RE: [PATCH] mm/memory hotplug/unplug: Optimize zone contiguous check when changing pfn range

[Liu, Yuan1]

Hi David & Mike

I merged this patch into v6.19-rc8 for validation and observed that unplugging 256 GB takes 3 seconds, while unplugging 512 GB takes 7 seconds. I believe this performance regression in memory unplug is not caused by this patch.

Best Regards,
Liu, Yuan1

> -----Original Message-----
> From: Liu, Yuan1 <yuan1.liu@xxxxxxxxx>
> Sent: Thursday, March 19, 2026 5:56 PM
> To: David Hildenbrand <david@xxxxxxxxxx>; Oscar Salvador
> <osalvador@xxxxxxx>; Mike Rapoport <rppt@xxxxxxxxxx>; Wei Yang
> <richard.weiyang@xxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx; Hu, Yong <yong.hu@xxxxxxxxx>; Zou, Nanhai
> <nanhai.zou@xxxxxxxxx>; Liu, Yuan1 <yuan1.liu@xxxxxxxxx>; Tim Chen
> <tim.c.chen@xxxxxxxxxxxxxxx>; Zhuo, Qiuxu <qiuxu.zhuo@xxxxxxxxx>; Chen, Yu
> C <yu.c.chen@xxxxxxxxx>; Deng, Pan <pan.deng@xxxxxxxxx>; Li, Tianyou
> <tianyou.li@xxxxxxxxx>; Chen Zhang <zhangchen.kidd@xxxxxx>; linux-
> kernel@xxxxxxxxxxxxxxx
> Subject: [PATCH] mm/memory hotplug/unplug: Optimize zone contiguous check
> when changing pfn range
> 
> 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 a new zone's pages_with_memmap member, it is pages within the zone
> that
> have an online memmap. It includes present pages and memory holes that
> have
> a memmap. When spanned_pages == pages_with_online_memmap, pfn_to_page()
> can
> be performed without further checks on any pfn within the zone span.
> 
> 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      |      3s      |       70%      |
> |                +------+---------------+--------------+----------------+
> |                | 512G |      36s      |      7s      |       81%      |
> +----------------+------+---------------+--------------+----------------+
> 
> +----------------+------+---------------+--------------+----------------+
> |                | Size | Time (before) | Time (after) | Time Reduction |
> |                +------+---------------+--------------+----------------+
> | Unplug Memory  | 256G |      11s      |      4s      |       64%      |
> |                +------+---------------+--------------+----------------+
> |                | 512G |      36s      |      9s      |       75%      |
> +----------------+------+---------------+--------------+----------------+
> 
> [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 : v7.0-rc4
>     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@xxxxxxxxx>
> Reported-by: Chen Zhang <zhangchen.kidd@xxxxxx>
> Tested-by: Yuan Liu <yuan1.liu@xxxxxxxxx>
> Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Reviewed-by: Qiuxu Zhuo <qiuxu.zhuo@xxxxxxxxx>
> Reviewed-by: Yu C Chen <yu.c.chen@xxxxxxxxx>
> Reviewed-by: Pan Deng <pan.deng@xxxxxxxxx>
> Reviewed-by: Nanhai Zou <nanhai.zou@xxxxxxxxx>
> Reviewed-by: Yuan Liu <yuan1.liu@xxxxxxxxx>
> Co-developed-by: Tianyou Li <tianyou.li@xxxxxxxxx>
> Signed-off-by: Tianyou Li <tianyou.li@xxxxxxxxx>
> Signed-off-by: Yuan Liu <yuan1.liu@xxxxxxxxx>
> ---
>  Documentation/mm/physical_memory.rst |  6 +++++
>  include/linux/mmzone.h               | 22 ++++++++++++++-
>  mm/internal.h                        | 10 +++----
>  mm/memory_hotplug.c                  | 21 +++++----------
>  mm/mm_init.c                         | 40 +++++++++-------------------
>  5 files changed, 50 insertions(+), 49 deletions(-)
> 
> diff --git a/Documentation/mm/physical_memory.rst
> b/Documentation/mm/physical_memory.rst
> index b76183545e5b..d324da29ac11 100644
> --- a/Documentation/mm/physical_memory.rst
> +++ b/Documentation/mm/physical_memory.rst
> @@ -483,6 +483,12 @@ General
>    ``present_pages`` should use ``get_online_mems()`` to get a stable
> value. It
>    is initialized by ``calculate_node_totalpages()``.
