Re: [PATCH v3] mm/memory hotplug/unplug: Optimize zone->contiguous update when move pfn range

Li, Tianyou
2025-11-28T15:17:27Z

Thanks for your review David.

On 11/28/2025 8:01 PM, David Hildenbrand (Red Hat) wrote:
> On 11/19/25 15:06, 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%      |
>> +----------------+------+---------------+--------------+----------------+
>>
>>
>> [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 | 51 ++++++++++++++++++++++++++++++++++++++++++---
>>   1 file changed, 48 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 0be83039c3b5..aed1827a2778 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -723,6 +723,51 @@ 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;
>> +
>> +    /*
>> +     * 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 moved pages in
>> the range,
>> +     * the contiguous property is surely false.
>> +     */
>> +    if (nr_pages < (zone->spanned_pages - zone->present_pages)) {
>> +        zone->contiguous = false;
>> +        return true;
>> +    }
>> +
>> +    clear_zone_contiguous(zone);
>> +    return false;
>> +}
>> +
>>   #ifdef CONFIG_ZONE_DEVICE
>>   static void section_taint_zone_device(unsigned long pfn)
>>   {
>> @@ -752,8 +797,7 @@ void move_pfn_range_to_zone(struct zone *zone,
>> unsigned long start_pfn,
>>   {
>>       struct pglist_data *pgdat = zone->zone_pgdat;
>>       int nid = pgdat->node_id;
>> -
>> -    clear_zone_contiguous(zone);
>> +    const bool fast_path = check_zone_contiguous_fast(zone,
>> start_pfn, nr_pages);
>>         if (zone_is_empty(zone))
>>           init_currently_empty_zone(zone, start_pfn, nr_pages);
>> @@ -783,7 +827,8 @@ void move_pfn_range_to_zone(struct zone *zone,
>> unsigned long start_pfn,
>>                MEMINIT_HOTPLUG, altmap, migratetype,
>>                isolate_pageblock);
>>   -    set_zone_contiguous(zone);
>> +    if (!fast_path)
>> +        set_zone_contiguous(zone);
>>   }
>>     struct auto_movable_stats {
>
> Agreed with Mike that we should keep clearing+resetting the bit.

Got it. Worked with Yuan Liu to understand the risk that if set the
zone->contiguous before the pfn range fully initialized. It seems
pageblock_pfn_to_page code path could be affect thus potentially it is
not safe.

> Also, I don't particularly enjoy the "fast_path" terminology. Probably we
> want in the end something high-level like:
>
>
> bool definetly_contig;
>
> definetly_contig = clear_zone_contiguous_for_growing(zone, start_pfn,
> nr_pages);
>
> ...
>
> set_zone_contiguous(zone, definetly_contig);
>
>
> We could do something similar on the removal path then, where the zone
> will for sure stay contiguous if we are removing the first/last part.
>
>
> bool definetly_contig;
>
> stays_contiguous = clear_zone_contiguous_for_shrinking(zone,
> start_pfn, nr_pages);
>
> ...
>
> set_zone_contiguous(zone, definetly_contig);
>
>
>
> If we can come up for a better name for definetly_contig that would be
> nice.
>

Instead of a bool value, could the clear_zone_contiguous_for_growing
and clear_zone_contiguous_for_shrinking return a enum value to indicate
one of the three states: 1. DEFINITELY_CONTIGUOUS;
2. DEFINITELY_NOT_CONTIGUOUS; 3. UNDETERMINED_CONTIGUOUS? The
set_zone_contiguous took the state and skip the contiguous check if
DEFINITELY_CONTIGUOUS or DEFINITELY_NOT_CONTIGUOUS.

Regards,

Tianyou