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

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

On 11/19/2025 8:44 PM, Wei Yang wrote:
> On Wed, Nov 19, 2025 at 08:41:11PM +0800, Li, Tianyou wrote:
>> 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.
>>
> Thanks for your effort.
>
> While maybe wait a little for v3, let's see other's comment on v2 :-)


Thanks. While I am less worried if it might change back -:) In the 
scenarios we tested, the overlap case did not happen. I agree Mike's 
suggestion do simplify the code and reduce the maintaining efforts.

Regards,

Tianyou


>