[PATCH 2/2] 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 is 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 checking abs(delta), ensuring the test correctly compares the
absolute difference between system zswap and cgroup zswapped bytes.

Signed-off-by: Wilson Felipe Pereira <wfelipe@google.com>
---
 tools/testing/selftests/cgroup/test_zswap.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index 312d51887a1d8..7bc2519862f7e 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -624,7 +624,8 @@ static int test_no_kmem_bypass(const char *root)
 		if (stored_pages > stored_pages_threshold) {
 			int zswapped = cg_read_key_long(test_group, "memory.stat", "zswapped ");
 			int delta = stored_pages * page_size - zswapped;
-			int result_ok = delta < stored_pages * page_size / 4;
+			int result_ok = abs(delta) <
+					stored_pages * page_size / 4;
 
 			ret = result_ok ? KSFT_PASS : KSFT_FAIL;
 			break;
-- 
2.55.0.571.g244d577d93-goog