[PATCH v6 3/3] selftests/cgroup: test_zswap: fix implicit unsigned promotion bug in test_no_kmem_bypass

[Wilson Felipe Pereira <wfelipe@google.com>]

In test_no_kmem_bypass(), delta (stored_pages * page_size - zswapped) is
checked against stored_pages * page_size / 4 to verify that the pages
pushed to zswap belong to the test memory cgroup.

Due to slight stat update timing differences, delta can evaluate to a small
negative number (e.g. -5MB out of 1GB). Because delta was declared as a
signed int and stored_pages is an unsigned size_t, C's usual arithmetic
conversions implicitly promote a negative delta to a large unsigned 64-bit
integer, causing `delta < stored_pages * page_size / 4` to falsely evaluate
to 0 and fail the test.

Fix this by declaring delta and max_delta as signed 64-bit values (s64),
explicitly casting stored_pages, and comparing against a signed threshold,
ensuring negative deltas correctly evaluate to true.

Fixes: a549f9f31561 ("selftests: cgroup: add test_zswap with no kmem bypass test")
Signed-off-by: Wilson Felipe Pereira <wfelipe@google.com>
---
 tools/testing/selftests/cgroup/test_zswap.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index f4ee440b3c66..6e7ee11bc97a 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -632,11 +632,12 @@ static int test_no_kmem_bypass(const char *root)
 			break;
 		/* If memory was pushed to zswap, verify it belongs to memcg */
 		if (stored_pages > stored_pages_threshold) {
-			s64 zswapped = cg_read_key_s64(test_group, "memory.stat", "zswapped ");
-			int delta = stored_pages * page_size - zswapped;
-			int result_ok = delta < stored_pages * page_size / 4;
+			s64 zswapped = cg_read_key_s64(
+				test_group, "memory.stat", "zswapped ");
+			s64 delta = (s64)stored_pages * page_size - zswapped;
+			s64 max_delta = (s64)stored_pages * page_size / 4;
 
-			ret = result_ok ? KSFT_PASS : KSFT_FAIL;
+			ret = (delta < max_delta) ? KSFT_PASS : KSFT_FAIL;
 			break;
 		}
 	}
-- 
2.55.0.979.g7e5102b832-goog