Re: [PATCH] mm/memory hotplug/unplug: Optimize zone contiguous check when changing pfn range

[Sion Ji]

On Thu, Mar 19, 2026 at 05:56:22AM -0400, Yuan Liu 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 a new zone's pages_with_memmap member, it is pages within the zone that
> have an online memmap. It includes present pages and memory holes that have
> a memmap. When spanned_pages == pages_with_online_memmap, pfn_to_page() can
> be performed without further checks on any pfn within the zone span.
> 
> 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      |      3s      |       70%      |
> |                +------+---------------+--------------+----------------+
> |                | 512G |      36s      |      7s      |       81%      |
> +----------------+------+---------------+--------------+----------------+
> 
> +----------------+------+---------------+--------------+----------------+
> |                | Size | Time (before) | Time (after) | Time Reduction |
> |                +------+---------------+--------------+----------------+
> | Unplug Memory  | 256G |      11s      |      4s      |       64%      |
> |                +------+---------------+--------------+----------------+
> |                | 512G |      36s      |      9s      |       75%      |
> +----------------+------+---------------+--------------+----------------+
> 
> [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 : v7.0-rc4
>     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.

Since the above results were tested in a QEMU environment, we ran the 
tests in a real-world environment [1]. The kernel version is v7.0-rc4,
which is the same as the QEMU guest kernel. We configured remote memory
node using 2 and 4 CXL devices for the 256G and 512G tests, respectively. 

We replaced the QEMU hotplug commands with the daxctl [2]. We tested by
changing the value of /sys/devices/system/memory/auto_online_blocks. 

The following is the result when the kernel onlines added memory blocks:
(echo online > /sys/devices/system/memory/auto_online_blocks)

+----------------+------+---------------+--------------+----------------+
|                | Size | Time (before) | Time (after) | Time Reduction |
|                +------+---------------+--------------+----------------+
| Plug Memory    | 256G |      6.7s     |     4.4s     |      -35%      |
|                +------+---------------+--------------+----------------+
|                | 512G |      18s      |     8.7s     |      -52%      |
+----------------+------+---------------+--------------+----------------+

+----------------+------+---------------+--------------+----------------+
|                | Size | Time (before) | Time (after) | Time Reduction |
|                +------+---------------+--------------+----------------+
| Unplug Memory  | 256G |      2.8s     |     2.6s     |      -8%       |
|                +------+---------------+--------------+----------------+
|                | 512G |      5.2s     |      5s      |      -4%       |
+----------------+------+---------------+--------------+----------------+

[1] Platform        : Intel GNR-AP
    CPU             : Intel(R) Xeon(R) 6960P
    Memory          : Samsung DDR5-4800
    Hotplug devices : Samsung CXL 2.0 128G Devices
    Kernel          : v7.0-rc4
    

[2] daxctl commands to hotplug memory for a test:
    time daxctl reconfigure-device --force --mode=system-ram dax0.0 
                                                           (Plug Memory)
    time daxctl reconfigure-device --force --mode=devdax dax0.0 
                                                         (Unplug Memory)


The following is the result when the value is set to online_movable:
(echo online_movable > /sys/devices/system/memory/auto_online_blocks)

+----------------+------+---------------+--------------+----------------+
|                | Size | Time (before) | Time (after) | Time Reduction |
|                +------+---------------+--------------+----------------+
| Plug Memory    | 256G |      4.5s     |     4.4s     |       -3%      |
|                +------+---------------+--------------+----------------+
|                | 512G |     18.6s     |     6.6s     |      -65%      |
+----------------+------+---------------+--------------+----------------+

+----------------+------+---------------+--------------+----------------+
|                | Size | Time (before) | Time (after) | Time Reduction |
|                +------+---------------+--------------+----------------+
| Unplug Memory  | 256G |      2.2s     |     2.6s     |      +18%      |
|                +------+---------------+--------------+----------------+
|                | 512G |      5.2s     |     4.2s     |      -20%      |
+----------------+------+---------------+--------------+----------------+


FYI, it is the result when kernel does not automatically online memory.
(echo offline > /sys/devices/system/memory/auto_online_blocks)

+----------------+------+---------------+--------------+----------------+
|                | Size | Time (before) | Time (after) | Time Reduction |
|                +------+---------------+--------------+----------------+
| Plug Memory    | 256G |      3.3s     |     4.4s     |      +33%      |
|                +------+---------------+--------------+----------------+
|                | 512G |      6.7s     |     6.6s     |       -2%      |
+----------------+------+---------------+--------------+----------------+

+----------------+------+---------------+--------------+----------------+
|                | Size | Time (before) | Time (after) | Time Reduction |
|                +------+---------------+--------------+----------------+
| Unplug Memory  | 256G |      2.2s     |     2.6s     |      +18%      |
|                +------+---------------+--------------+----------------+
|                | 512G |      4.4s     |     4.2s     |       -5%      |
+----------------+------+---------------+--------------+----------------+


We hope this result is helpful.

Regards,
Sion Ji