Thanks for your comments Mike. Appreciated.
On Mon, Nov 17, 2025 at 11:30:52AM +0800, 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;The check for adjacency of the new range to the zone can be moved to the beginning of move_pfn_range_to_zone() and it will already optimize the common case when we hotplug memory to a contiguous zone.
Do you mean we can separate the update_zone_contiguous logic into two parts, one for fast path at the beginning of the move_pfn_range_to_zone, and the other for slow path after the memmep_init_range?
+ } + + /* + * 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;Let's just check that we don't add enough pages to cover the hole if (nr_new_pages < old_absent_pages) return; and if we do go to the slow path and walk the pageblocks.
I'd like to avoid of the slow path as much as possible. The check 'if (nr_new_pages < old_absent_pages)' is more strict if overlap happens. I am OK to simplify it if there is no overlap cases or to reduce the maintaining efforts.
Thanks & Regards,
Tianyou
+ + set_zone_contiguous(zone); +} +