Add basic KUnit tests for the generic aspects of the lazy MMU mode:
ensure that it appears active when it should, depending on how
enable/disable and pause/resume pairs are nested.

Signed-off-by: Kevin Brodsky <redacted>
---
  mm/Kconfig                     | 12 ++++++
  mm/Makefile                    |  1 +
  mm/tests/lazy_mmu_mode_kunit.c | 71 ++++++++++++++++++++++++++++++++++
  3 files changed, 84 insertions(+)
  create mode 100644 mm/tests/lazy_mmu_mode_kunit.c

diff --git a/mm/Kconfig b/mm/Kconfig
index 62073bd61544..ac48deb44884 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig

@@ -1471,6 +1471,18 @@ config ARCH_HAS_LAZY_MMU_MODE
  	  MMU-related architectural state to be deferred until the mode is
  	  exited. See <linux/pgtable.h> for details.
  
+config LAZY_MMU_MODE_KUNIT_TEST
+	tristate "KUnit tests for the lazy MMU mode" if !KUNIT_ALL_TESTS
+	depends on ARCH_HAS_LAZY_MMU_MODE
+	depends on KUNIT
+	default KUNIT_ALL_TESTS
+	help
+	  Enable this option to check that the lazy MMU mode interface behaves
+	  as expected. Only tests for the generic interface are included (not
+	  architecture-specific behaviours).
+
+	  If unsure, say N.
+
  source "mm/damon/Kconfig"
  
  endmenu

diff --git a/mm/Makefile b/mm/Makefile
index 2d0570a16e5b..9175f8cc6565 100644
--- a/mm/Makefile
+++ b/mm/Makefile

@@ -147,3 +147,4 @@ obj-$(CONFIG_SHRINKER_DEBUG) += shrinker_debug.o
  obj-$(CONFIG_EXECMEM) += execmem.o
  obj-$(CONFIG_TMPFS_QUOTA) += shmem_quota.o
  obj-$(CONFIG_PT_RECLAIM) += pt_reclaim.o
+obj-$(CONFIG_LAZY_MMU_MODE_KUNIT_TEST) += tests/lazy_mmu_mode_kunit.o

diff --git a/mm/tests/lazy_mmu_mode_kunit.c b/mm/tests/lazy_mmu_mode_kunit.c
new file mode 100644
index 000000000000..2720eb995714
--- /dev/null
+++ b/mm/tests/lazy_mmu_mode_kunit.c

@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <kunit/test.h>
+#include <linux/pgtable.h>
+
+static void expect_not_active(struct kunit *test)
+{
+	KUNIT_EXPECT_FALSE(test, is_lazy_mmu_mode_active());
+}
+
+static void expect_active(struct kunit *test)
+{
+	KUNIT_EXPECT_TRUE(test, is_lazy_mmu_mode_active());
+}
+
+static void lazy_mmu_mode_active(struct kunit *test)
+{
+	expect_not_active(test);
+
+	lazy_mmu_mode_enable();
+	expect_active(test);
+
+	{
+		/* Nested section */
+		lazy_mmu_mode_enable();
+		expect_active(test);
+
+		lazy_mmu_mode_disable();
+		expect_active(test);
+	}
+
+	{
+		/* Paused section */
+		lazy_mmu_mode_pause();
+		expect_not_active(test);
+
+		{
+			/* No effect (paused) */
+			lazy_mmu_mode_enable();
+			expect_not_active(test);
+
+			lazy_mmu_mode_disable();
+			expect_not_active(test);
+
+			lazy_mmu_mode_pause();
+			expect_not_active(test);
+
+			lazy_mmu_mode_resume();
+			expect_not_active(test);
+		}
+
+		lazy_mmu_mode_resume();
+		expect_active(test);
+	}
+
+	lazy_mmu_mode_disable();
+	expect_not_active(test);
+}
+
+static struct kunit_case lazy_mmu_mode_test_cases[] = {
+	KUNIT_CASE(lazy_mmu_mode_active),
+	{}
+};
+
+static struct kunit_suite lazy_mmu_mode_test_suite = {
+	.name = "lazy_mmu_mode",
+	.test_cases = lazy_mmu_mode_test_cases,
+};
+kunit_test_suite(lazy_mmu_mode_test_suite);
+
+MODULE_DESCRIPTION("Tests for the lazy MMU mode");
+MODULE_LICENSE("GPL");