mm/damon: test and document power-of-2 min_region_sz requirement

Since commit c80f46ac228b ("mm/damon/core: disallow non-power of two min_region_sz"), min_region_sz is always restricted to be a power of two. Add a kunit test to confirm the functionality. Also, the change adds a restriction to addr_unit parameter. Clarify it on the document.

SeongJae Park (2):
mm/damon/tests/core-kunit: add a test for damon_commit_ctx() Docs/mm/damon/design: document the power-of-two limitation for
addr_unit

 Documentation/mm/damon/design.rst |  2 ++
 mm/damon/tests/core-kunit.h       | 22 ++++++++++++++++++++++
 2 files changed, 24 insertions(+)

base-commit: 4380f0b6370ead5000b8d155b25a86cb59d68c06

2.47.3

Add a kunit test for confirming the change that is made on commit c80f46ac228b ("mm/damon/core: disallow non-power of two min_region_sz") functions as expected.

---
 mm/damon/tests/core-kunit.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

index 44a983fca9501..65181c6dac1b4 100644
@@ -1060,6 +1060,27 @@ static void damon_test_commit_target_regions(struct kunit *test)
 			(unsigned long[][2]) {{3, 8}, {8, 10}}, 2);
 }
 
+static void damon_test_commit_ctx(struct kunit *test)
+{
+	struct damon_ctx *src, *dst;
+
+	src = damon_new_ctx();
+	if (!src)
+		kunit_skip(test, "src alloc fail");
+	dst = damon_new_ctx();
+	if (!dst) {
+		damon_destroy_ctx(src);
+		kunit_skip(test, "dst alloc fail");
+	}
+	/* Only power of two min_region_sz is allowed. */
+	src->min_region_sz = 4096;
+	KUNIT_EXPECT_EQ(test, damon_commit_ctx(dst, src), 0);
+	src->min_region_sz = 4095;
+	KUNIT_EXPECT_EQ(test, damon_commit_ctx(dst, src), -EINVAL);
+	damon_destroy_ctx(src);
+	damon_destroy_ctx(dst);
+}
+
 static void damos_test_filter_out(struct kunit *test)
 {
 	struct damon_target *t;

@@ -1316,6 +1337,7 @@ static struct kunit_case damon_test_cases[] = {
 	KUNIT_CASE(damos_test_commit_pageout),
 	KUNIT_CASE(damos_test_commit_migrate_hot),
 	KUNIT_CASE(damon_test_commit_target_regions),
+	KUNIT_CASE(damon_test_commit_ctx),
 	KUNIT_CASE(damos_test_filter_out),
 	KUNIT_CASE(damon_test_feed_loop_next_input),
 	KUNIT_CASE(damon_test_set_filters_default_reject),

2.47.3

The min_region_sz is set as max(DAMON_MIN_REGION_SZ / addr_unit, 1). DAMON_MIN_REGION_SZ is the same to PAGE_SIZE, and addr_unit is what the user can arbitrarily set. Commit c80f46ac228b ("mm/damon/core: disallow non-power of two min_region_sz") made min_region_sz to always be a power of two. Hence, addr_unit should be a power of two when it is smaller than PAGE_SIZE. While 'addr_unit' is a user-exposed parameter, the rule is not documented. This can confuse users. Specifically, if the user sets addr_unit as a value that is smaller than PAGE_SIZE and not a power of two, the setup will explicitly fail.

Document the rule on the design document. Usage documents reference the design document for detail, so updating only the design document should suffice.

---
 Documentation/mm/damon/design.rst | 2 ++
 1 file changed, 2 insertions(+)

index 28d932ceaf7ed..29fff20b3c2a9 100644
@@ -150,6 +150,8 @@ address on the given address space.  Support of ``address unit`` parameter is
 up to each operations set implementation.  ``paddr`` is the only operations set
 implementation that supports the parameter.
 
+If the value is smaller than ``PAGE_SIZE``, only a power of two should be used.
+
 .. _damon_core_logic:
 
 Core Logics

2.47.3