Re: [PATCH V3 1/3] arm64/mm/hotplug: Register boot memory hot remove notifier earlier

[Gavin Shan]

Hi Anshuman,

On 9/21/20 10:05 PM, Anshuman Khandual wrote:
> This moves memory notifier registration earlier in the boot process from
> device_initcall() to early_initcall() which will help in guarding against
> potential early boot memory offline requests. Even though there should not
> be any actual offlinig requests till memory block devices are initialized
> with memory_dev_init() but then generic init sequence might just change in
> future. Hence an early registration for the memory event notifier would be
> helpful. While here, just skip the registration if CONFIG_MEMORY_HOTREMOVE
> is not enabled and also call out when memory notifier registration fails.
>
> 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 | 14 ++++++++++++--
>   1 file changed, 12 insertions(+), 2 deletions(-)

With the following nit-picky comments resolved:

Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>

> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 75df62fea1b6..df3b7415b128 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1499,7 +1499,17 @@ static struct notifier_block prevent_bootmem_remove_nb = {
>   
>   static int __init prevent_bootmem_remove_init(void)
>   {
> -	return register_memory_notifier(&prevent_bootmem_remove_nb);
> +	int ret = 0;
> +
> +	if (!IS_ENABLED(CONFIG_MEMORY_HOTREMOVE))
> +		return ret;
> +
> +	ret = register_memory_notifier(&prevent_bootmem_remove_nb);
> +	if (!ret)
> +		return ret;
> +
> +	pr_err("Notifier registration failed - boot memory can be removed\n");
> +	return ret;
>   }

It might be cleaner if the duplicated return statements can be
avoided. Besides, it's always nice to print the errno even though
zero is always returned from register_memory_notifier(). So I guess
you probably need something like below:

        ret = register_memory_notifier(&prevent_bootmem_remove_nb);
        if (ret)
            pr_err("%s: Error %d registering notifier\n", __func__, ret)

        return ret;


register_memory_notifier                   # 0 is returned on !CONFIG_MEMORY_HOTPLUG_SPARSE
   blocking_notifier_chain_register
      notifier_chain_register              # 0 is always returned
      
> -device_initcall(prevent_bootmem_remove_init);
> +early_initcall(prevent_bootmem_remove_init);
>   #endif

Cheers,
Gavin