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

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

Hi All,

Rebased with latest master. Add "Reported-by" from customer. Looking 
forward to any comments/suggestions. Thanks.

Regards,

Tianyou


On 11/17/2025 11:30 AM, 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;
> +	}
> +
> +	/*
> +	 * 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;
> +
> +	set_zone_contiguous(zone);
> +}
> +
>   #ifdef CONFIG_ZONE_DEVICE
>   static void section_taint_zone_device(unsigned long pfn)
>   {
> @@ -752,6 +793,10 @@ 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;
> +	bool old_contiguous = zone->contiguous;
> +	unsigned long old_start_pfn = zone->zone_start_pfn;
> +	unsigned long old_nr_pages = zone->spanned_pages;
> +	unsigned long old_absent_pages = zone->spanned_pages - zone->present_pages;
>   
>   	clear_zone_contiguous(zone);
>   
> @@ -783,7 +828,8 @@ void move_pfn_range_to_zone(struct zone *zone, unsigned long start_pfn,
>   			 MEMINIT_HOTPLUG, altmap, migratetype,
>   			 isolate_pageblock);
>   
> -	set_zone_contiguous(zone);
> +	update_zone_contiguous(zone, old_contiguous, old_start_pfn, old_nr_pages,
> +				old_absent_pages, start_pfn, nr_pages);
>   }
>   
>   struct auto_movable_stats {
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 7712d887b696..04fdd949fe49 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -2263,26 +2263,43 @@ void __init init_cma_pageblock(struct page *page)
>   }
>   #endif
>   
> -void set_zone_contiguous(struct zone *zone)
> +/*
> + * Check if all pageblocks in the given PFN range belong to the given zone.
> + * The given range is expected to be within the zone's pfn range, otherwise
> + * false is returned.
> + */
> +bool check_zone_contiguous(struct zone *zone, unsigned long start_pfn,
> +			    unsigned long nr_pages)
>   {
> -	unsigned long block_start_pfn = zone->zone_start_pfn;
> +	unsigned long end_pfn = start_pfn + nr_pages;
> +	unsigned long block_start_pfn = start_pfn;
>   	unsigned long block_end_pfn;
>   
> +	if (start_pfn < zone->zone_start_pfn || end_pfn > zone_end_pfn(zone))
> +		return false;
> +
>   	block_end_pfn = pageblock_end_pfn(block_start_pfn);
> -	for (; block_start_pfn < zone_end_pfn(zone);
> +	for (; block_start_pfn < end_pfn;
>   			block_start_pfn = block_end_pfn,
>   			 block_end_pfn += pageblock_nr_pages) {
>   
> -		block_end_pfn = min(block_end_pfn, zone_end_pfn(zone));
> +		block_end_pfn = min(block_end_pfn, end_pfn);
>   
>   		if (!__pageblock_pfn_to_page(block_start_pfn,
>   					     block_end_pfn, zone))
> -			return;
> +			return false;
>   		cond_resched();
>   	}
>   
> -	/* We confirm that there is no hole */
> -	zone->contiguous = true;
> +	return true;
> +}
> +
> +void set_zone_contiguous(struct zone *zone)
> +{
> +	unsigned long start_pfn = zone->zone_start_pfn;
> +	unsigned long nr_pages = zone->spanned_pages;
> +
> +	zone->contiguous = check_zone_contiguous(zone, start_pfn, nr_pages);
>   }
>   
>   /*