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

[Mike Rapoport <rppt@kernel.org>]

On Mon, Nov 17, 2025 at 11:30:52AM +0800, 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      |      3s      |       70%      |
> |                +------+---------------+--------------+----------------+
> |                | 512G |      33s      |      8s      |       76%      |
> +----------------+------+---------------+--------------+----------------+
> 
> [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>
> Signed-off-by: Tianyou Li <tianyou.li@intel.com>
> ---
>  mm/internal.h       |  3 +++
>  mm/memory_hotplug.c | 48 ++++++++++++++++++++++++++++++++++++++++++++-
>  mm/mm_init.c        | 31 ++++++++++++++++++++++-------
>  3 files changed, 74 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 1561fc2ff5b8..734caae6873c 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -734,6 +734,9 @@ void set_zone_contiguous(struct zone *zone);
>  bool pfn_range_intersects_zones(int nid, unsigned long start_pfn,
>  			   unsigned long nr_pages);
>  
> +bool check_zone_contiguous(struct zone *zone, unsigned long start_pfn,
> +			   unsigned long nr_pages);
> +
>  static inline void clear_zone_contiguous(struct zone *zone)
>  {
>  	zone->contiguous = false;
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 0be83039c3b5..96c003271b8e 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -723,6 +723,47 @@ static void __meminit resize_pgdat_range(struct pglist_data *pgdat, unsigned lon
>  
>  }
>  
> +static void __meminit update_zone_contiguous(struct zone *zone,
> +			bool old_contiguous, unsigned long old_start_pfn,
> +			unsigned long old_nr_pages, unsigned long old_absent_pages,
> +			unsigned long new_start_pfn, unsigned long new_nr_pages)
> +{
> +	unsigned long old_end_pfn = old_start_pfn + old_nr_pages;
> +	unsigned long new_end_pfn = new_start_pfn + new_nr_pages;
> +	unsigned long new_filled_pages = 0;
> +
> +	/*
> +	 * If the moved pfn range does not intersect with the old zone span,
> +	 * the contiguous property is surely false.
> +	 */
> +	if (new_end_pfn < old_start_pfn || new_start_pfn > old_end_pfn)
> +		return;
> +
> +	/*
> +	 * If the moved pfn range is adjacent to the old zone span,
> +	 * check the range to the left or to the right
> +	 */
> +	if (new_end_pfn == old_start_pfn || new_start_pfn == old_end_pfn) {
> +		zone->contiguous = old_contiguous &&
> +			check_zone_contiguous(zone, new_start_pfn, new_nr_pages);
> +		return;

The check for adjacency of the new range to the zone can be moved to the
beginning of move_pfn_range_to_zone() and it will already optimize the
common case when we hotplug memory to a contiguous zone.

> +	}
> +
> +	/*
> +	 * If old zone's hole larger than the new filled pages, the contiguous
> +	 * property is surely false.
> +	 */
> +	new_filled_pages = new_end_pfn - old_start_pfn;
> +	if (new_start_pfn > old_start_pfn)
> +		new_filled_pages -= new_start_pfn - old_start_pfn;
> +	if (new_end_pfn > old_end_pfn)
> +		new_filled_pages -= new_end_pfn - old_end_pfn;
> +	if (new_filled_pages < old_absent_pages)
> +		return;

Let's just check that we don't add enough pages to cover the hole

	if (nr_new_pages < old_absent_pages)
		return;

and if we do go to the slow path and walk the pageblocks.

> +
> +	set_zone_contiguous(zone);
> +}
> +

-- 
Sincerely yours,
Mike.