mm/damon/tests/core-kunit: skip damon_test_nr_accesses_to_accesses_bp() if aggr_interval is zero

The aggregation interval of test purpose damon_attrs for
damon_test_nr_accesses_to_accesses_bp() becomes zero on 32 bit
architecture, since size of int and long types are same.  As a result,
damon_nr_accesses_to_accesses_bp() call with the test data triggers
divide-by-zero exception.  damon_nr_accesses_to_accesses_bp() shouldn't
be called with such data, and the non-test code avoids that by checking
the case on damon_update_monitoring_results().  Skip the test code in
the case, and add an explicit caution of the case on the comment for the
test target function.

Link: https://lkml.kernel.org/r/20240905162423.74053-1-sj@kernel.org
Fixes: 5e06ad59 ("mm/damon/core-test: test max_nr_accesses overflow caused divide-by-zero")
Signed-off-by: SeongJae Park <sj@kernel.org>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Closes: https://lore.kernel.org/c771b962-a58f-435b-89e4-1211a9323181@roeck-us.net
Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: David Gow <davidgow@google.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/damon/core.c

@@ -552,7 +552,13 @@ static unsigned int damon_accesses_bp_to_nr_accesses(
 	return accesses_bp * damon_max_nr_accesses(attrs) / 10000;
 }
 
-/* convert nr_accesses to access ratio in bp (per 10,000) */
+/*
+ * Convert nr_accesses to access ratio in bp (per 10,000).
+ *
+ * Callers should ensure attrs.aggr_interval is not zero, like
+ * damon_update_monitoring_results() does .  Otherwise, divide-by-zero would
+ * happen.
+ */
 static unsigned int damon_nr_accesses_to_accesses_bp(
 		unsigned int nr_accesses, struct damon_attrs *attrs)
 {

mm/damon/tests/core-kunit.h

@@ -320,6 +320,18 @@ static void damon_test_nr_accesses_to_accesses_bp(struct kunit *test)
 		.aggr_interval = ((unsigned long)UINT_MAX + 1) * 10
 	};
 
+	/*
+	 * In some cases such as 32bit architectures where UINT_MAX is
+	 * ULONG_MAX, attrs.aggr_interval becomes zero.  Calling
+	 * damon_nr_accesses_to_accesses_bp() in the case will cause
+	 * divide-by-zero.  Such case is prohibited in normal execution since
+	 * the caution is documented on the comment for the function, and
+	 * damon_update_monitoring_results() does the check.  Skip the test in
+	 * the case.
+	 */
+	if (!attrs.aggr_interval)
+		kunit_skip(test, "aggr_interval is zero.");
+
 	KUNIT_EXPECT_EQ(test, damon_nr_accesses_to_accesses_bp(123, &attrs), 0);
 }