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

[Oscar Salvador <osalvador@suse.de>]

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:

> ---
>  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?

> +{
> +	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.

>  	/* 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.

>  	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?

>  }
 

-- 
Oscar Salvador
SUSE Labs