> 
> +``pages_with_online_memmap``
> +  The pages_with_online_memmap is pages within the zone that have an
> online
> +  memmap. It includes present pages and memory holes that have a memmap.
> When
> +  spanned_pages == pages_with_online_memmap, pfn_to_page() can be
> performed
> +  without further checks on any pfn within the zone span.
> +
>  ``present_early_pages``
>    The present pages existing within the zone located on memory available
> since
>    early boot, excluding hotplugged memory. Defined only when
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 3e51190a55e4..c7a136ce55c7 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -943,6 +943,11 @@ struct zone {
>  	 * cma pages is present pages that are assigned for CMA use
>  	 * (MIGRATE_CMA).
>  	 *
> +	 * pages_with_online_memmap is pages within the zone that have an
> online
> +	 * memmap. It includes present pages and memory holes that have a
> memmap.
> +	 * When spanned_pages == pages_with_online_memmap, pfn_to_page() can
> be
> +	 * performed without further checks on any pfn within the zone span.
> +	 *
>  	 * So present_pages may be used by memory hotplug or memory power
>  	 * management logic to figure out unmanaged pages by checking
>  	 * (present_pages - managed_pages). And managed_pages should be used
> @@ -967,6 +972,7 @@ struct zone {
>  	atomic_long_t		managed_pages;
>  	unsigned long		spanned_pages;
>  	unsigned long		present_pages;
> +	unsigned long		pages_with_online_memmap;
>  #if defined(CONFIG_MEMORY_HOTPLUG)
>  	unsigned long		present_early_pages;
>  #endif
> @@ -1051,7 +1057,6 @@ struct zone {
>  	bool			compact_blockskip_flush;
>  #endif
> 
> -	bool			contiguous;
> 
>  	CACHELINE_PADDING(_pad3_);
>  	/* Zone statistics */
> @@ -1124,6 +1129,21 @@ static inline bool zone_spans_pfn(const struct zone
> *zone, unsigned long pfn)
>  	return zone->zone_start_pfn <= pfn && pfn < zone_end_pfn(zone);
>  }
> 
> +/**
> + * zone_is_contiguous - test whether a zone is contiguous
> + * @zone: the zone to test.
> + *
> + * In a contiguous zone, it is valid to call pfn_to_page() on any pfn in
> the
> + * spanned zone without requiring pfn_valid() or pfn_to_online_page()
> checks.
> + *
> + * Returns: true if contiguous, otherwise false.
