[RFC PATCH V3 15/17] prctl: Introduce new prctl to control scanning

Raghavendra K T
2025-08-14T15:36:15Z

A new scalar value (PTEAScanScale) to control per task PTE A bit scanning
is introduced.

0 : scanning disabled
1-10 : scanning enabled.

In future PTEAScanScale could be used to control aggressiveness of
scanning.

CC: linux-doc@vger.kernel.org
CC: Jonathan Corbet <corbet@lwn.net>
CC: linux-fsdevel@vger.kernel.org

Suggested-by: David Rientjes <rientjes@google.com>
Signed-off-by: Raghavendra K T <raghavendra.kt@amd.com>
---
Documentation/filesystems/proc.rst | 2 ++
fs/proc/task_mmu.c | 4 ++++
include/linux/mm_types.h | 3 +++
include/uapi/linux/prctl.h | 7 +++++++
kernel/fork.c | 4 ++++
kernel/sys.c | 25 +++++++++++++++++++++++++
mm/kscand.c | 5 +++++
7 files changed, 50 insertions(+)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 5236cb52e357..0e99d1ca229a 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -205,6 +205,7 @@ read the file /proc/PID/status::
VmLib: 1412 kB
VmPTE: 20 kb
VmSwap: 0 kB
+ PTEAScanScale: 0
HugetlbPages: 0 kB
CoreDumping: 0
THP_enabled: 1
@@ -288,6 +289,7 @@ It's slow but very precise.
VmPTE size of page table entries
VmSwap amount of swap used by anonymous private data
(shmem swap usage is not included)
+ PTEAScanScale Integer representing async PTE A bit scan agrression
HugetlbPages size of hugetlb memory portions
CoreDumping process's memory is currently being dumped
(killing the process may lead to a corrupted core)
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 751479eb128f..05be24e4bc4f 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -79,6 +79,10 @@ void task_mem(struct seq_file *m, struct mm_struct *mm)
" kB\nVmPTE:\t", mm_pgtables_bytes(mm) >> 10, 8);
SEQ_PUT_DEC(" kB\nVmSwap:\t", swap);
seq_puts(m, " kB\n");
+#ifdef CONFIG_KSCAND
+ seq_put_decimal_ull_width(m, "PTEAScanScale:\t", mm->pte_scan_scale, 8);
+ seq_puts(m, "\n");
+#endif
hugetlb_report_usage(m, mm);
}
#undef SEQ_PUT_DEC
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index e3d8f11a5a04..798e6053eebe 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1151,6 +1151,9 @@ struct mm_struct {
#ifdef CONFIG_KSCAND
/* Tracks promotion node. XXX: use nodemask */
int target_node;
+
+ /* Integer representing PTE A bit scan aggression (0-10) */
+ unsigned int pte_scan_scale;
#endif
/*
* An operation with batched TLB flushing is going on. Anything
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 43dec6eed559..6b5877865e08 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -371,4 +371,11 @@ struct prctl_mm_map {
# define PR_FUTEX_HASH_GET_SLOTS 2
# define PR_FUTEX_HASH_GET_IMMUTABLE 3

+/* Set/get PTE A bit scan scale */
+#define PR_SET_PTE_A_SCAN_SCALE 79
+#define PR_GET_PTE_A_SCAN_SCALE 80
+# define PR_PTE_A_SCAN_SCALE_MIN 0
+# define PR_PTE_A_SCAN_SCALE_MAX 10
+# define PR_PTE_A_SCAN_SCALE_DEFAULT 8
+
#endif /* _LINUX_PRCTL_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index a13043de91b0..bb780215024c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -106,6 +106,7 @@
#include <uapi/linux/pidfd.h>
#include <linux/pidfs.h>
#include <linux/tick.h>
+#include <linux/prctl.h>

#include <asm/pgalloc.h>
#include <linux/uaccess.h>
@@ -1050,6 +1051,9 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
futex_mm_init(mm);
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !defined(CONFIG_SPLIT_PMD_PTLOCKS)
mm->pmd_huge_pte = NULL;
+#endif
+#ifdef CONFIG_KSCAND
+ mm->pte_scan_scale = PR_PTE_A_SCAN_SCALE_DEFAULT;
#endif
mm_init_uprobes_state(mm);
hugetlb_count_init(mm);
diff --git a/kernel/sys.c b/kernel/sys.c
index adc0de0aa364..f6c893b22bc6 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2147,6 +2147,19 @@ static int prctl_set_auxv(struct mm_struct *mm, unsigned long addr,

return 0;
}
+#ifdef CONFIG_KSCAND
+static int prctl_pte_scan_scale_write(unsigned int scale)
+{
+ scale = clamp(scale, PR_PTE_A_SCAN_SCALE_MIN, PR_PTE_A_SCAN_SCALE_MAX);
+ current->mm->pte_scan_scale = scale;
+ return 0;
+}
+
+static unsigned int prctl_pte_scan_scale_read(void)
+{
+ return current->mm->pte_scan_scale;
+}
+#endif

static int prctl_set_mm(int opt, unsigned long addr,
unsigned long arg4, unsigned long arg5)
@@ -2824,6 +2837,18 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
case PR_FUTEX_HASH:
error = futex_hash_prctl(arg2, arg3, arg4);
break;
+#ifdef CONFIG_KSCAND
+ case PR_SET_PTE_A_SCAN_SCALE:
+ if (arg3 || arg4 || arg5)
+ return -EINVAL;
+ error = prctl_pte_scan_scale_write((unsigned int) arg2);
+ break;
+ case PR_GET_PTE_A_SCAN_SCALE:
+ if (arg2 || arg3 || arg4 || arg5)
+ return -EINVAL;
+ error = prctl_pte_scan_scale_read();
+ break;
+#endif
default:
trace_task_prctl_unknown(option, arg2, arg3, arg4, arg5);
error = -EINVAL;
diff --git a/mm/kscand.c b/mm/kscand.c
index 273306f47553..8aef6021c6ba 100644
--- a/mm/kscand.c
+++ b/mm/kscand.c
@@ -1306,6 +1306,11 @@ static unsigned long kscand_scan_mm_slot(void)
goto outerloop;
}

+ if (!mm->pte_scan_scale) {
+ next_mm = true;
+ goto outerloop;
+ }
+
mm_target_node = READ_ONCE(mm->target_node);
if (mm_target_node != mm_slot_target_node)
WRITE_ONCE(mm->target_node, mm_slot_target_node);
--
2.34.1