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

Li, Tianyou
2025-11-19T12:41:25Z

On 11/19/2025 7:42 PM, Wei Yang wrote:
> On Wed, Nov 19, 2025 at 12:07:18PM +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 | 2s | 80% |
>> | +------+---------------+--------------+----------------+
>> | | 512G | 33s | 6s | 81% |
>> +----------------+------+---------------+--------------+----------------+
>>
> Nice

Thanks for your time to review.

>> [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;
>> + }
>> +
> Mike's suggestion is easier for me to understand :-)

OK :-), with the clear votes now, I will change it in patch v3 real
quick. Thanks.

>> + 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 {
>> --
>> 2.47.1
>>