> + */
> +static inline bool zone_is_contiguous(const struct zone *zone)
> +{
> +	return READ_ONCE(zone->spanned_pages) ==
> +		READ_ONCE(zone->pages_with_online_memmap);
> +}
> +
>  static inline bool zone_is_initialized(const struct zone *zone)
>  {
>  	return zone->initialized;
> diff --git a/mm/internal.h b/mm/internal.h
> index cb0af847d7d9..7c4c8ab68bde 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -793,21 +793,17 @@ extern struct page *__pageblock_pfn_to_page(unsigned
> long start_pfn,
>  static inline struct page *pageblock_pfn_to_page(unsigned long start_pfn,
>  				unsigned long end_pfn, struct zone *zone)
>  {
> -	if (zone->contiguous)
> +	if (zone_is_contiguous(zone) && zone_spans_pfn(zone, start_pfn)) {
> +		VM_BUG_ON(end_pfn > zone_end_pfn(zone));
>  		return pfn_to_page(start_pfn);
> +	}
> 
>  	return __pageblock_pfn_to_page(start_pfn, end_pfn, zone);
>  }
> 
> -void set_zone_contiguous(struct zone *zone);
>  bool pfn_range_intersects_zones(int nid, unsigned long start_pfn,
>  			   unsigned long nr_pages);
> 
> -static inline void clear_zone_contiguous(struct zone *zone)
> -{
> -	zone->contiguous = false;
> -}
> -
>  extern int __isolate_free_page(struct page *page, unsigned int order);
>  extern void __putback_isolated_page(struct page *page, unsigned int
> order,
>  				    int mt);
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index bc805029da51..2ba7a394a64b 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -492,11 +492,11 @@ static void shrink_zone_span(struct zone *zone,
> unsigned long start_pfn,
>  		pfn = find_smallest_section_pfn(nid, zone, end_pfn,
>  						zone_end_pfn(zone));
>  		if (pfn) {
> -			zone->spanned_pages = zone_end_pfn(zone) - pfn;
> +			WRITE_ONCE(zone->spanned_pages, zone_end_pfn(zone) -
> pfn);
>  			zone->zone_start_pfn = pfn;
>  		} else {
>  			zone->zone_start_pfn = 0;
> -			zone->spanned_pages = 0;
> +			WRITE_ONCE(zone->spanned_pages, 0);
>  		}
>  	} else if (zone_end_pfn(zone) == end_pfn) {
>  		/*
> @@ -508,10 +508,10 @@ static void shrink_zone_span(struct zone *zone,
> unsigned long start_pfn,
>  		pfn = find_biggest_section_pfn(nid, zone, zone-
> >zone_start_pfn,
>  					       start_pfn);
>  		if (pfn)
> -			zone->spanned_pages = pfn - zone->zone_start_pfn + 1;
> +			WRITE_ONCE(zone->spanned_pages, pfn - zone-
> >zone_start_pfn + 1);
>  		else {
>  			zone->zone_start_pfn = 0;
> -			zone->spanned_pages = 0;
> +			WRITE_ONCE(zone->spanned_pages, 0);
>  		}
>  	}
>  }
> @@ -565,18 +565,13 @@ void remove_pfn_range_from_zone(struct zone *zone,
> 
>  	/*
>  	 * Zone shrinking code cannot properly deal with ZONE_DEVICE. So
> -	 * we will not try to shrink the zones - which is okay as
> -	 * set_zone_contiguous() cannot deal with ZONE_DEVICE either way.
> +	 * we will not try to shrink the zones.
>  	 */
>  	if (zone_is_zone_device(zone))
>  		return;
> 
> -	clear_zone_contiguous(zone);
> -
>  	shrink_zone_span(zone, start_pfn, start_pfn + nr_pages);
>  	update_pgdat_span(pgdat);
> -
> -	set_zone_contiguous(zone);
>  }
> 
>  /**
> @@ -753,8 +748,6 @@ 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);
> -
>  	if (zone_is_empty(zone))
>  		init_currently_empty_zone(zone, start_pfn, nr_pages);
>  	resize_zone_range(zone, start_pfn, nr_pages);
> @@ -782,8 +775,6 @@ void move_pfn_range_to_zone(struct zone *zone,
> unsigned long start_pfn,
>  	memmap_init_range(nr_pages, nid, zone_idx(zone), start_pfn, 0,
