mm/damon: hide kdamond and kdamond_lock from API callers

index 26fb8e90dff6..5b7ea7082134 100644
@@ -972,6 +972,7 @@ bool damon_initialized(void);
 int damon_start(struct damon_ctx **ctxs, int nr_ctxs, bool exclusive);
 int damon_stop(struct damon_ctx **ctxs, int nr_ctxs);
 bool damon_is_running(struct damon_ctx *ctx);
+int damon_kdamond_pid(struct damon_ctx *ctx);
 
 int damon_call(struct damon_ctx *ctx, struct damon_call_control *control);
 int damos_walk(struct damon_ctx *ctx, struct damos_walk_control *control);

index 729a5f7fac94..81b998d32074 100644
@@ -1442,6 +1442,23 @@ bool damon_is_running(struct damon_ctx *ctx)
 	return running;
 }
 
+/**
+ * damon_kdamond_pid() - Return pid of a given DAMON context's worker thread.
+ * @ctx:	The DAMON context of the question.
+ *
+ * Return: pid if @ctx is running, negative error code otherwise.
+ */
+int damon_kdamond_pid(struct damon_ctx *ctx)
+{
+	int pid = -EINVAL;
+
+	mutex_lock(&ctx->kdamond_lock);
+	if (ctx->kdamond)
+		pid = ctx->kdamond->pid;
+	mutex_unlock(&ctx->kdamond_lock);
+	return pid;
+}
+
 /*
  * damon_call_handle_inactive_ctx() - handle DAMON call request that added to
  *				      an inactive context.

index 95fd9375a7d8..4de25708b05a 100644
@@ -1819,10 +1819,9 @@ static ssize_t pid_show(struct kobject *kobj,
 	if (!ctx)
 		goto out;
 
-	mutex_lock(&ctx->kdamond_lock);
-	if (ctx->kdamond)
-		pid = ctx->kdamond->pid;
-	mutex_unlock(&ctx->kdamond_lock);
+	pid = damon_kdamond_pid(ctx);
+	if (pid < 0)
+		pid = -1;
 out:
 	mutex_unlock(&damon_sysfs_lock);
 	return sysfs_emit(buf, "%d\n", pid);

index 8296f984b428..bedb9134d286 100644
@@ -405,7 +405,9 @@ static int damon_lru_sort_turn(bool on)
 	err = damon_start(&ctx, 1, true);
 	if (err)
 		return err;
-	kdamond_pid = ctx->kdamond->pid;
+	kdamond_pid = damon_kdamond_pid(ctx);
+	if (kdamond_pid < 0)
+		return kdamond_pid;
 	return damon_call(ctx, &call_control);
 }
 

index 8463a5a5032f..55df43e241c5 100644
@@ -307,7 +307,9 @@ static int damon_reclaim_turn(bool on)
 	err = damon_start(&ctx, 1, true);
 	if (err)
 		return err;
-	kdamond_pid = ctx->kdamond->pid;
+	kdamond_pid = damon_kdamond_pid(ctx);
+	if (kdamond_pid < 0)
+		return kdamond_pid;
 	return damon_call(ctx, &call_control);
 }
 

index 5b7ea7082134..e6930d8574d3 100644
@@ -759,23 +759,20 @@ struct damon_attrs {
  * of the monitoring.
  *
  * @attrs:		Monitoring attributes for accuracy/overhead control.
- * @kdamond:		Kernel thread who does the monitoring.
- * @kdamond_lock:	Mutex for the synchronizations with @kdamond.
  *
- * For each monitoring context, one kernel thread for the monitoring is
- * created.  The pointer to the thread is stored in @kdamond.
+ * For each monitoring context, one kernel thread for the monitoring, namely
+ * kdamond, is created.  The pid of kdamond can be retrieved using
+ * damon_kdamond_pid().
  *
- * Once started, the monitoring thread runs until explicitly required to be
- * terminated or every monitoring target is invalid.  The validity of the
- * targets is checked via the &damon_operations.target_valid of @ops.  The
- * termination can also be explicitly requested by calling damon_stop().
- * The thread sets @kdamond to NULL when it terminates. Therefore, users can
- * know whether the monitoring is ongoing or terminated by reading @kdamond.
- * Reads and writes to @kdamond from outside of the monitoring thread must
- * be protected by @kdamond_lock.
+ * Once started, kdamond runs until explicitly required to be terminated or
+ * every monitoring target is invalid.  The validity of the targets is checked
+ * via the &damon_operations.target_valid of @ops.  The termination can also be
+ * explicitly requested by calling damon_stop().  To know if a kdamond is
+ * running, damon_is_running() can be used.
  *
- * Note that the monitoring thread protects only @kdamond via @kdamond_lock.
- * Accesses to other fields must be protected by themselves.
+ * While the kdamond is running, all accesses to &struct damon_ctx from a
+ * thread other than the kdamond should be made using safe DAMON APIs,
+ * including damon_call() and damos_walk().
  *
  * @ops:	Set of monitoring operations for given use cases.
  * @addr_unit:	Scale factor for core to ops address conversion.

@@ -816,10 +813,12 @@ struct damon_ctx {
 	struct damos_walk_control *walk_control;
 	struct mutex walk_control_lock;
 
-/* public: */
+	/* Working thread of the given DAMON context */
 	struct task_struct *kdamond;
+	/* Protects @kdamond field access */
 	struct mutex kdamond_lock;
 
+/* public: */
 	struct damon_operations ops;
 	unsigned long addr_unit;
 	unsigned long min_sz_region;