Re: [PATCH v7 2/2] mm/memory_hotplug: optimize zone contiguous check when changing pfn range

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

>  /*
>   * Only struct pages that correspond to ranges defined by memblock.memory
>   * are zeroed and initialized by going through __init_single_page() during
> @@ -822,22 +844,27 @@ void __meminit init_deferred_page(unsigned long pfn, int nid)
>   *   zone/node above the hole except for the trailing pages in the last
>   *   section that will be appended to the zone/node below.
>   */
> -static void __init init_unavailable_range(unsigned long spfn,
> -					  unsigned long epfn,
> -					  int zone, int node)
> +static unsigned long __init init_unavailable_range(unsigned long spfn,
> +						   unsigned long epfn,
> +						   int zone, int node)
>  {
>  	unsigned long pfn;
> -	u64 pgcnt = 0;
> +	u64 pgcnt = 0, online_pgcnt = 0;
> +	unsigned long last_subsection = -1;
> +	bool is_online = false;
>  
>  	for_each_valid_pfn(pfn, spfn, epfn) {
>  		__init_single_page(pfn_to_page(pfn), pfn, zone, node);
>  		__SetPageReserved(pfn_to_page(pfn));
> +		if (unavailable_pfn_is_online(pfn, &last_subsection, &is_online))
> +			online_pgcnt++;

Can we avoid these helpers?

		const unsigned long subsection = pfn & PAGE_SUBSECTION_MASK;

		/* We can have section-sized online holes with VMEMMAP. */
		if (IS_ENMABLED(CONFIG_SPARSEMEM_VMEMMAP) &&
		    subsection != last_subsection) {
			is_online = pfn_to_online_page(pfn);
			last_subsection = subsection;
		}
		if (is_online)
			online_pgcnt++;

An alternative is an inner loop that just walks in SUBSECTION chunks until epfn.
That would probably be even cleaner and faster.

I remember !vmemmap always only has early sections when they are actually
online. We could extent the comment to clarify that.


(I have patches to clean that init code up)


-- 
Cheers,

David