Re: [PATCH v2] mm/memory hotplug/unplug: Optimize zone->contiguous update when move pfn range

["Li, Tianyou" <tianyou.li@intel.com>]

Hi All,

Patch v2 with the changes suggested from David and Mike, which 
simplified the code and also performance get further improved on 
Icelake. Appreciated for your review. Thanks.

Regards,

Tianyou


On 11/19/2025 12:07 PM, Tianyou Li wrote:
> When invoke move_pfn_range_to_zone, it will update the zone->contiguous by
> checking the new zone's pfn range from the beginning to the end, regardless
> the previous state of the old zone. When the zone's pfn range is large, the
> cost of traversing the pfn range to update the zone->contiguous could be
> significant.
>
> Add fast paths to quickly detect cases where zone is definitely not
> contiguous without scanning the new zone. The cases are: when the new range
> did not overlap with previous range, the contiguous should be false; if the
> new range adjacent with the previous range, just need to check the new
> range; if the new added pages could not fill the hole of previous zone, the
> contiguous should be false.
>
> The following test cases of memory hotplug for a VM [1], tested in the
> environment [2], show that this optimization can significantly reduce the
> memory hotplug time [3].
>
> +----------------+------+---------------+--------------+----------------+
> |                | Size | Time (before) | Time (after) | Time Reduction |
> |                +------+---------------+--------------+----------------+
> | Memory Hotplug | 256G |      10s      |      2s      |       80%      |
> |                +------+---------------+--------------+----------------+
> |                | 512G |      33s      |      6s      |       81%      |
> +----------------+------+---------------+--------------+----------------+
>
> [1] Qemu commands to hotplug 512G memory for a VM:
>      object_add memory-backend-ram,id=hotmem0,size=512G,share=on
>      device_add virtio-mem-pci,id=vmem1,memdev=hotmem0,bus=port1
>      qom-set vmem1 requested-size 512G
>
> [2] Hardware     : Intel Icelake server
>      Guest Kernel : v6.18-rc2
>      Qemu         : v9.0.0
>
>      Launch VM    :
>      qemu-system-x86_64 -accel kvm -cpu host \
>      -drive file=./Centos10_cloud.qcow2,format=qcow2,if=virtio \
>      -drive file=./seed.img,format=raw,if=virtio \
>      -smp 3,cores=3,threads=1,sockets=1,maxcpus=3 \
>      -m 2G,slots=10,maxmem=2052472M \
>      -device pcie-root-port,id=port1,bus=pcie.0,slot=1,multifunction=on \
>      -device pcie-root-port,id=port2,bus=pcie.0,slot=2 \
>      -nographic -machine q35 \
>      -nic user,hostfwd=tcp::3000-:22
>
>      Guest kernel auto-onlines newly added memory blocks:
>      echo online > /sys/devices/system/memory/auto_online_blocks
>
> [3] The time from typing the QEMU commands in [1] to when the output of
>      'grep MemTotal /proc/meminfo' on Guest reflects that all hotplugged
>      memory is recognized.
>
> Reported-by: Nanhai Zou <nanhai.zou@intel.com>
> Reported-by: Chen Zhang <zhangchen.kidd@jd.com>
> Tested-by: Yuan Liu <yuan1.liu@intel.com>
> Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
> Reviewed-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
> Reviewed-by: Yu C Chen <yu.c.chen@intel.com>
> Reviewed-by: Pan Deng <pan.deng@intel.com>
> Reviewed-by: Nanhai Zou <nanhai.zou@intel.com>
> Reviewed-by: Yuan Liu <yuan1.liu@intel.com>
> Signed-off-by: Tianyou Li <tianyou.li@intel.com>
> ---
>   mm/memory_hotplug.c | 57 ++++++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 54 insertions(+), 3 deletions(-)
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 0be83039c3b5..8f126f20ca47 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -723,6 +723,57 @@ static void __meminit resize_pgdat_range(struct pglist_data *pgdat, unsigned lon
>   
>   }
>   
> +static bool __meminit check_zone_contiguous_fast(struct zone *zone,
> +			unsigned long start_pfn, unsigned long nr_pages)
> +{
> +	const unsigned long end_pfn = start_pfn + nr_pages;
> +	unsigned long nr_filled_pages;
> +
> +	/*
> +	 * Given the moved pfn range's contiguous property is always true,
> +	 * under the conditional of empty zone, the contiguous property should
> +	 * be true.
> +	 */
> +	if (zone_is_empty(zone)) {
> +		zone->contiguous = true;
> +		return true;
> +	}
> +
> +	/*
> +	 * If the moved pfn range does not intersect with the original zone span,
> +	 * the contiguous property is surely false.
> +	 */
> +	if (end_pfn < zone->zone_start_pfn || start_pfn > zone_end_pfn(zone)) {
> +		zone->contiguous = false;
> +		return true;
> +	}
> +
> +	/*
> +	 * If the moved pfn range is adjacent to the original zone span, given
> +	 * the moved pfn range's contiguous property is always true, the zone's
> +	 * contiguous property inherited from the original value.
> +	 */
> +	if (end_pfn == zone->zone_start_pfn || start_pfn == zone_end_pfn(zone))
> +		return true;
> +
> +	/*
> +	 * If the original zone's hole larger than the new filled pages, the
> +	 * contiguous property is surely false.
> +	 */
> +	nr_filled_pages = end_pfn - zone->zone_start_pfn;
> +	if (start_pfn > zone->zone_start_pfn)
> +		nr_filled_pages -= start_pfn - zone->zone_start_pfn;
> +	if (end_pfn > zone_end_pfn(zone))
> +		nr_filled_pages -= end_pfn - zone_end_pfn(zone);
> +	if (nr_filled_pages < (zone->spanned_pages - zone->present_pages)) {
> +		zone->contiguous = false;
> +		return true;
> +	}
> +
> +	clear_zone_contiguous(zone);
> +	return false;
> +}
> +
>   #ifdef CONFIG_ZONE_DEVICE
>   static void section_taint_zone_device(unsigned long pfn)
>   {
> @@ -752,8 +803,7 @@ void move_pfn_range_to_zone(struct zone *zone, unsigned long start_pfn,
>   {
>   	struct pglist_data *pgdat = zone->zone_pgdat;
>   	int nid = pgdat->node_id;
> -
> -	clear_zone_contiguous(zone);
> +	const bool fast_path = check_zone_contiguous_fast(zone, start_pfn, nr_pages);
>   
>   	if (zone_is_empty(zone))
>   		init_currently_empty_zone(zone, start_pfn, nr_pages);
> @@ -783,7 +833,8 @@ void move_pfn_range_to_zone(struct zone *zone, unsigned long start_pfn,
>   			 MEMINIT_HOTPLUG, altmap, migratetype,
>   			 isolate_pageblock);
>   
> -	set_zone_contiguous(zone);
> +	if (!fast_path)
> +		set_zone_contiguous(zone);
>   }
>   
>   struct auto_movable_stats {