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Thu, 06 Aug 2026 01:32:51 -0700 (PDT) Date: Thu, 6 Aug 2026 10:32:49 +0200 From: Michal =?utf-8?Q?Koutn=C3=BD?= To: Wilson Felipe Pereira Cc: 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 Subject: Re: [PATCH 2/2] selftests/cgroup: test_zswap: fix implicit unsigned promotion bug in test_no_kmem_bypass Message-ID: References: <20260804042053.56940-1-wfelipe@google.com> <20260804042053.56940-3-wfelipe@google.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="lcbrsqi5muuudvb7" Content-Disposition: inline In-Reply-To: <20260804042053.56940-3-wfelipe@google.com> X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: m6wqgwscm4py87srn7h3okizo5uruys7 X-Rspamd-Queue-Id: 7E60220005 X-HE-Tag: 1786005173-440471 X-HE-Meta: U2FsdGVkX1+wyhQ0fPl61w7EPn7loOSF8Sl2wzbXHvmdfYU6dAdlOVbcfGF7iL2tVfscCDIYxyTiEz3oS+uTB1btaXzxHgiJYDno4FULHg6LdlJn6PGypqs0tCS0Fb94/R0dLg6YJOZCfortOTtJLs1MH+oOFfzrZ9yMOXB/VzjEeLQMasJUWOasi4Za9nbMmgKo+fP736srYvXjbQhLzxqCSrsNSQ25iJPOzcTSNTbpnDkKnwc6IIbwLdv9JdVXYQGNb326ty4CsCkXnJOtRltbwyDLzy3IRRIgX11BHk4RvhaMEg6DDmit23WNcgmK4BkvDRKcm7Cg/hf5Oijqv0Z17Yi03Pgrv0OHf7AvBztrB51aTVRR9iLJmHP6oPxhIBF2AZuValam7OzXAGKG8ltvknj4MPlZAz/dp8sqjDJvp/MMBz3qvpTDQFWVsM8FQKpU+/mzluTTvEnk7zJ+hEeXNXS3MypNWlrC1suxCjp2f8VrE+iD2XRDpd9DJZogu2/dfsvx2Fpq3GQ9BJM6g6Sgoc4hamVale3ml8z8kUtV3bxFAv1vcSwCyAgnCgiD+TjHAjKT5mVXk6LeKyjFqIpMtbCc1E/mL3TV2CuOuOF/iV6ycMgiZJOA8iff+eIqkBluSfjl5ew1Szl6XEgoDH1lrKdSTBbMBs9Mh5DFhRQKjKPM/2Tj7S4bprHJgIV22735Qrh1kqyOmENKqy4/l9ddE0lKltFJq1wGWWKxAdp7FvwvJimvynZMx+kJzSh3v5UUCjXmEE76gNJcaVJPYkC1nJn36kwCltbUxBoQUYaHRzxNIIpHmhm3ydifPnek301NZ9YD4hcEfKzGE54kxUVcqknZ7IUrPN/IXz+NxY1kjvxQAA7j8P9OoQFp1FzZwxow2R8HiDQIfkcG/2YT6fL4y0yEO0Hyqdk6VtrOsMvBxij96q3jC9x1UdCQtQo4T5SmK67lxHy4A97wcof 18JnVW2C 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: --lcbrsqi5muuudvb7 Content-Type: text/plain; protected-headers=v1; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Subject: Re: [PATCH 2/2] selftests/cgroup: test_zswap: fix implicit unsigned promotion bug in test_no_kmem_bypass MIME-Version: 1.0 On Tue, Aug 04, 2026 at 04:20:45AM +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. Thanks for the breakdown. > Fix this by checking abs(delta), ensuring the test correctly compares the > absolute difference between system zswap and cgroup zswapped bytes. I still think the delta quantity has some meaning here and taking the abs() changes the semantics of the checked inequality. What about making both zswapped and delta `long`s? (Under similar reasoning, there's already a possible loss after squashing cg_read_key_long() result into the `int`.) In any way, also Fixes: a549f9f31561a ("selftests: cgroup: add test_zswap with no kmem bypas= s test") Thanks, Michal --lcbrsqi5muuudvb7 Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iJEEABYKADkWIQRCE24Fn/AcRjnLivR+PQLnlNv4CAUCanRGrBsUgAAAAAAEAA5t YW51MiwyLjUrMS4xMiwyLDIACgkQfj0C55Tb+AjG7wEAubwExH9IrXsRlyXqhce7 oitcuCe9bJ98x6HC+nOhwGYBAP1BPxXMx8oSYQJbEG2gUmqt3VXQUyW00jKZMzWe GagA =B29m -----END PGP SIGNATURE----- --lcbrsqi5muuudvb7--