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

Li, Tianyou
2025-11-18T10:31:50Z

On 11/18/2025 5:35 PM, Li, Tianyou wrote:
> 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?
>

Re-think your idea, it's doable consider the check_zone_contiguous is
not necessary. We can have a function check_zone_contiguous_fast, which
need to pass the zone, start_pfn and nr_pages, return a boolean value to
indicate the fast path or not. The code changes minimized. Will send the
patch v2 soon.

>
>>> +    }
>>> +
>>> +    /*
>>> +     * 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);
>>> +}
>>> +