Re: [PATCH v2] mm/memory hotplug/unplug: Optimize zone contiguous check when changing pfn range

[Mike Rapoport <rppt@kernel.org>]

On Wed, Apr 01, 2026 at 03:01:55AM -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/Documentation/mm/physical_memory.rst b/Documentation/mm/physical_memory.rst
> index b76183545e5b..e47e96ef6a6d 100644
> --- a/Documentation/mm/physical_memory.rst
> +++ b/Documentation/mm/physical_memory.rst
> @@ -483,6 +483,17 @@ General
>    ``present_pages`` should use ``get_online_mems()`` to get a stable value. It
>    is initialized by ``calculate_node_totalpages()``.
>  
> +``pages_with_online_memmap``
> +  Tracks pages within the zone that have an online memmap (present pages and

Please spell out "memory map" rather then memmap in the documentation and
in the comments.

> +  memory holes whose memmap has been initialized). When ``spanned_pages`` ==
> +  ``pages_with_online_memmap``, ``pfn_to_page()`` can be performed without
> +  further checks on any PFN within the zone span.
> +
> +  Note: this counter may temporarily undercount when pages with an online
> +  memmap exist outside the current zone span. Growing the zone to cover such
> +  pages and later shrinking it back may result in a "too small" value. This is
> +  safe: it merely prevents detecting a contiguous zone.
> +
>  ``present_early_pages``
>    The present pages existing within the zone located on memory available since
>    early boot, excluding hotplugged memory. Defined only when

...

> +/*
> + * Initialize unavailable range [spfn, epfn) while accounting only the pages
> + * that fall within the zone span towards pages_with_online_memmap. Pages
> + * outside the zone span are still initialized but not accounted.
> + */
> +static void __init init_unavailable_range_for_zone(struct zone *zone,
> +						   unsigned long spfn,
> +						   unsigned long epfn)
> +{
> +	int nid = zone_to_nid(zone);
> +	int zid = zone_idx(zone);
> +	unsigned long in_zone_start;
> +	unsigned long in_zone_end;
> +
> +	in_zone_start = clamp(spfn, zone->zone_start_pfn, zone_end_pfn(zone));
> +	in_zone_end = clamp(epfn, zone->zone_start_pfn, zone_end_pfn(zone));
> +
> +	if (spfn < in_zone_start)
> +		init_unavailable_range(spfn, in_zone_start, zid, nid);
> +
> +	if (in_zone_start < in_zone_end)
> +		zone->pages_with_online_memmap +=
> +			init_unavailable_range(in_zone_start, in_zone_end,
> +					       zid, nid);
> +
> +	if (in_zone_end < epfn)
> +		init_unavailable_range(in_zone_end, epfn, zid, nid);
>  }

I think we can make it simpler, see below.

>  /*
> @@ -956,9 +986,10 @@ static void __init memmap_init_zone_range(struct zone *zone,
>  	memmap_init_range(end_pfn - start_pfn, nid, zone_id, start_pfn,
>  			  zone_end_pfn, MEMINIT_EARLY, NULL, MIGRATE_MOVABLE,
>  			  false);
> +	zone->pages_with_online_memmap += end_pfn - start_pfn;
>  
>  	if (*hole_pfn < start_pfn)
> -		init_unavailable_range(*hole_pfn, start_pfn, zone_id, nid);
> +		init_unavailable_range_for_zone(zone, *hole_pfn, start_pfn);

Here *hole_pfn is either inside zone span or below it and in the second
case it's enough to adjust page count returned by init_unavailable_range()
by (zone_start_pfn - *hole_pfn).

>  	*hole_pfn = end_pfn;
>  }
> @@ -996,8 +1027,11 @@ static void __init memmap_init(void)
>  #else
>  	end_pfn = round_up(end_pfn, MAX_ORDER_NR_PAGES);
>  #endif
> -	if (hole_pfn < end_pfn)
> -		init_unavailable_range(hole_pfn, end_pfn, zone_id, nid);
> +	if (hole_pfn < end_pfn) {
> +		struct zone *zone = &NODE_DATA(nid)->node_zones[zone_id];
> +
> +		init_unavailable_range_for_zone(zone, hole_pfn, end_pfn);

Here we know that the range is not in any zone span.

> +	}
>  }
>  

-- 
Sincerely yours,
Mike.