Re: [PATCH v6 1/2] mm/memory_hotplug: optimize zone contiguous check when changing pfn range

["David Hildenbrand (Arm)" <david@kernel.org>]

>>> +static void __init update_zone_online_memmap_pages(struct zone *zone,
>>> +						   unsigned long start_pfn,
>>> +						   unsigned long end_pfn,
>>> +						   unsigned long *hole_pfn)
>>> +{
>>> +#ifdef CONFIG_SPARSEMEM_VMEMMAP
>>> +	unsigned long zone_start_pfn = zone->zone_start_pfn;
>>> +	unsigned long zone_end_pfn = zone_start_pfn + zone->spanned_pages;
>>
>> These two can be const.
> 
> Will do.
> 
>>> +	unsigned long sub_start, sub_end;
>>> +
>>> +	sub_start = max(ALIGN_DOWN(start_pfn, PAGES_PER_SUBSECTION),
>>> +			zone_start_pfn);
>>> +	sub_end = min(ALIGN(end_pfn, PAGES_PER_SUBSECTION), zone_end_pfn);
>>
>> Hm, I don't immediately understand why we do the PAGES_PER_SUBSECTION
>> thing
>> here. Why is that required?
> 
> pages_with_online_memmap counts all PFNs where pfn_to_online_page() is
> valid. With CONFIG_SPARSEMEM_VMEMMAP, pfn_section_valid() operates at
> PAGES_PER_SUBSECTION granularity — when any page in a subsection has
> memory, the entire subsection is valid/online. So we align to subsection
> boundaries to include hole pages within partially-populated subsections.

But we must never account exceeding the zone range. So I don't understand why we
would have to care about PAGES_PER_SUBSECTION here at all?

-- 
Cheers,

David