Re: [PATCH V4 2/3] arm64/mm/hotplug: Enable MEM_OFFLINE event handling

[Gavin Shan]

Hi Anshuman,

On 9/29/20 11:54 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. While here, also call out the first section being attempted
> for offline or got offlined.
>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Marc Zyngier <maz@xxxxxxxxxx>
> Cc: Steve Capper <steve.capper@xxxxxxx>
> Cc: Mark Brown <broonie@xxxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
>   arch/arm64/mm/mmu.c | 29 +++++++++++++++++++++++++++--
>   1 file changed, 27 insertions(+), 2 deletions(-)

This looks good to me except a nit and it can be improved if
that looks reasonable and only when you get a chance for
respin.

Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>

> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 4e70f4fea06c..90a30f5ebfc0 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1482,13 +1482,38 @@ 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) {
> +		unsigned long start = PFN_PHYS(pfn);
> +		unsigned long end = start + (1UL << PA_SECTION_SHIFT);
> +
>   		ms = __pfn_to_section(pfn);
> -		if (early_section(ms))
> +		if (!early_section(ms))
> +			continue;
> +

The discussion here is irrelevant to this patch itself. It seems
early_section() is coarse, which means all memory detected during
boot time won't be hotpluggable?

> +		if (action == MEM_GOING_OFFLINE) {
> +			pr_warn("Boot memory [%lx %lx] offlining attempted\n", start, end);
>   			return NOTIFY_BAD;
> +		} else if (action == MEM_OFFLINE) {
> +			/*
> +			 * 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 [%lx %lx] offlined\n", start, end);
> +
> +			/*
> +			 * 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;
>   }

I think NOTIFY_BAD is returned for MEM_OFFLINE wouldn't be a
bad idea, even the core isn't handling the errno. With this,
the code can be simplified. However, it's not a big deal and
you probably evaluate and change when you need another respin:

    pr_warn("Boot memory [%lx %lx] %s\n",
            (action == MEM_GOING_OFFLINE) ? "offlining attempted" : "offlined",
            start, end);
    return NOTIFY_BAD;

Cheers,
Gavin