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h=Date:From:To:Cc:Subject:In-Reply-To:References; b=Lv6frWmIMBhsZT2PbPJmz9srQDtxWdSn9v59muHBsrLCyGSrOM4BF+Dwp/XHqJ9Hr ZIhqg1lFK5NXXOOVJHWCGB3GvpzX42wDAjbROLuEi8b3RvB1druPR2CWA+ZgLTPwzl XHf38yM4beeoq2eVsBsaiwhmWV0YcnfpmxdTt6Q4= Date: Wed, 2 Sep 2026 18:04:08 -0700 From: Andrew Morton To: Joshua Hahn Cc: Nhat Pham , Johannes Weiner , Yosry Ahmed , Krush Chavan , "Chengming Zhou" , "Tejun Heo" , Michal =?ISO-8859-1?Q?Koutn=FD?= , "Shuah Khan" , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH v2] selftests/cgroup: account for zswap shrinker writeback Message-Id: <20260902180408.05b925ce05359cd4d7f96894@linux-foundation.org> In-Reply-To: <20260902194521.3652178-1-joshua.hahnjy@gmail.com> References: <20260902194521.3652178-1-joshua.hahnjy@gmail.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 776A98000A X-Stat-Signature: dii3q8874bdpsnrcng4gkg8h7m1hu6ts X-Rspam-User: X-HE-Tag: 1788397450-460503 X-HE-Meta: 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 xlGKsSZT PDJRjSnrqd/36Xu2FWQausxQ1Lyn8qY+FiHu4gMACXDFFHNZ4hWlgoj9gyd/fp4RUjD9+3btLISVuKzT/dMxM+gNvzMNJaFRAE9hBd9qhD7i8oOKDCg09TgVWZYd6kDCE6OF3I6Ccclj9QjQcUt00DAuXUHwvIkqSpOmpmBGiFCiuYFt3BTvGYbzm9GiUNLaDuyYAjsyB7Y/t5MIN9XdSrUH2cTy0wx+J/MjehsflsdYdV4kp8X2cMZusqTMjwhzF3y3Pwg6XxoRD9CjNZ6q46QHUoxKi84lHEbVID60EWmCuXSqWWUk1g7V6sI7KuPVvoZWoBXHZ3m+96wk3+JZQigtPx7ZcrEXW6rz57w0CqADMwyUo0/3tz0O2Ng== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, 2 Sep 2026 12:45:20 -0700 Joshua Hahn wrote: > The test_no_invasive_cgroup_shrink selftest checks that when a cgroup > has zswapped out more memory than memory.zswap.max, it does not > trigger writeback for other cgroups. To do this, it compares the > writeback count in a control cgroup and makes sure that it is 0, > and then checks the writeback count in an aggressor cgroup who does > expect to see writeback. > > However, when the zswap shrinker is enabled, the victim cgroup can see > legitimate writebacks not triggered by the aggressor. In some Meta CI > tests, we have seen this failure mode happen. > > Instead of checking that the victim cgroup has 0 writeback, compare the > writeback values before and after the aggressor runs and check that > the victim cgroup did not perform any additional writeback. Note that > this can still lead to probabilistic failures if writebacks take longer > than 5 seconds, but this should fix the systematic failure case. > > Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure") Thanks. I do support backporting selftests fixes. We want selftests to work well in the kernel with which they are shipped. But, as ever, we should include a clear explanation of the userspace-visible runtime effects of the bug. Seems the short answer is "false-positive test failures" but we can perhaps do a little better than that. Quoting the failure messages would be a good way - very recognizable. > Reported-by: Krush Chavan Relatedly, is there a Link:/Closes:?