On Mon, Aug 24, 2026 at 03:33:58AM +0000, Wilson Felipe Pereira <[email protected]> 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 <[email protected]> > --- > 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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