>  			 MEMINIT_HOTPLUG, altmap, migratetype,
>  			 isolate_pageblock);
> -
> -	set_zone_contiguous(zone);
>  }
> 
>  struct auto_movable_stats {
> @@ -1079,6 +1070,8 @@ void adjust_present_page_count(struct page *page,
> struct memory_group *group,
>  	if (early_section(__pfn_to_section(page_to_pfn(page))))
>  		zone->present_early_pages += nr_pages;
>  	zone->present_pages += nr_pages;
> +	WRITE_ONCE(zone->pages_with_online_memmap,
> +		READ_ONCE(zone->pages_with_online_memmap) + nr_pages);
>  	zone->zone_pgdat->node_present_pages += nr_pages;
> 
>  	if (group && movable)
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index df34797691bd..96690e550024 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -946,6 +946,7 @@ static void __init memmap_init_zone_range(struct zone
> *zone,
>  	unsigned long zone_start_pfn = zone->zone_start_pfn;
>  	unsigned long zone_end_pfn = zone_start_pfn + zone->spanned_pages;
>  	int nid = zone_to_nid(zone), zone_id = zone_idx(zone);
> +	unsigned long zone_hole_start, zone_hole_end;
> 
>  	start_pfn = clamp(start_pfn, zone_start_pfn, zone_end_pfn);
>  	end_pfn = clamp(end_pfn, zone_start_pfn, zone_end_pfn);
> @@ -957,8 +958,19 @@ static void __init memmap_init_zone_range(struct zone
> *zone,
>  			  zone_end_pfn, MEMINIT_EARLY, NULL, MIGRATE_MOVABLE,
>  			  false);
> 
> -	if (*hole_pfn < start_pfn)
> +	WRITE_ONCE(zone->pages_with_online_memmap,
> +		   READ_ONCE(zone->pages_with_online_memmap) +
> +		   (end_pfn - start_pfn));
> +
> +	if (*hole_pfn < start_pfn) {
>  		init_unavailable_range(*hole_pfn, start_pfn, zone_id, nid);
> +		zone_hole_start = clamp(*hole_pfn, zone_start_pfn,
> zone_end_pfn);
> +		zone_hole_end = clamp(start_pfn, zone_start_pfn,
> zone_end_pfn);
> +		if (zone_hole_start < zone_hole_end)
> +			WRITE_ONCE(zone->pages_with_online_memmap,
> +				   READ_ONCE(zone->pages_with_online_memmap) +
> +				   (zone_hole_end - zone_hole_start));
> +	}
> 
>  	*hole_pfn = end_pfn;
>  }
> @@ -2261,28 +2273,6 @@ void __init init_cma_pageblock(struct page *page)
>  }
>  #endif
> 
> -void set_zone_contiguous(struct zone *zone)
> -{
> -	unsigned long block_start_pfn = zone->zone_start_pfn;
> -	unsigned long block_end_pfn;
> -
> -	block_end_pfn = pageblock_end_pfn(block_start_pfn);
> -	for (; block_start_pfn < zone_end_pfn(zone);
> -			block_start_pfn = block_end_pfn,
> -			 block_end_pfn += pageblock_nr_pages) {
> -
> -		block_end_pfn = min(block_end_pfn, zone_end_pfn(zone));
> -
> -		if (!__pageblock_pfn_to_page(block_start_pfn,
> -					     block_end_pfn, zone))
> -			return;
> -		cond_resched();
> -	}
> -
> -	/* We confirm that there is no hole */
> -	zone->contiguous = true;
> -}
> -
>  /*
>   * Check if a PFN range intersects multiple zones on one or more
>   * NUMA nodes. Specify the @nid argument if it is known that this
> @@ -2311,7 +2301,6 @@ bool pfn_range_intersects_zones(int nid, unsigned
> long start_pfn,
>  static void __init mem_init_print_info(void);
>  void __init page_alloc_init_late(void)
>  {
> -	struct zone *zone;
>  	int nid;
> 
>  #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
> @@ -2345,9 +2334,6 @@ void __init page_alloc_init_late(void)
>  	for_each_node_state(nid, N_MEMORY)
>  		shuffle_free_memory(NODE_DATA(nid));
> 
> -	for_each_populated_zone(zone)
> -		set_zone_contiguous(zone);
> -
>  	/* Initialize page ext after all struct pages are initialized. */
>  	if (deferred_struct_pages)
>  		page_ext_init();
> --
> 2.47.3