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

Li, Tianyou
2025-11-17T02:38:15Z

Hi All,

Rebased with latest master. Add "Reported-by" from customer. Looking
forward to any comments/suggestions. Thanks.

Regards,

Tianyou

On 11/17/2025 11:30 AM, 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;
> + }
> +
> + /*
> + * 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;
> +
> + set_zone_contiguous(zone);
> +}
> +
> #ifdef CONFIG_ZONE_DEVICE
> static void section_taint_zone_device(unsigned long pfn)
> {
> @@ -752,6 +793,10 @@ 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;
> + bool old_contiguous = zone->contiguous;
> + unsigned long old_start_pfn = zone->zone_start_pfn;
> + unsigned long old_nr_pages = zone->spanned_pages;
> + unsigned long old_absent_pages = zone->spanned_pages - zone->present_pages;
>
> clear_zone_contiguous(zone);
>
> @@ -783,7 +828,8 @@ void move_pfn_range_to_zone(struct zone *zone, unsigned long start_pfn,
> MEMINIT_HOTPLUG, altmap, migratetype,
> isolate_pageblock);
>
> - set_zone_contiguous(zone);
> + update_zone_contiguous(zone, old_contiguous, old_start_pfn, old_nr_pages,
> + old_absent_pages, start_pfn, nr_pages);
> }
>
> struct auto_movable_stats {
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 7712d887b696..04fdd949fe49 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -2263,26 +2263,43 @@ void __init init_cma_pageblock(struct page *page)
> }
> #endif
>
> -void set_zone_contiguous(struct zone *zone)
> +/*
> + * Check if all pageblocks in the given PFN range belong to the given zone.
> + * The given range is expected to be within the zone's pfn range, otherwise
> + * false is returned.
> + */
> +bool check_zone_contiguous(struct zone *zone, unsigned long start_pfn,
> + unsigned long nr_pages)
> {
> - unsigned long block_start_pfn = zone->zone_start_pfn;
> + unsigned long end_pfn = start_pfn + nr_pages;
> + unsigned long block_start_pfn = start_pfn;
> unsigned long block_end_pfn;
>
> + if (start_pfn < zone->zone_start_pfn || end_pfn > zone_end_pfn(zone))
> + return false;
> +
> block_end_pfn = pageblock_end_pfn(block_start_pfn);
> - for (; block_start_pfn < zone_end_pfn(zone);
> + for (; block_start_pfn < end_pfn;
> block_start_pfn = block_end_pfn,
> block_end_pfn += pageblock_nr_pages) {
>
> - block_end_pfn = min(block_end_pfn, zone_end_pfn(zone));
> + block_end_pfn = min(block_end_pfn, end_pfn);
>
> if (!__pageblock_pfn_to_page(block_start_pfn,
> block_end_pfn, zone))
> - return;
> + return false;
> cond_resched();
> }
>
> - /* We confirm that there is no hole */
> - zone->contiguous = true;
> + return true;
> +}
> +
> +void set_zone_contiguous(struct zone *zone)
> +{
> + unsigned long start_pfn = zone->zone_start_pfn;
> + unsigned long nr_pages = zone->spanned_pages;
> +
> + zone->contiguous = check_zone_contiguous(zone, start_pfn, nr_pages);
> }
>
> /*