[PATCH RFC v2 3/4] mm/ksm: introduce force_madvise knob

[Oleksandr Natalenko <oleksandr@redhat.com>]

Present a new sysfs knob to mark task's anonymous memory as mergeable.

To force merging task's VMAs, its PID is echoed in a write-only file:

   # echo PID > /sys/kernel/mm/ksm/force_madvise

Force unmerging is done similarly, but with "minus" sign:

   # echo -PID > /sys/kernel/mm/ksm/force_madvise

"0" or "-0" can be used to control the current task.

To achieve this, previously introduced ksm_enter()/ksm_leave() helpers
are used in the "store" handler.

Signed-off-by: Oleksandr Natalenko <oleksandr@redhat.com>
---
 mm/ksm.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)

diff --git a/mm/ksm.c b/mm/ksm.c
index e9f3901168bb..22c59fb03d3a 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2879,10 +2879,77 @@ static void wait_while_offlining(void)
 
 #define KSM_ATTR_RO(_name) \
 	static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
+#define KSM_ATTR_WO(_name) \
+	static struct kobj_attribute _name##_attr = __ATTR_WO(_name)
 #define KSM_ATTR(_name) \
 	static struct kobj_attribute _name##_attr = \
 		__ATTR(_name, 0644, _name##_show, _name##_store)
 
+static ssize_t force_madvise_store(struct kobject *kobj,
+				     struct kobj_attribute *attr,
+				     const char *buf, size_t count)
+{
+	int err;
+	pid_t pid;
+	bool merge = true;
+	struct task_struct *tsk;
+	struct mm_struct *mm;
+	struct vm_area_struct *vma;
+
+	err = kstrtoint(buf, 10, &pid);
+	if (err)
+		return -EINVAL;
+
+	if (pid < 0) {
+		pid = abs(pid);
+		merge = false;
+	}
+
+	if (!pid && *buf == '-')
+		merge = false;
+
+	rcu_read_lock();
+	if (pid) {
+		tsk = find_task_by_vpid(pid);
+		if (!tsk) {
+			err = -ESRCH;
+			rcu_read_unlock();
+			goto out;
+		}
+	} else {
+		tsk = current;
+	}
+
+	tsk = tsk->group_leader;
+
+	get_task_struct(tsk);
+	rcu_read_unlock();
+
+	mm = get_task_mm(tsk);
+	if (!mm) {
+		err = -EINVAL;
+		goto out_put_task_struct;
+	}
+	down_write(&mm->mmap_sem);
+	vma = mm->mmap;
+	while (vma) {
+		if (merge)
+			ksm_enter(vma->vm_mm, vma, &vma->vm_flags);
+		else
+			ksm_leave(vma, vma->vm_start, vma->vm_end, &vma->vm_flags);
+		vma = vma->vm_next;
+	}
+	up_write(&mm->mmap_sem);
+	mmput(mm);
+
+out_put_task_struct:
+	put_task_struct(tsk);
+
+out:
+	return err ? err : count;
+}
+KSM_ATTR_WO(force_madvise);
+
 static ssize_t sleep_millisecs_show(struct kobject *kobj,
 				    struct kobj_attribute *attr, char *buf)
 {
@@ -3185,6 +3252,7 @@ static ssize_t full_scans_show(struct kobject *kobj,
 KSM_ATTR_RO(full_scans);
 
 static struct attribute *ksm_attrs[] = {
+	&force_madvise_attr.attr,
 	&sleep_millisecs_attr.attr,
 	&pages_to_scan_attr.attr,
 	&run_attr.attr,
-- 
2.21.0