[RFC PATCH V3 01/17] mm: Add kscand kthread for PTE A bit scan

Raghavendra K T
2025-08-14T15:33:47Z

Also add a config option for the same.

High level design:

While (1):
Scan the slowtier pages belonging to VMAs of a task.
Add to migation list.

A separate thread:
Migrate scanned pages to a toptier node based on heuristics.

The overall code is influenced by khugepaged design.

Signed-off-by: Raghavendra K T <raghavendra.kt@amd.com>
---
mm/Kconfig | 8 +++
mm/Makefile | 1 +
mm/kscand.c | 163 ++++++++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 172 insertions(+)
create mode 100644 mm/kscand.c

diff --git a/mm/Kconfig b/mm/Kconfig
index 781be3240e21..d1e5be76a96e 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -750,6 +750,14 @@ config KSM
until a program has madvised that an area is MADV_MERGEABLE, and
root has set /sys/kernel/mm/ksm/run to 1 (if CONFIG_SYSFS is set).

+config KSCAND
+ bool "Enable PTE A bit scanning and Migration"
+ depends on NUMA_BALANCING
+ help
+ Enable PTE A bit scanning of page. The option creates a separate
+ kthread for scanning and migration. Accessed slow-tier pages are
+ migrated to a regular NUMA node to reduce hot page access latency.
+
config DEFAULT_MMAP_MIN_ADDR
int "Low address space to protect from user allocation"
depends on MMU
diff --git a/mm/Makefile b/mm/Makefile
index 1a7a11d4933d..a16ef2ff3da1 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -97,6 +97,7 @@ obj-$(CONFIG_FAIL_PAGE_ALLOC) += fail_page_alloc.o
obj-$(CONFIG_MEMTEST) += memtest.o
obj-$(CONFIG_MIGRATION) += migrate.o
obj-$(CONFIG_NUMA) += memory-tiers.o
+obj-$(CONFIG_KSCAND) += kscand.o
obj-$(CONFIG_DEVICE_MIGRATION) += migrate_device.o
obj-$(CONFIG_TRANSPARENT_HUGEPAGE) += huge_memory.o khugepaged.o
obj-$(CONFIG_PAGE_COUNTER) += page_counter.o
diff --git a/mm/kscand.c b/mm/kscand.c
new file mode 100644
index 000000000000..f7bbbc70c86a
--- /dev/null
+++ b/mm/kscand.c
@@ -0,0 +1,163 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/mm.h>
+#include <linux/mm_types.h>
+#include <linux/sched.h>
+#include <linux/sched/mm.h>
+#include <linux/mmu_notifier.h>
+#include <linux/swap.h>
+#include <linux/mm_inline.h>
+#include <linux/kthread.h>
+#include <linux/string.h>
+#include <linux/delay.h>
+#include <linux/cleanup.h>
+
+#include <asm/pgalloc.h>
+#include "internal.h"
+
+static struct task_struct *kscand_thread __read_mostly;
+static DEFINE_MUTEX(kscand_mutex);
+
+/* How long to pause between two scan cycles */
+static unsigned int kscand_scan_sleep_ms __read_mostly = 20;
+
+/* Max number of mms to scan in one scan cycle */
+#define KSCAND_MMS_TO_SCAN (4 * 1024UL)
+static unsigned long kscand_mms_to_scan __read_mostly = KSCAND_MMS_TO_SCAN;
+
+bool kscand_scan_enabled = true;
+static bool need_wakeup;
+
+static unsigned long kscand_sleep_expire;
+
+static DECLARE_WAIT_QUEUE_HEAD(kscand_wait);
+
+/* Data structure to keep track of current mm under scan */
+struct kscand_scan {
+ struct list_head mm_head;
+};
+
+struct kscand_scan kscand_scan = {
+ .mm_head = LIST_HEAD_INIT(kscand_scan.mm_head),
+};
+
+static inline int kscand_has_work(void)
+{
+ return !list_empty(&kscand_scan.mm_head);
+}
+
+static inline bool kscand_should_wakeup(void)
+{
+ bool wakeup = kthread_should_stop() || need_wakeup ||
+ time_after_eq(jiffies, kscand_sleep_expire);
+
+ need_wakeup = false;
+
+ return wakeup;
+}
+
+static void kscand_wait_work(void)
+{
+ const unsigned long scan_sleep_jiffies =
+ msecs_to_jiffies(kscand_scan_sleep_ms);
+
+ if (!scan_sleep_jiffies)
+ return;
+
+ kscand_sleep_expire = jiffies + scan_sleep_jiffies;
+
+ /* Allows kthread to pause scanning */
+ wait_event_timeout(kscand_wait, kscand_should_wakeup(),
+ scan_sleep_jiffies);
+}
+static void kscand_do_scan(void)
+{
+ unsigned long iter = 0, mms_to_scan;
+
+ mms_to_scan = READ_ONCE(kscand_mms_to_scan);
+
+ while (true) {
+ if (unlikely(kthread_should_stop()) ||
+ !READ_ONCE(kscand_scan_enabled))
+ break;
+
+ if (kscand_has_work())
+ msleep(100);
+
+ iter++;
+
+ if (iter >= mms_to_scan)
+ break;
+ cond_resched();
+ }
+}
+
+static int kscand(void *none)
+{
+ while (true) {
+ if (unlikely(kthread_should_stop()))
+ break;
+
+ while (!READ_ONCE(kscand_scan_enabled)) {
+ cpu_relax();
+ kscand_wait_work();
+ }
+
+ kscand_do_scan();
+
+ kscand_wait_work();
+ }
+ return 0;
+}
+
+static int start_kscand(void)
+{
+ struct task_struct *kthread;
+
+ guard(mutex)(&kscand_mutex);
+
+ if (kscand_thread)
+ return 0;
+
+ kthread = kthread_run(kscand, NULL, "kscand");
+ if (IS_ERR(kscand_thread)) {
+ pr_err("kscand: kthread_run(kscand) failed\n");
+ return PTR_ERR(kthread);
+ }
+
+ kscand_thread = kthread;
+ pr_info("kscand: Successfully started kscand");
+
+ if (!list_empty(&kscand_scan.mm_head))
+ wake_up_interruptible(&kscand_wait);
+
+ return 0;
+}
+
+static int stop_kscand(void)
+{
+ guard(mutex)(&kscand_mutex);
+
+ if (kscand_thread) {
+ kthread_stop(kscand_thread);
+ kscand_thread = NULL;
+ }
+
+ return 0;
+}
+
+static int __init kscand_init(void)
+{
+ int err;
+
+ err = start_kscand();
+ if (err)
+ goto err_kscand;
+
+ return 0;
+
+err_kscand:
+ stop_kscand();
+
+ return err;
+}
+subsys_initcall(kscand_init);
--
2.34.1