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Tue, 01 Sep 2026 01:57:05 -0700 (PDT) Date: Tue, 1 Sep 2026 10:57:03 +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 v5 2/2] selftests/cgroup: test_zswap: fix implicit unsigned promotion bug in test_no_kmem_bypass Message-ID: References: <20260901052250.154532-1-wfelipe@google.com> <20260901052250.154532-3-wfelipe@google.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="nuwu7tagmytj5izv" Content-Disposition: inline In-Reply-To: <20260901052250.154532-3-wfelipe@google.com> X-Stat-Signature: kcpbnh1b45e54xztfmiw3xfyg99ihadt X-Rspamd-Queue-Id: DF8A640005 X-Rspamd-Server: rspam02 X-Rspam-User: X-HE-Tag: 1788253027-33416 X-HE-Meta: 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 S5kmK6EW 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: --nuwu7tagmytj5izv Content-Type: text/plain; protected-headers=v1; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Subject: Re: [PATCH v5 2/2] selftests/cgroup: test_zswap: fix implicit unsigned promotion bug in test_no_kmem_bypass MIME-Version: 1.0 On Tue, Sep 01, 2026 at 05:22:28AM +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 long long 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 > Acked-by: Michal Koutn=FD (The long long helper is a non-trivial change, so the ack should be stripped on this version (unless you can convince me ;-).) I'm afraid this bitness propagated too broadly. The long vs long long difference is only relevant on 32b, right? And the bit-width issue only appears because of the product, not the values read from memory.stat [1]. I'd consider three ways forward: a) Consolidate the helpers into one cg_read_key_s64() (same explicit size regardless of arch), b) keep single arch-dependent helper and tackle product(s) only, c) do not bother with 32b in these selftests (I have a hunch that they're not so relevant on such archs). WDYT? Michal [1] At least the values derived from available memory, memory events could grow indefinitely. It's true that kernel uses explicit u64 for all of the entries. --nuwu7tagmytj5izv Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iJEEABYKADkWIQRCE24Fn/AcRjnLivR+PQLnlNv4CAUCapaTXBsUgAAAAAAEAA5t YW51MiwyLjUrMS4xMiwyLDIACgkQfj0C55Tb+Ag6rAD9F+z0oXfuQAC+NJQ52/69 Wy9h9jBokjfFxG+tYku4HusA/A3LjZsf4+90ciLSyVy6BFPvXQx3uN958bN5VesH mjUK =8Zx/ -----END PGP SIGNATURE----- --nuwu7tagmytj5izv--