Re: [PATCH v8 3/3] mm/memory hotplug/unplug: Optimize zone->contiguous update when changes pfn range

[Li, Tianyou]

On 1/22/2026 7:43 PM, Mike Rapoport wrote:
> Hi,
>
> On Tue, Jan 20, 2026 at 10:33:46PM +0800, Tianyou Li wrote:
> > When invoke move_pfn_range_to_zone or remove_pfn_range_from_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 |
> > |                +------+---------------+--------------+----------------+
> > | Plug Memory    | 256G |      10s      |      2s      |       80%      |
> > |                +------+---------------+--------------+----------------+
> > |                | 512G |      33s      |      6s      |       81%      |
> > +----------------+------+---------------+--------------+----------------+
> >
> > +----------------+------+---------------+--------------+----------------+
> > |                | Size | Time (before) | Time (after) | Time Reduction |
> > |                +------+---------------+--------------+----------------+
> > | Unplug Memory  | 256G |      10s      |      2s      |       80%      |
> > |                +------+---------------+--------------+----------------+
> > |                | 512G |      34s      |      6s      |       82%      |
> > +----------------+------+---------------+--------------+----------------+
> >
> > [1] Qemu commands to hotplug 256G/512G memory for a VM:
> >      object_add memory-backend-ram,id=hotmem0,size=256G/512G,share=on
> >      device_add virtio-mem-pci,id=vmem1,memdev=hotmem0,bus=port1
> >      qom-set vmem1 requested-size 256G/512G (Plug Memory)
> >      qom-set vmem1 requested-size 0G (Unplug Memory)
> >
> > [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@xxxxxxxxx>
> > Reported-by: Chen Zhang <zhangchen.kidd@xxxxxx>
> > Tested-by: Yuan Liu <yuan1.liu@xxxxxxxxx>
> > Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > Reviewed-by: Qiuxu Zhuo <qiuxu.zhuo@xxxxxxxxx>
> > Reviewed-by: Yu C Chen <yu.c.chen@xxxxxxxxx>
> > Reviewed-by: Pan Deng <pan.deng@xxxxxxxxx>
> > Reviewed-by: Nanhai Zou <nanhai.zou@xxxxxxxxx>
> > Reviewed-by: Yuan Liu <yuan1.liu@xxxxxxxxx>
> > Signed-off-by: Tianyou Li <tianyou.li@xxxxxxxxx>
> > ---
> ...
>
> > +int online_memory_block_pages(unsigned long start_pfn, unsigned long nr_pages,
> > +			unsigned long nr_vmemmap_pages, struct zone *zone,
> > +			struct memory_group *group)
> >   {
> > +	const bool contiguous = zone->contiguous;
> > +	enum zone_contig_state new_contiguous_state;
> >   	int ret;
> >   
> > +	/*
> > +	 * Calculate the new zone contig state before move_pfn_range_to_zone()
> > +	 * sets the zone temporarily to non-contiguous.
> > +	 */
> > +	new_contiguous_state = zone_contig_state_after_growing(zone, start_pfn,
> > +							       nr_pages);
> > +
> >   	if (nr_vmemmap_pages) {
> >   		ret = mhp_init_memmap_on_memory(start_pfn, nr_vmemmap_pages, zone);
> >   		if (ret)
> > -			return ret;
> > +			goto restore_zone_contig;
> But zone_contig_state_after_growing() does not change zone->contiguous. Why
> do we need to save and restore it?

Move_pfn_range_to_zone() will clear the zone contiguous state and it was 
invoked by online_pages(). If error occurs after 
move_pfn_range_to_zone() called like in online_pages(), I think we'd 
better to restore the original value if previous zone contiguous state 
is true.


> >   	}
> >   
> >   	ret = online_pages(start_pfn + nr_vmemmap_pages,
> > @@ -1271,7 +1320,7 @@ int online_memory_block_pages(unsigned long start_pfn,
> >   	if (ret) {
> >   		if (nr_vmemmap_pages)
> >   			mhp_deinit_memmap_on_memory(start_pfn, nr_vmemmap_pages);
> > -		return ret;
> > +		goto restore_zone_contig;
> >   	}
> >   
> >   	/*
> > @@ -1282,6 +1331,15 @@ int online_memory_block_pages(unsigned long start_pfn,
> >   		adjust_present_page_count(pfn_to_page(start_pfn), group,
> >   					  nr_vmemmap_pages);
> >   
> > +	/*
> > +	 * Now that the ranges are indicated as online, check whether the whole
> > +	 * zone is contiguous.
> > +	 */
> > +	set_zone_contiguous(zone, new_contiguous_state);
> > +	return 0;
> > +
> > +restore_zone_contig:
> > +	zone->contiguous = contiguous;
> >   	return ret;
> >   }