Re: [PATCH v5 1/2] mm/memory hotplug/unplug: Optimize zone->contiguous update when changes pfn range

["Li, Tianyou" <tianyou.li@intel.com>]

On 12/11/2025 1:07 PM, Oscar Salvador wrote:
> On Mon, Dec 08, 2025 at 11:25:43PM +0800, Tianyou Li wrote:
>> When invoke move_pfn_range_to_zone or remove_pfn_range_from_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 |
>> |                +------+---------------+--------------+----------------+
>> | Plug Memory    | 256G |      10s      |      2s      |       80%      |
>> |                +------+---------------+--------------+----------------+
>> |                | 512G |      33s      |      6s      |       81%      |
>> +----------------+------+---------------+--------------+----------------+
>>
>> +----------------+------+---------------+--------------+----------------+
>> |                | Size | Time (before) | Time (after) | Time Reduction |
>> |                +------+---------------+--------------+----------------+
>> | Unplug Memory  | 256G |      10s      |      2s      |       80%      |
>> |                +------+---------------+--------------+----------------+
>> |                | 512G |      34s      |      6s      |       82%      |
>> +----------------+------+---------------+--------------+----------------+
>>
>> [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 : 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>
> Overall this looks good to me, thanks Tianyou Li for working on this.
> Just some minor comments below:


Very appreciated Oscar for your time to review the patch! Yuan and I 
will work on the patch v6 to address all your comments/suggestions.

We will test the patch v6 across the weekend. If any other comments 
during those days, we will need more days probably. Thanks.


>> ---
>>   mm/internal.h       |  8 +++++-
>>   mm/memory_hotplug.c | 64 ++++++++++++++++++++++++++++++++++++++++++---
>>   mm/mm_init.c        | 13 +++++++--
>>   3 files changed, 79 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 1561fc2ff5b8..1b5bba6526d4 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -730,7 +730,13 @@ static inline struct page *pageblock_pfn_to_page(unsigned long start_pfn,
>>   	return __pageblock_pfn_to_page(start_pfn, end_pfn, zone);
>>   }
>>   
>> -void set_zone_contiguous(struct zone *zone);
>> +enum zone_contig_state {
>> +	ZONE_CONTIG_YES,
>> +	ZONE_CONTIG_NO,
>> +	ZONE_CONTIG_MAYBE,
>> +};
>> +
>> +void set_zone_contiguous(struct zone *zone, enum zone_contig_state state);
>>   bool pfn_range_intersects_zones(int nid, unsigned long start_pfn,
>>   			   unsigned long nr_pages);
>>   
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 0be83039c3b5..d711f6e2c87f 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -544,6 +544,28 @@ static void update_pgdat_span(struct pglist_data *pgdat)
>>   	pgdat->node_spanned_pages = node_end_pfn - node_start_pfn;
>>   }
>>   
>> +static enum zone_contig_state __meminit zone_contig_state_after_shrinking(
>> +		struct zone *zone, unsigned long start_pfn, unsigned long nr_pages)
> Why do we need the __meminit? These functions are only used from memory-hotplug
> code so we should not need it?


Got it. Will remove the __meminit in patch v6.


>> +{
>> +	const unsigned long end_pfn = start_pfn + nr_pages;
>> +
>> +	/*
>> +	 * If the removed pfn range inside the original zone span, the contiguous
>> +	 * property is surely false.
>> +	 */
>> +	if (start_pfn > zone->zone_start_pfn && end_pfn < zone_end_pfn(zone))
>> +		return ZONE_CONTIG_NO;
>> +
>> +	/* If the removed pfn range is at the beginning or end of the
>> +	 * original zone span, the contiguous property is preserved when
>> +	 * the original zone is contiguous.
>> +	 */
>> +	if (start_pfn == zone->zone_start_pfn || end_pfn == zone_end_pfn(zone))
>> +		return zone->contiguous ? ZONE_CONTIG_YES : ZONE_CONTIG_MAYBE;
>> +
>> +	return ZONE_CONTIG_MAYBE;
>> +}
>> +
>>   void remove_pfn_range_from_zone(struct zone *zone,
>>   				      unsigned long start_pfn,
>>   				      unsigned long nr_pages)
>> @@ -551,6 +573,7 @@ void remove_pfn_range_from_zone(struct zone *zone,
>>   	const unsigned long end_pfn = start_pfn + nr_pages;
>>   	struct pglist_data *pgdat = zone->zone_pgdat;
>>   	unsigned long pfn, cur_nr_pages;
>> +	enum zone_contig_state contiguous_state = ZONE_CONTIG_MAYBE;
> I think that new_contiguous_state is clearer, but I do not have a strong
> opinion here.


Will do in the patch v6.


>>   	/* Poison struct pages because they are now uninitialized again. */
>>   	for (pfn = start_pfn; pfn < end_pfn; pfn += cur_nr_pages) {
>> @@ -571,12 +594,13 @@ void remove_pfn_range_from_zone(struct zone *zone,
>>   	if (zone_is_zone_device(zone))
>>   		return;
>>   
>> +	contiguous_state = zone_contig_state_after_shrinking(zone, start_pfn, nr_pages);
>>   	clear_zone_contiguous(zone);
>>   
>>   	shrink_zone_span(zone, start_pfn, start_pfn + nr_pages);
>>   	update_pgdat_span(pgdat);
>>   
>> -	set_zone_contiguous(zone);
>> +	set_zone_contiguous(zone, contiguous_state);
>>   }
>>   
> ...
>> @@ -752,7 +809,8 @@ 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;
>> -
>> +	const enum zone_contig_state contiguous_state =
>> +		zone_contig_state_after_growing(zone, start_pfn, nr_pages);
> Same comment from remove_pfn_range_from_zone.


Will do in the patch v6.


>>   	clear_zone_contiguous(zone);
>>   
>>   	if (zone_is_empty(zone))
>> @@ -783,7 +841,7 @@ void move_pfn_range_to_zone(struct zone *zone, unsigned long start_pfn,
>>   			 MEMINIT_HOTPLUG, altmap, migratetype,
>>   			 isolate_pageblock);
>>   
>> -	set_zone_contiguous(zone);
>> +	set_zone_contiguous(zone, contiguous_state);
>>   }
>>   
>>   struct auto_movable_stats {
>> diff --git a/mm/mm_init.c b/mm/mm_init.c
>> index 7712d887b696..e296bd9fac9e 100644
>> --- a/mm/mm_init.c
>> +++ b/mm/mm_init.c
>> @@ -2263,11 +2263,19 @@ void __init init_cma_pageblock(struct page *page)
>>   }
>>   #endif
>>   
>> -void set_zone_contiguous(struct zone *zone)
>> +void set_zone_contiguous(struct zone *zone, enum zone_contig_state state)
>>   {
>>   	unsigned long block_start_pfn = zone->zone_start_pfn;
>>   	unsigned long block_end_pfn;
>>   
>> +	if (state == ZONE_CONTIG_YES) {
>> +		zone->contiguous = true;
>> +		return;
>> +	}
>> +
>> +	if (state == ZONE_CONTIG_NO)
>> +		return;
>> +
>>   	block_end_pfn = pageblock_end_pfn(block_start_pfn);
>>   	for (; block_start_pfn < zone_end_pfn(zone);
>>   			block_start_pfn = block_end_pfn,
>> @@ -2283,6 +2291,7 @@ void set_zone_contiguous(struct zone *zone)
>>   
>>   	/* We confirm that there is no hole */
>>   	zone->contiguous = true;
>> +
> Not needed?


My mistake, thanks. Will do in the patch v6.


Regards,

Tianyou


>>   }
>   
>