Re: [PATCH v9 2/2] mm/memory_hotplug: optimize zone contiguous check when changing pfn range

[Mike Rapoport <rppt@kernel.org>]

Hi,

On Mon, Sep 14, 2026 at 03:29:29AM -0400, Yuan Liu wrote:
> When move_pfn_range_to_zone() or remove_pfn_range_from_zone() updates a
> zone, set_zone_contiguous() rescans the entire zone pageblock-by-pageblock
> to rebuild zone->contiguous. For large zones this is a significant cost
> during memory hotplug and hot-unplug.
>
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 1533aebafb68..d40a8ff23370 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -817,22 +817,39 @@ 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 next_chunk_pfn __maybe_unused = spfn;
>  	unsigned long pfn;
> -	u64 pgcnt = 0;
> +	u64 online_pgcnt = 0, pgcnt = 0;
> +	bool is_online = true;
>  
>  	for_each_valid_pfn(pfn, spfn, epfn) {
>  		__init_single_page(pfn_to_page(pfn), pfn, zone, node);
>  		__SetPageReserved(pfn_to_page(pfn));
>  		pgcnt++;
> +
> +		/*
> +		 * With vmemmap, at this stage all pages in an early section
> +		 * have a valid memmap and are marked as online. However, only
> +		 * subsections in the subsection map are actually online.
> +		 */

I'm having trouble parsing this comment. Shouldn't is say that some
subsections in a section can be offline because of holes?

Other than that

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> +#ifdef CONFIG_SPARSEMEM_VMEMMAP
> +		if (pfn >= next_chunk_pfn) {
> +			is_online = pfn_section_valid(__pfn_to_section(pfn), pfn);
> +			next_chunk_pfn = min(SUBSECTION_ALIGN_UP(pfn + 1), epfn);
> +		}
> +#endif
> +		if (is_online)
> +			online_pgcnt++;
>  	}
>  
>  	if (pgcnt)
>  		pr_info("On node %d, zone %s: %lld pages in unavailable ranges\n",
>  			node, zone_names[zone], pgcnt);
> +	return online_pgcnt;
>  }
>  
>  /*

-- 
Sincerely yours,
Mike.