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Message-ID: <0f679956-fd29-4165-a136-18f68e517fb5@intel.com>
Date: Sat, 24 Jan 2026 20:43:51 +0800
From: "Li, Tianyou" <tianyou.li@...el.com>
To: Mike Rapoport <rppt@...nel.org>
CC: David Hildenbrand <david@...nel.org>, Oscar Salvador <osalvador@...e.de>,
Wei Yang <richard.weiyang@...il.com>, Michal Hocko <mhocko@...e.com>,
<linux-mm@...ck.org>, Yong Hu <yong.hu@...el.com>, Nanhai Zou
<nanhai.zou@...el.com>, Yuan Liu <yuan1.liu@...el.com>, Tim Chen
<tim.c.chen@...ux.intel.com>, Qiuxu Zhuo <qiuxu.zhuo@...el.com>, Yu C Chen
<yu.c.chen@...el.com>, Pan Deng <pan.deng@...el.com>, Chen Zhang
<zhangchen.kidd@...com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 3/3] mm/memory hotplug/unplug: Optimize
zone->contiguous update when changes pfn range
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@...el.com>
>> Reported-by: Chen Zhang <zhangchen.kidd@...com>
>> Tested-by: Yuan Liu <yuan1.liu@...el.com>
>> Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
>> Reviewed-by: Qiuxu Zhuo <qiuxu.zhuo@...el.com>
>> Reviewed-by: Yu C Chen <yu.c.chen@...el.com>
>> Reviewed-by: Pan Deng <pan.deng@...el.com>
>> Reviewed-by: Nanhai Zou <nanhai.zou@...el.com>
>> Reviewed-by: Yuan Liu <yuan1.liu@...el.com>
>> Signed-off-by: Tianyou Li <tianyou.li@...el.com>
>> ---
> ...
>
>> +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;
>> }
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