Re: [PATCH v3] mm/memory hotplug/unplug: Optimize zone contiguous check when changing pfn range

David Hildenbrand (Arm)
2026-04-08T07:36:23Z

On 4/8/26 05:16, Yuan Liu wrote:
> When move_pfn_range_to_zone() or remove_pfn_range_from_zone() updates a
> zone, set_zone_contiguous() rescans the entire zone pageblock-by-pageblock
> to rebuild zone->contiguous. For large zones this is a significant cost
> during memory hotplug and hot-unplug.
>
> Add a new zone member pages_with_online_memmap that tracks the number of
> pages within the zone span that have an online memory map (including present
> pages and memory holes whose memory map has been initialized). When
> spanned_pages == pages_with_online_memmap the zone is contiguous and
> pfn_to_page() can be called on any PFN in the zone span without further
> pfn_valid() checks.
>
> Only pages that fall within the current zone span are accounted towards
> pages_with_online_memmap. A "too small" value is safe, it merely prevents
> detecting a contiguous zone.
>
> 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.
>
> 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>
> Co-developed-by: Tianyou Li <tianyou.li@intel.com>
> Signed-off-by: Tianyou Li <tianyou.li@intel.com>
> Signed-off-by: Yuan Liu <yuan1.liu@intel.com>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> ---

[...]

> @@ -842,7 +842,7 @@ overlap_memmap_init(unsigned long zone, unsigned long *pfn)
> * zone/node above the hole except for the trailing pages in the last
> * section that will be appended to the zone/node below.
> */
> -static void __init init_unavailable_range(unsigned long spfn,
> +static unsigned long __init init_unavailable_range(unsigned long spfn,
> unsigned long epfn,
> int zone, int node)
> {
> @@ -858,6 +858,7 @@ static void __init init_unavailable_range(unsigned long spfn,
> if (pgcnt)
> pr_info("On node %d, zone %s: %lld pages in unavailable ranges\n",
> node, zone_names[zone], pgcnt);
> + return pgcnt;
> }
>
> /*
> @@ -956,9 +957,22 @@ static void __init memmap_init_zone_range(struct zone *zone,
> memmap_init_range(end_pfn - start_pfn, nid, zone_id, start_pfn,
> zone_end_pfn, MEMINIT_EARLY, NULL, MIGRATE_MOVABLE,
> false);
> + zone->pages_with_online_memmap += end_pfn - start_pfn;
>
> - if (*hole_pfn < start_pfn)
> - init_unavailable_range(*hole_pfn, start_pfn, zone_id, nid);
> + if (*hole_pfn < start_pfn) {
> + unsigned long pgcnt;
> +
> + if (*hole_pfn < zone_start_pfn) {
> + init_unavailable_range(*hole_pfn, zone_start_pfn,
> + zone_id, nid);
> + pgcnt = init_unavailable_range(zone_start_pfn,
> + start_pfn, zone_id, nid);

Indentation of parameters.

> + } else {
> + pgcnt = init_unavailable_range(*hole_pfn, start_pfn,
> + zone_id, nid);

Same here.

> + }
> + zone->pages_with_online_memmap += pgcnt;
> + }

Maybe something like the following could make it nicer to read, just a
thought.

unsigned long hole_start_pfn = *hole_pfn;

if (hole_start_pfn < zone_start_pfn) {
init_unavailable_range(hole_start_pfn, zone_start_pfn,
zone_id, nid);
hole_start_pfn = zone_start_pfn;
}
pgcnt = init_unavailable_range(hole_start_pfn, start_pfn,
zone_id, nid);

LGTM, thanks!

--
Cheers,

David