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Fri, 7 Mar 2025 04:47:20 +0000 Message-ID: Date: Thu, 6 Mar 2025 20:47:18 -0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 33/42] x86/resctrl: resctrl_exit() teardown resctrl but leave the mount point To: James Morse , , CC: Thomas Gleixner , Ingo Molnar , Borislav Petkov , H Peter Anvin , Babu Moger , , "D Scott Phillips OS" , , , , , , Jamie Iles , Xin Hao , , , , David Hildenbrand , Rex Nie , "Dave Martin" , Koba Ko , Shanker Donthineni , Shaopeng Tan , "Tony Luck" References: <20250207181823.6378-1-james.morse@arm.com> <20250207181823.6378-34-james.morse@arm.com> <91026839-2f2e-4562-aa77-6901148c89ad@intel.com> <0b2c17fe-3998-4def-81eb-a910d79fbc0f@arm.com> From: Reinette Chatre Content-Language: en-US In-Reply-To: <0b2c17fe-3998-4def-81eb-a910d79fbc0f@arm.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW4PR03CA0282.namprd03.prod.outlook.com (2603:10b6:303:b5::17) To SJ2PR11MB7573.namprd11.prod.outlook.com (2603:10b6:a03:4d2::10) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ2PR11MB7573:EE_|MN0PR11MB5961:EE_ X-MS-Office365-Filtering-Correlation-Id: 2bbe8043-1c62-411c-0242-08dd5d332547 X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|366016|1800799024; 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MPAM has an error interrupt which indicates >>>>> the MPAM driver has gone haywire. Should this occur tasks could run with >>>>> the wrong control values, leading to bad performance for important tasks. >>>>> The MPAM driver needs a way to tell resctrl that no further configuration >>>>> should be attempted. >>>>> >>>>> Using resctrl_exit() for this leaves the system in a funny state as >>>>> resctrl is still mounted, but cannot be un-mounted because the sysfs >>>>> directory that is typically used has been removed. Dave Martin suggests >>>>> this may cause systemd trouble in the future as not all filesystems >>>>> can be unmounted. >>>>> >>>>> Add calls to remove all the files and directories in resctrl, and >>>>> remove the sysfs_remove_mount_point() call that leaves the system >>>>> in a funny state. When triggered, this causes all the resctrl files >>>>> to disappear. resctrl can be unmounted, but not mounted again. >>> >>>>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c >>>>> index 6e30283358d4..424622d2f959 100644 >>>>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c >>>>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c >>>>> @@ -4371,9 +4375,12 @@ int __init resctrl_init(void) >>>>> >>>> >>>> Could you please add the kerneldoc you proposed in >>>> https://lore.kernel.org/lkml/f2ecb501-bc65-49a9-903d-80ba1737845f@arm.com/ ? >>> >>> Huh. The way that is indented means I copied it out the file - I'm not sure went wrong >>> there. Thanks for fishing out the link! >>> >>> >>>>> void __exit resctrl_exit(void) >>>>> { >>>>> + mutex_lock(&rdtgroup_mutex); >>>>> + rdtgroup_destroy_root(); >>>>> + mutex_unlock(&rdtgroup_mutex); >>>>> + >>>>> debugfs_remove_recursive(debugfs_resctrl); >>>>> unregister_filesystem(&rdt_fs_type); >>>>> - sysfs_remove_mount_point(fs_kobj, "resctrl"); >>>>> >>>>> resctrl_mon_resource_exit(); >>>>> } >>>> >>>> It is difficult for me to follow the kernfs reference counting required >>>> to make this work. Specifically, the root kn is "destroyed" here but it >>>> is required to stick around until unmount when the rest of the files >>>> are removed. >>> >>> This drops resctrl's reference to all of the files, which would make the files disappear. >>> unmount is what calls kernfs_kill_sb(), which gets rid of the root of the filesystem. >> >> My concern is mostly with the kernfs_remove() calls in the rdt_kill_sb()->rmdir_all_sub() >> flow. For example: >> kernfs_remove(kn_info); >> kernfs_remove(kn_mongrp); >> kernfs_remove(kn_mondata); >> >> As I understand the above require the destroyed root to still be around. > > Right - because rdt_get_tree() has these global pointers into the hierarchy, but doesn't > take a reference. rmdir_all_sub() relies on always being called before > rdtgroup_destroy_root(). > > The point hack would be for rdtgroup_destroy_root() to NULL out those global pointers, (I > note they are left dangling) - that would make a subsequent call to rmdir_all_sub() harmless. > > A better fix would be to pull out all the filesystem relevant parts from rdt_kill_sb(), > make that safe for multiple calls and get resctrl_exit() to call that. > A call to rdt_kill_sb() after resctrl_exit() would just cleanup the super-block. > This will leave things in a more predictable state. > > Since V7 has been posted already I try to keep things coherent by copying this exchange and responded to you there [1]. Reinette [1] https://lore.kernel.org/lkml/053d8a62-022b-4bf8-8e47-651e7c3a2d59@intel.com/