Re: [PATCH v3 2/2] selftests/cgroup: test_zswap: fix implicit unsigned promotion bug in test_no_kmem_bypass

[Michal Koutný]

On Thu, Aug 27, 2026 at 03:47:42AM +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 | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)

Thanks!

Acked-by: Michal Koutný <mkoutny@xxxxxxxx>