samples/damon/wsse: start and stop DAMON as the user requests

Start running DAMON to monitor accesses of a process that the user
specified via 'target_pid' parameter, when 'y' is passed to 'enable'
parameter.  Stop running DAMON when 'n' is passed to 'enable' parameter. 
Estimating the working set size from DAMON's monitoring results and
reporting it to the user will be implemented by the following commit.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: SeongJae Park <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

samples/damon/wsse.c

@@ -27,15 +27,48 @@ static bool enable __read_mostly;
 module_param_cb(enable, &enable_param_ops, &enable, 0600);
 MODULE_PARM_DESC(enable, "Enable or disable DAMON_SAMPLE_WSSE");
 
+static struct damon_ctx *ctx;
+static struct pid *target_pidp;
+
 static int damon_sample_wsse_start(void)
 {
+	struct damon_target *target;
+
 	pr_info("start\n");
-	return 0;
+
+	ctx = damon_new_ctx();
+	if (!ctx)
+		return -ENOMEM;
+	if (damon_select_ops(ctx, DAMON_OPS_VADDR)) {
+		damon_destroy_ctx(ctx);
+		return -EINVAL;
+	}
+
+	target = damon_new_target();
+	if (!target) {
+		damon_destroy_ctx(ctx);
+		return -ENOMEM;
+	}
+	damon_add_target(ctx, target);
+	target_pidp = find_get_pid(target_pid);
+	if (!target_pidp) {
+		damon_destroy_ctx(ctx);
+		return -EINVAL;
+	}
+	target->pid = target_pidp;
+
+	return damon_start(&ctx, 1, true);
 }
 
 static void damon_sample_wsse_stop(void)
 {
 	pr_info("stop\n");
+	if (ctx) {
+		damon_stop(&ctx, 1);
+		damon_destroy_ctx(ctx);
+	}
+	if (target_pidp)
+		put_pid(target_pidp);
 }
 
 static int damon_sample_wsse_enable_store(