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

Wei Yang
2025-11-19T12:44:20Z

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 :-)

--
Wei Yang
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