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Subject: Re: [PATH v2] cgroup: add cgroup_favordynmods= command-line option Content-Language: en-US To: Luiz Capitulino , Waiman Long , tj@kernel.org, lizefan.x@bytedance.com, hannes@cmpxchg.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Cc: lcapitulino@gmail.com References: <20230906005712.66461-1-luizcap@amazon.com> <5487ed0a-8483-0a92-c7c1-9ca3ed8e6162@oracle.com> From: Kamalesh Babulal In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MAXPR01CA0118.INDPRD01.PROD.OUTLOOK.COM (2603:1096:a00:5d::36) To DM6PR10MB3001.namprd10.prod.outlook.com (2603:10b6:5:69::23) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM6PR10MB3001:EE_|DS0PR10MB7127:EE_ X-MS-Office365-Filtering-Correlation-Id: f6e23042-a063-4966-78c1-08dbb039dcb6 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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definitions=2023-09-08_03,2023-09-05_01,2023-05-22_02 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxscore=0 bulkscore=0 mlxlogscore=921 suspectscore=0 phishscore=0 spamscore=0 malwarescore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2308100000 definitions=main-2309080064 X-Proofpoint-GUID: EVcOhKNhsDpg2AxpAdS90fMKMwaC-Z3C X-Proofpoint-ORIG-GUID: EVcOhKNhsDpg2AxpAdS90fMKMwaC-Z3C Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/8/23 00:17, Luiz Capitulino wrote: [...] >>>> Consider a case where the kernel is compiled with >>>> CONFIG_CGROUP_FAVOR_DYNMODS=n and kernel command line is passed with >>>> cgroup_favordynmods=true, this would set the have_favordynmods to true. >>>> In cgroup_favordynmods_setup(), should it return 0 with a pr_warn(), >>>> when CONFIG_CGROUP_FAVOR_DYNMODS=n in the above case, or is this >>>> expected behavior? >>> >>> According to the documentation of __setup: >>> >>> /* >>>   * NOTE: __setup functions return values: >>>   * @fn returns 1 (or non-zero) if the option argument is "handled" >>>   * and returns 0 if the option argument is "not handled". >>>   */ >>> >>> So the return value should tell whether the input parameter is a recognizable true or false value, not whether it is true or false. kstrtobool returns 0 if it is a recognizable T/F value or -EINVAL otherwise. So the check is correct. I did double check that before I ack'ed the patch. >>> >> >> Apologies for not being clear in the previous email. It was in two parts, >> where the first one was more of a question, where if a kernel is compiled >> with CONFIG_CGROUP_FAVOR_DYNMODS config option disabled and the user >> attempts to pass cgroup_favordynmods=true in the kernel command line. >> >> In this scenario, the have_favordynmods is set to true regardless of >> the CONFIG_CGROUP_FAVOR_DYNMODS config option being enabled/disabled in >> the kernel. This allows the user to set CGRP_ROOT_FAVOR_DYNMODS flag >> without enabling the CONFIG_CGROUP_FAVOR_DYNMODS kernel config. > > Correct, that's exactly the goal of this patch: to give users the > option to enable/disable favordynmods at boot-time regardless of > CONFIG_FAVOR_DYNMODS. > > This is especially useful with cgroup v1 where remounting with > favordynmods is not supported. Thank you so much for explaining. I understand the idea of the patch better now. >> Shouldn't the cgroup_favordynmods kernel parameter be valid only when >> the kernel is compiled with CONFIG_CGROUP_FAVOR_DYNMODS=y and allows the >> user to only disable it in the kernel command line instead of allowing >> them to set/unset have_favordynmods when CONFIG_CGROUP_FAVOR_DYNMODS is >> disabled. > > This was my first idea as well, but since we'd allow for enabling why > not allow for disabling as well? Besides, the resulting code is > fairly simple. Agreed, If it's independent of CONFIG_CGROUP_FAVOR_DYNMODS config option, providing both enable and disable, is useful. >> If the above assumption is right, that's where the second part was of >> email, where I was suggesting the restriction by using ifdef guards in >> cgroup_favordynmods_setup(), something like: >> >> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c >> index 2b7d74304606..5c7d1a0b1dbe 100644 >> --- a/kernel/cgroup/cgroup.c >> +++ b/kernel/cgroup/cgroup.c >> @@ -6768,7 +6768,11 @@ __setup("cgroup_debug", enable_cgroup_debug); >> >>   static int __init cgroup_favordynmods_setup(char *str) >>   { >> +#ifdef CGROUP_FAVOR_DYNMODS >>          return (kstrtobool(str, &have_favordynmods) == 0); >> +#endif >> +       pr_warn("Favor Dynmods not supported\n"); >> +       return 0; >>   } > > Why should we do this? What's the benefit for the user? This code was constructed on the idea of have_favordynmods, should be available only when the kernel is compiled with CONFIG_CGROUP_FAVOR_DYNMODS and it's of no benefit. >>   __setup("cgroup_favordynmods=", cgroup_favordynmods_setup); >> -- Thanks, Kamalesh