Re: [PATCH 4/9] mm, memory_hotplug: get rid of is_zone_device_section

[Jerome Glisse <jglisse@redhat.com>]

On Mon, Apr 10, 2017 at 01:03:46PM +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> device memory hotplug hooks into regular memory hotplug only half way.
> It needs memory sections to track struct pages but there is no
> need/desire to associate those sections with memory blocks and export
> them to the userspace via sysfs because they cannot be onlined anyway.
> 
> This is currently expressed by for_device argument to arch_add_memory
> which then makes sure to associate the given memory range with
> ZONE_DEVICE. register_new_memory then relies on is_zone_device_section
> to distinguish special memory hotplug from the regular one. While this
> works now, later patches in this series want to move __add_zone outside
> of arch_add_memory path so we have to come up with something else.
> 
> Add want_memblock down the __add_pages path and use it to control
> whether the section->memblock association should be done. arch_add_memory
> then just trivially want memblock for everything but for_device hotplug.
> 
> remove_memory_section doesn't need is_zone_device_section either. We can
> simply skip all the memblock specific cleanup if there is no memblock
> for the given section.
> 
> This shouldn't introduce any functional change.
> 
> Cc: Dan Williams <dan.j.williams@gmail.com>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---

[...]

> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 342332f29364..1570b3eea493 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -493,7 +493,7 @@ static int __meminit __add_zone(struct zone *zone, unsigned long phys_start_pfn)
>  }
>  
>  static int __meminit __add_section(int nid, struct zone *zone,
> -					unsigned long phys_start_pfn)
> +					unsigned long phys_start_pfn, bool want_memblock)
>  {
>  	int ret;
>  
> @@ -510,7 +510,10 @@ static int __meminit __add_section(int nid, struct zone *zone,
>  	if (ret < 0)
>  		return ret;
>  
> -	return register_new_memory(nid, __pfn_to_section(phys_start_pfn));
> +	if (want_memblock)
> +		ret = register_new_memory(nid, __pfn_to_section(phys_start_pfn));
> +
> +	return ret;
>  }

The above is wrong for ZONE_DEVICE sparse_add_one_section() will return a
positive value (on success) thus ret > 0 and other function in the hotplug
path will interpret positive value as an error.

I suggest something like:
	if (!want_memblock)
		return 0;

	return register_new_memory(nid, __pfn_to_section(phys_start_pfn));
}

instead (also avoid a > 80 columns warning message).

Cheers,
Jérôme