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Message-ID: <aXINUc0ZJSJusel2@kernel.org>
Date: Thu, 22 Jan 2026 13:43:13 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Tianyou Li <tianyou.li@...el.com>
Cc: David Hildenbrand <david@...nel.org>,
Oscar Salvador <osalvador@...e.de>,
Wei Yang <richard.weiyang@...il.com>,
Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
Yong Hu <yong.hu@...el.com>, Nanhai Zou <nanhai.zou@...el.com>,
Yuan Liu <yuan1.liu@...el.com>,
Tim Chen <tim.c.chen@...ux.intel.com>,
Qiuxu Zhuo <qiuxu.zhuo@...el.com>, Yu C Chen <yu.c.chen@...el.com>,
Pan Deng <pan.deng@...el.com>, Chen Zhang <zhangchen.kidd@...com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 3/3] mm/memory hotplug/unplug: Optimize
zone->contiguous update when changes pfn range
Hi,
On Tue, Jan 20, 2026 at 10:33:46PM +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@...el.com>
> Reported-by: Chen Zhang <zhangchen.kidd@...com>
> Tested-by: Yuan Liu <yuan1.liu@...el.com>
> Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
> Reviewed-by: Qiuxu Zhuo <qiuxu.zhuo@...el.com>
> Reviewed-by: Yu C Chen <yu.c.chen@...el.com>
> Reviewed-by: Pan Deng <pan.deng@...el.com>
> Reviewed-by: Nanhai Zou <nanhai.zou@...el.com>
> Reviewed-by: Yuan Liu <yuan1.liu@...el.com>
> Signed-off-by: Tianyou Li <tianyou.li@...el.com>
> ---
...
> +int online_memory_block_pages(unsigned long start_pfn, unsigned long nr_pages,
> + unsigned long nr_vmemmap_pages, struct zone *zone,
> + struct memory_group *group)
> {
> + const bool contiguous = zone->contiguous;
> + enum zone_contig_state new_contiguous_state;
> int ret;
>
> + /*
> + * Calculate the new zone contig state before move_pfn_range_to_zone()
> + * sets the zone temporarily to non-contiguous.
> + */
> + new_contiguous_state = zone_contig_state_after_growing(zone, start_pfn,
> + nr_pages);
> +
> if (nr_vmemmap_pages) {
> ret = mhp_init_memmap_on_memory(start_pfn, nr_vmemmap_pages, zone);
> if (ret)
> - return ret;
> + goto restore_zone_contig;
But zone_contig_state_after_growing() does not change zone->contiguous. Why
do we need to save and restore it?
> }
>
> ret = online_pages(start_pfn + nr_vmemmap_pages,
> @@ -1271,7 +1320,7 @@ int online_memory_block_pages(unsigned long start_pfn,
> if (ret) {
> if (nr_vmemmap_pages)
> mhp_deinit_memmap_on_memory(start_pfn, nr_vmemmap_pages);
> - return ret;
> + goto restore_zone_contig;
> }
>
> /*
> @@ -1282,6 +1331,15 @@ int online_memory_block_pages(unsigned long start_pfn,
> adjust_present_page_count(pfn_to_page(start_pfn), group,
> nr_vmemmap_pages);
>
> + /*
> + * Now that the ranges are indicated as online, check whether the whole
> + * zone is contiguous.
> + */
> + set_zone_contiguous(zone, new_contiguous_state);
> + return 0;
> +
> +restore_zone_contig:
> + zone->contiguous = contiguous;
> return ret;
> }
--
Sincerely yours,
Mike.
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