.
> 
> This enables MEM_OFFLINE memory event handling. It will help intercept any
> possible error condition such as if boot memory some how still got offlined
> even after an expilicit notifier failure, potentially by a future change in
> generic hotplug framework. This would help detect such scenarious and help
> debug further.
> 
> It also adds a validation function which scans entire boot memory and makes
> sure that early memory sections are online. This check is essential for the
> memory notifier to work properly as it cannot prevent boot memory offlining
> if they are not online to begin with. But this additional sanity check is
> enabled only with DEBUG_VM.

Could you please split this in separate patches rather than having a
single one doing three somewhat related things?

> --- a/arch/arm64/kernel/setup.c
> +++ b/arch/arm64/kernel/setup.c
> @@ -376,6 +376,14 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
>  			"This indicates a broken bootloader or old kernel\n",
>  			boot_args[1], boot_args[2], boot_args[3]);
>  	}
> +
> +	/*
> +	 * Register the memory notifier which will prevent boot
> +	 * memory offlining requests - early enough. But there
> +	 * should not be any actual offlinig request till memory
> +	 * block devices are initialized with memory_dev_init().
> +	 */
> +	memory_hotremove_notifier();

Why can this not be an early_initcall()? As you said, memory_dev_init()
is called much later, after the SMP was initialised.

You could even combine this with validate_bootmem_online_state() in a
single early_initcall() which, after checking, registers the notifier.

-- 
Catalin