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Mon, 24 Aug 2026 05:37:12 -0700 (PDT) Date: Mon, 24 Aug 2026 14:37:11 +0200 From: Michal =?utf-8?Q?Koutn=C3=BD?= To: Wilson Felipe Pereira Cc: Andrew Morton , Johannes Weiner , Yosry Ahmed , Nhat Pham , Chengming Zhou , Tejun Heo , Shuah Khan , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 2/2] selftests/cgroup: test_zswap: fix implicit unsigned promotion bug in test_no_kmem_bypass Message-ID: References: <20260824033533.2147900-1-wfelipe@google.com> <20260824033533.2147900-3-wfelipe@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="pfcylgei2cxs7jeq" Content-Disposition: inline In-Reply-To: <20260824033533.2147900-3-wfelipe@google.com> --pfcylgei2cxs7jeq Content-Type: text/plain; protected-headers=v1; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Subject: Re: [PATCH v2 2/2] selftests/cgroup: test_zswap: fix implicit unsigned promotion bug in test_no_kmem_bypass MIME-Version: 1.0 On Mon, Aug 24, 2026 at 03:33:58AM +0000, Wilson Felipe Pereira 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. >=20 > Due to slight stat update timing differences, delta can evaluate to a sma= ll > 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 evalua= te > to 0 and fail the test. >=20 > Fix this by declaring zswapped and delta as signed longs and comparing > against a signed threshold, ensuring negative deltas correctly evaluate > to true. >=20 > Fixes: a549f9f31561a ("selftests: cgroup: add test_zswap with no kmem byp= ass test") > Signed-off-by: Wilson Felipe Pereira > --- > tools/testing/selftests/cgroup/test_zswap.c | 8 +++++--- > 1 file changed, 5 insertions(+), 3 deletions(-) >=20 > 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 =3D cg_read_key_long(test_group, "memory.stat", "zswappe= d "); > - int delta =3D stored_pages * page_size - zswapped; > - int result_ok =3D delta < stored_pages * page_size / 4; > + long zswapped =3D cg_read_key_long( > + test_group, "memory.stat", "zswapped "); > + long delta =3D stored_pages * page_size - zswapped; > + long max_delta =3D (long)(stored_pages * page_size / 4); > + int result_ok =3D 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 =3D (long)stored_pages * page_size - zswapped; long max_delta =3D stored_pages * page_size / 4; > =20 > ret =3D result_ok ? KSFT_PASS : KSFT_FAIL; ret =3D (delta < max_delta) ? KSFT_PASS : KSFT_FAIL; Thanks, Michal --pfcylgei2cxs7jeq Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iJEEABYKADkWIQRCE24Fn/AcRjnLivR+PQLnlNv4CAUCaow68xsUgAAAAAAEAA5t YW51MiwyLjUrMS4xMiwyLDIACgkQfj0C55Tb+Ah3wQEAkD4CY79HnHFL3zUOh2xO fMcsSq7vsvnmYX7s4mYh8CsBAMi2iZwoDAROyEEUXSL3lUZmgimfzFx6ViqPPxyO Z8UD =UNwY -----END PGP SIGNATURE----- --pfcylgei2cxs7jeq--