Re: [PATCH v2 2/2] selftests/cgroup: test_zswap: fix implicit unsigned promotion bug in test_no_kmem_bypass

[Michal Koutný]

On Mon, Aug 24, 2026 at 03:33:58AM +0000, Wilson Felipe Pereira <wfelipe@xxxxxxxxxx> wrote:
> 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 declaring zswapped and delta as signed longs and comparing
> against a signed threshold, ensuring negative deltas correctly evaluate
> to true.
> 
> Fixes: a549f9f31561a ("selftests: cgroup: add test_zswap with no kmem bypass test")
> Signed-off-by: Wilson Felipe Pereira <wfelipe@xxxxxxxxxx>
> ---
>  tools/testing/selftests/cgroup/test_zswap.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
> index a7ff525c1267..15cc1dfdd1f4 100644
> --- a/tools/testing/selftests/cgroup/test_zswap.c
> +++ b/tools/testing/selftests/cgroup/test_zswap.c
> @@ -621,9 +621,11 @@ 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) {
> -			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;
> +			long zswapped = cg_read_key_long(
> +				test_group, "memory.stat", "zswapped ");
> +			long delta = stored_pages * page_size - zswapped;
> +			long max_delta = (long)(stored_pages * page_size / 4);
> +			int result_ok = delta < max_delta;

I'd rewrite it like below for higher type confidence (mainly, an explict
cast of stored_pages so that subtraction is signed; possibly, implicit
cast of expression assinged to max_delta):

			long delta = (long)stored_pages * page_size - zswapped;
			long max_delta = stored_pages * page_size / 4;


>  
>  			ret = result_ok ? KSFT_PASS : KSFT_FAIL;
			ret = (delta < max_delta) ? KSFT_PASS : KSFT_FAIL;

Thanks,
Michal

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