mm/damon/sysfs-test: add a unit test for damon_sysfs_set_targets()

damon_sysfs_set_targets() had a bug that can result in unexpected memory
usage and monitoring overhead increase.  The bug has fixed by a previous
commit.  Add a unit test for avoiding a similar bug of future.

Link: https://lkml.kernel.org/r/20231022210735.46409-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/damon/Kconfig

@@ -59,6 +59,18 @@ config DAMON_SYSFS
 	  This builds the sysfs interface for DAMON.  The user space can use
 	  the interface for arbitrary data access monitoring.
 
+config DAMON_SYSFS_KUNIT_TEST
+	bool "Test for damon debugfs interface" if !KUNIT_ALL_TESTS
+	depends on DAMON_SYSFS && KUNIT=y
+	default KUNIT_ALL_TESTS
+	help
+	  This builds the DAMON sysfs interface Kunit test suite.
+
+	  For more information on KUnit and unit tests in general, please refer
+	  to the KUnit documentation.
+
+	  If unsure, say N.
+
 config DAMON_DBGFS
 	bool "DAMON debugfs interface (DEPRECATED!)"
 	depends on DAMON_VADDR && DAMON_PADDR && DEBUG_FS

mm/damon/sysfs-test.h

@@ -0,0 +1,86 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Data Access Monitor Unit Tests
+ *
+ * Author: SeongJae Park <sj@kernel.org>
+ */
+
+#ifdef CONFIG_DAMON_SYSFS_KUNIT_TEST
+
+#ifndef _DAMON_SYSFS_TEST_H
+#define _DAMON_SYSFS_TEST_H
+
+#include <kunit/test.h>
+
+static unsigned int nr_damon_targets(struct damon_ctx *ctx)
+{
+	struct damon_target *t;
+	unsigned int nr_targets = 0;
+
+	damon_for_each_target(t, ctx)
+		nr_targets++;
+
+	return nr_targets;
+}
+
+static int __damon_sysfs_test_get_any_pid(int min, int max)
+{
+	struct pid *pid;
+	int i;
+
+	for (i = min; i <= max; i++) {
+		pid = find_get_pid(i);
+		if (pid) {
+			put_pid(pid);
+			return i;
+		}
+	}
+	return -1;
+}
+
+static void damon_sysfs_test_set_targets(struct kunit *test)
+{
+	struct damon_sysfs_targets *sysfs_targets;
+	struct damon_sysfs_target *sysfs_target;
+	struct damon_ctx *ctx;
+
+	sysfs_targets = damon_sysfs_targets_alloc();
+	sysfs_targets->nr = 1;
+	sysfs_targets->targets_arr = kmalloc_array(1,
+			sizeof(*sysfs_targets->targets_arr), GFP_KERNEL);
+
+	sysfs_target = damon_sysfs_target_alloc();
+	sysfs_target->pid = __damon_sysfs_test_get_any_pid(12, 100);
+	sysfs_target->regions = damon_sysfs_regions_alloc();
+	sysfs_targets->targets_arr[0] = sysfs_target;
+
+	ctx = damon_new_ctx();
+
+	damon_sysfs_set_targets(ctx, sysfs_targets);
+	KUNIT_EXPECT_EQ(test, 1u, nr_damon_targets(ctx));
+
+	sysfs_target->pid = __damon_sysfs_test_get_any_pid(
+			sysfs_target->pid + 1, 200);
+	damon_sysfs_set_targets(ctx, sysfs_targets);
+	KUNIT_EXPECT_EQ(test, 1u, nr_damon_targets(ctx));
+
+	damon_destroy_ctx(ctx);
+	kfree(sysfs_targets->targets_arr);
+	kfree(sysfs_targets);
+	kfree(sysfs_target);
+}
+
+static struct kunit_case damon_sysfs_test_cases[] = {
+	KUNIT_CASE(damon_sysfs_test_set_targets),
+	{},
+};
+
+static struct kunit_suite damon_sysfs_test_suite = {
+	.name = "damon-sysfs",
+	.test_cases = damon_sysfs_test_cases,
+};
+kunit_test_suite(damon_sysfs_test_suite);
+
+#endif /* _DAMON_SYSFS_TEST_H */
+
+#endif /* CONFIG_DAMON_SYSFS_KUNIT_TEST */

mm/damon/sysfs.c

@@ -1836,3 +1836,5 @@ out:
 	return err;
 }
 subsys_initcall(damon_sysfs_init);
+
+#include "sysfs-test.h"