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

Li, Tianyou
2025-11-18T09:36:06Z

Thanks for your comments Mike. Appreciated.

On 11/18/2025 1:13 PM, Mike Rapoport wrote:
> On Mon, Nov 17, 2025 at 11:30:52AM +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 | 3s | 70% |
>> | +------+---------------+--------------+----------------+
>> | | 512G | 33s | 8s | 76% |
>> +----------------+------+---------------+--------------+----------------+
>>
>> [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>
>> Signed-off-by: Tianyou Li <tianyou.li@intel.com>
>> ---
>> mm/internal.h | 3 +++
>> mm/memory_hotplug.c | 48 ++++++++++++++++++++++++++++++++++++++++++++-
>> mm/mm_init.c | 31 ++++++++++++++++++++++-------
>> 3 files changed, 74 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 1561fc2ff5b8..734caae6873c 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -734,6 +734,9 @@ void set_zone_contiguous(struct zone *zone);
>> bool pfn_range_intersects_zones(int nid, unsigned long start_pfn,
>> unsigned long nr_pages);
>>
>> +bool check_zone_contiguous(struct zone *zone, unsigned long start_pfn,
>> + unsigned long nr_pages);
>> +
>> static inline void clear_zone_contiguous(struct zone *zone)
>> {
>> zone->contiguous = false;
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 0be83039c3b5..96c003271b8e 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -723,6 +723,47 @@ static void __meminit resize_pgdat_range(struct pglist_data *pgdat, unsigned lon
>>
>> }
>>
>> +static void __meminit update_zone_contiguous(struct zone *zone,
>> + bool old_contiguous, unsigned long old_start_pfn,
>> + unsigned long old_nr_pages, unsigned long old_absent_pages,
>> + unsigned long new_start_pfn, unsigned long new_nr_pages)
>> +{
>> + unsigned long old_end_pfn = old_start_pfn + old_nr_pages;
>> + unsigned long new_end_pfn = new_start_pfn + new_nr_pages;
>> + unsigned long new_filled_pages = 0;
>> +
>> + /*
>> + * If the moved pfn range does not intersect with the old zone span,
>> + * the contiguous property is surely false.
>> + */
>> + if (new_end_pfn < old_start_pfn || new_start_pfn > old_end_pfn)
>> + return;
>> +
>> + /*
>> + * If the moved pfn range is adjacent to the old zone span,
>> + * check the range to the left or to the right
>> + */
>> + if (new_end_pfn == old_start_pfn || new_start_pfn == old_end_pfn) {
>> + zone->contiguous = old_contiguous &&
>> + check_zone_contiguous(zone, new_start_pfn, new_nr_pages);
>> + return;
> The check for adjacency of the new range to the zone can be moved to the
> beginning of move_pfn_range_to_zone() and it will already optimize the
> common case when we hotplug memory to a contiguous zone.

Do you mean we can separate the update_zone_contiguous logic into two
parts, one for fast path at the beginning of the move_pfn_range_to_zone,
and the other for slow path after the memmep_init_range?

>> + }
>> +
>> + /*
>> + * If old zone's hole larger than the new filled pages, the contiguous
>> + * property is surely false.
>> + */
>> + new_filled_pages = new_end_pfn - old_start_pfn;
>> + if (new_start_pfn > old_start_pfn)
>> + new_filled_pages -= new_start_pfn - old_start_pfn;
>> + if (new_end_pfn > old_end_pfn)
>> + new_filled_pages -= new_end_pfn - old_end_pfn;
>> + if (new_filled_pages < old_absent_pages)
>> + return;
> Let's just check that we don't add enough pages to cover the hole
>
> if (nr_new_pages < old_absent_pages)
> return;
>
> and if we do go to the slow path and walk the pageblocks.

I'd like to avoid of the slow path as much as possible. The check 'if
(nr_new_pages < old_absent_pages)' is more strict if overlap happens. I
am OK to simplify it if there is no overlap cases or to reduce the
maintaining efforts.

Thanks & Regards,

Tianyou

>> +
>> + set_zone_contiguous(zone);
>> +}
>> +