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Wed, 17 Jun 2026 05:27:07 -0700 (PDT) Received: from localhost.localdomain ([62.77.90.70]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49230a9b45bsm130662975e9.15.2026.06.17.05.27.06 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 17 Jun 2026 05:27:07 -0700 (PDT) Date: Wed, 17 Jun 2026 14:27:05 +0200 From: Michal =?utf-8?Q?Koutn=C3=BD?= To: Li Wang Cc: akpm@linux-foundation.org, tj@kernel.org, longman@redhat.com, roman.gushchin@linux.dev, hannes@cmpxchg.org, yosry@kernel.org, jiayuan.chen@linux.dev, nphamcs@gmail.com, chengming.zhou@linux.dev, shuah@kernel.org, linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, Michal Hocko , Muchun Song , Shakeel Butt , Yosry Ahmed Subject: Re: [PATCH v7 6/8] selftest/cgroup: fix zswap test_no_invasive_cgroup_shrink on large pagesize system Message-ID: References: <20260424040059.12940-1-li.wang@linux.dev> <20260424040059.12940-7-li.wang@linux.dev> MIME-Version: 1.0 Content-Type: multipart/signed; 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protected-headers=v1; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Subject: Re: [PATCH v7 6/8] selftest/cgroup: fix zswap test_no_invasive_cgroup_shrink on large pagesize system MIME-Version: 1.0 On Fri, Apr 24, 2026 at 12:00:57PM +0800, Li Wang wrote: > test_no_invasive_cgroup_shrink sets up two cgroups: wb_group, which is > expected to trigger zswap writeback, and a control group (renamed to > zw_group), Aha, it should stand for zswap writeback? Then zwb_group to avoid (my) confusion with zsw_group :-) Although the original names were already well descriptive (both groups are expected to have some zswap). > which should only have pages sitting in zswap without any > writeback. >=20 > There are two problems with the current test: >=20 > 1) The data patterns are reversed. wb_group uses allocate_bytes(), which > writes only a single byte per page =E2=80=94 trivially compressible, > especially by zstd =E2=80=94 so compressed pages fit within zswap.max = and > writeback is never triggered. Meanwhile, the control group uses > getrandom() to produce hard-to-compress data, but it is the group > that does *not* need writeback. >=20 > 2) The test uses fixed sizes (10K zswap.max, 10MB allocation) that are > too small on systems with large PAGE_SIZE (e.g. 64K), failing to > build enough memory pressure to trigger writeback reliably. >=20 > Fix both issues by: > - Swapping the data patterns: fill wb_group pages with partially > random data (getrandom for page_size/4 bytes) to resist compression > and trigger writeback, and fill zw_group pages with simple repeated > data to stay compressed in zswap. I'd have expected that having both equal (i.e. both random to fill up more easily) is what tests the effect zswap.max upon writeback most precisely. > - Making all size parameters PAGE_SIZE-aware: set allocation size to > PAGE_SIZE * 1024, memory.zswap.max to PAGE_SIZE, and memory.max to > allocation_size / 2 for both cgroups. Makes sense. > - Allocating memory inline instead of via cg_run() so the pages > remain resident throughout the test. What is the residency good for? (It doesn't matter AFAICS, so the change seems gratuitous and code diverges from test_zswap_usage().) --npkgor3mtzc7eyn7 Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iJEEABYKADkWIQRCE24Fn/AcRjnLivR+PQLnlNv4CAUCajKSlRsUgAAAAAAEAA5t YW51MiwyLjUrMS4xMiwyLDIACgkQfj0C55Tb+AjrLAD9GBYJlDuakfCppd9zM7AV pVNB8ypWNBY5Zblvu5XqtjwBAP+ZAbviWaThkRKtDNFKUvOG/ItMxC0RjON1YZma WNkB =KrLy -----END PGP SIGNATURE----- --npkgor3mtzc7eyn7--