Re: [PATCH v3] mm/memory hotplug/unplug: Optimize zone->contiguous update when move pfn range
From: Li, Tianyou
Date: Fri Nov 28 2025 - 10:17:27 EST
Thanks for your review David.
On 11/28/2025 8:01 PM, David Hildenbrand (Red Hat) wrote:
On 11/19/25 15:06, 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 | 2s | 80% |
| +------+---------------+--------------+----------------+
| | 512G | 33s | 6s | 81% |
+----------------+------+---------------+--------------+----------------+
[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@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>
Signed-off-by: Tianyou Li <tianyou.li@xxxxxxxxx>
---
mm/memory_hotplug.c | 51 ++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 48 insertions(+), 3 deletions(-)
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 0be83039c3b5..aed1827a2778 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -723,6 +723,51 @@ static void __meminit resize_pgdat_range(struct pglist_data *pgdat, unsigned lon
}
+static bool __meminit check_zone_contiguous_fast(struct zone *zone,
+ unsigned long start_pfn, unsigned long nr_pages)
+{
+ const unsigned long end_pfn = start_pfn + nr_pages;
+
+ /*
+ * Given the moved pfn range's contiguous property is always true,
+ * under the conditional of empty zone, the contiguous property should
+ * be true.
+ */
+ if (zone_is_empty(zone)) {
+ zone->contiguous = true;
+ return true;
+ }
+
+ /*
+ * If the moved pfn range does not intersect with the original zone span,
+ * the contiguous property is surely false.
+ */
+ if (end_pfn < zone->zone_start_pfn || start_pfn > zone_end_pfn(zone)) {
+ zone->contiguous = false;
+ return true;
+ }
+
+ /*
+ * If the moved pfn range is adjacent to the original zone span, given
+ * the moved pfn range's contiguous property is always true, the zone's
+ * contiguous property inherited from the original value.
+ */
+ if (end_pfn == zone->zone_start_pfn || start_pfn == zone_end_pfn(zone))
+ return true;
+
+ /*
+ * If the original zone's hole larger than the moved pages in the range,
+ * the contiguous property is surely false.
+ */
+ if (nr_pages < (zone->spanned_pages - zone->present_pages)) {
+ zone->contiguous = false;
+ return true;
+ }
+
+ clear_zone_contiguous(zone);
+ return false;
+}
+
#ifdef CONFIG_ZONE_DEVICE
static void section_taint_zone_device(unsigned long pfn)
{
@@ -752,8 +797,7 @@ 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);
+ const bool fast_path = check_zone_contiguous_fast(zone, start_pfn, nr_pages);
if (zone_is_empty(zone))
init_currently_empty_zone(zone, start_pfn, nr_pages);
@@ -783,7 +827,8 @@ void move_pfn_range_to_zone(struct zone *zone, unsigned long start_pfn,
MEMINIT_HOTPLUG, altmap, migratetype,
isolate_pageblock);
- set_zone_contiguous(zone);
+ if (!fast_path)
+ set_zone_contiguous(zone);
}
struct auto_movable_stats {
Agreed with Mike that we should keep clearing+resetting the bit.
Got it. Worked with Yuan Liu to understand the risk that if set the zone->contiguous before the pfn range fully initialized. It seems pageblock_pfn_to_page code path could be affect thus potentially it is not safe.
Also, I don't particularly enjoy the "fast_path" terminology. Probably we
want in the end something high-level like:
bool definetly_contig;
definetly_contig = clear_zone_contiguous_for_growing(zone, start_pfn, nr_pages);
...
set_zone_contiguous(zone, definetly_contig);
We could do something similar on the removal path then, where the zone
will for sure stay contiguous if we are removing the first/last part.
bool definetly_contig;
stays_contiguous = clear_zone_contiguous_for_shrinking(zone, start_pfn, nr_pages);
...
set_zone_contiguous(zone, definetly_contig);
If we can come up for a better name for definetly_contig that would be nice.
Instead of a bool value, could the clear_zone_contiguous_for_growing and clear_zone_contiguous_for_shrinking return a enum value to indicate one of the three states: 1. DEFINITELY_CONTIGUOUS; 2. DEFINITELY_NOT_CONTIGUOUS; 3. UNDETERMINED_CONTIGUOUS? The set_zone_contiguous took the state and skip the contiguous check if DEFINITELY_CONTIGUOUS or DEFINITELY_NOT_CONTIGUOUS.
Regards,
Tianyou