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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> 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> X-Rspamd-Queue-Id: 91726C000C X-Rspam-User: X-Stat-Signature: 4yupz7pasug9cjrb6p1pmtatbo5q8aex X-Rspamd-Server: rspam06 X-HE-Tag: 1787575034-748429 X-HE-Meta: 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 ENnzh6jv 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: --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--