Re: [PATCH V3 2/3] arm64/mm/hotplug: Enable MEM_OFFLINE event handling

From: Gavin Shan <gshan@redhat.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>,
	linux-arm-kernel@lists.infradead.org
Cc: Mark Rutland <mark.rutland@arm.com>,
	Will Deacon <will@kernel.com>,
	Steve Capper <steve.capper@arm.com>,
	catalin.marinas@arm.com, linux-kernel@vger.kernel.org,
	Mark Brown <broonie@kernel.org>, Marc Zyngier <maz@kernel.org>,
	will@kernel.org
Subject: Re: [PATCH V3 2/3] arm64/mm/hotplug: Enable MEM_OFFLINE event handling
Date: Wed, 23 Sep 2020 16:31:34 +1000	[thread overview]
Message-ID: <d4e90ed2-f348-0d80-b3d7-d156abe05a90@redhat.com> (raw)
In-Reply-To: <1600689908-28213-3-git-send-email-anshuman.khandual@arm.com>

Hi Anshuman,

On 9/21/20 10:05 PM, Anshuman Khandual wrote:
> 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 explicit notifier failure, potentially by a future change in
> generic hot plug framework. This would help detect such scenarios and help
> debug further.
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Steve Capper <steve.capper@arm.com>
> Cc: Mark Brown <broonie@kernel.org>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---

I'm not sure if it makes sense since MEM_OFFLINE won't be triggered
after NOTIFY_BAD is returned from MEM_GOING_OFFLINE. NOTIFY_BAD means
the whole offline process is stopped. It would be guranteed by generic
framework from syntax standpoint.

However, this looks good if MEM_OFFLINE is triggered without calling
into MEM_GOING_OFFLINE previously, but it would be a bug from generic
framework.

>   arch/arm64/mm/mmu.c | 37 ++++++++++++++++++++++++++++++++-----
>   1 file changed, 32 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index df3b7415b128..6b171bd88bcf 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1482,13 +1482,40 @@ static int prevent_bootmem_remove_notifier(struct notifier_block *nb,
>   	unsigned long end_pfn = arg->start_pfn + arg->nr_pages;
>   	unsigned long pfn = arg->start_pfn;
>   
> -	if (action != MEM_GOING_OFFLINE)
> +	if ((action != MEM_GOING_OFFLINE) && (action != MEM_OFFLINE))
>   		return NOTIFY_OK;
>   
> -	for (; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
> -		ms = __pfn_to_section(pfn);
> -		if (early_section(ms))
> -			return NOTIFY_BAD;
> +	if (action == MEM_GOING_OFFLINE) {
> +		for (; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
> +			ms = __pfn_to_section(pfn);
> +			if (early_section(ms)) {
> +				pr_warn("Boot memory offlining attempted\n");
> +				return NOTIFY_BAD;
> +			}
> +		}
> +	} else if (action == MEM_OFFLINE) {
> +		for (; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
> +			ms = __pfn_to_section(pfn);
> +			if (early_section(ms)) {
> +
> +				/*
> +				 * This should have never happened. Boot memory
> +				 * offlining should have been prevented by this
> +				 * very notifier. Probably some memory removal
> +				 * procedure might have changed which would then
> +				 * require further debug.
> +				 */
> +				pr_err("Boot memory offlined\n");
> +
> +				/*
> +				 * Core memory hotplug does not process a return
> +				 * code from the notifier for MEM_OFFLINE event.
> +				 * Error condition has been reported. Report as
> +				 * ignored.
> +				 */
> +				return NOTIFY_DONE;
> +			}
> +		}
>   	}
>   	return NOTIFY_OK;
>   }
> 

It's pretty much irrelevant comment if the patch doesn't make sense:
the logical block for MEM_GOING_OFFLINE would be reused by MEM_OFFLINE
as they looks similar except the return value and error message :)

Cheers,
Gavin

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