linux-kernel - Re: [PATCH v3 31/38] x86/resctrl: resctrl_exit() teardown resctrl but leave the mount point

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Message-ID: <20735ea0-2a3a-4230-92c6-6007c0777e24@intel.com>
Date: Mon, 8 Jul 2024 10:47:57 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: James Morse <james.morse@....com>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>
CC: Fenghua Yu <fenghua.yu@...el.com>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, H Peter Anvin
	<hpa@...or.com>, Babu Moger <Babu.Moger@....com>,
	<shameerali.kolothum.thodi@...wei.com>, D Scott Phillips OS
	<scott@...amperecomputing.com>, <carl@...amperecomputing.com>,
	<lcherian@...vell.com>, <bobo.shaobowang@...wei.com>,
	<tan.shaopeng@...itsu.com>, <baolin.wang@...ux.alibaba.com>, Jamie Iles
	<quic_jiles@...cinc.com>, Xin Hao <xhao@...ux.alibaba.com>,
	<peternewman@...gle.com>, <dfustini@...libre.com>, <amitsinght@...vell.com>,
	David Hildenbrand <david@...hat.com>, Rex Nie <rex.nie@...uarmicro.com>,
	"Dave Martin" <dave.martin@....com>, Shaopeng Tan
	<tan.shaopeng@...fujitsu.com>
Subject: Re: [PATCH v3 31/38] x86/resctrl: resctrl_exit() teardown resctrl but
 leave the mount point

Hi James,

On 7/4/24 9:41 AM, James Morse wrote:
> Hi Reinette,
> 
> On 28/06/2024 17:53, Reinette Chatre wrote:
>> On 6/14/24 8:00 AM, James Morse wrote:
>>> resctrl_exit() was intended for use when the 'resctrl' module was unloaded.
>>> resctrl can't be built as a module, and the kernfs helpers are not exported
>>> so this is unlikely to change. 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 impoartant tasks.
>>
>> impoartant -> important
>>
>>> 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.
> 
>> I am not familiar with these flows so I would like to confirm ...
>> In this scenario the resctrl filesystem will be unregistered, are
>> you saying that it is possible to unmount a filesystem after it has
>> been unregistered?
> 
> Counter-intuitively: yes.
> 
> The rules are described in fs/filesystems.c: We can access the members of the struct
> file_system_type if the list lock is held, or a reference is held to the module. This is
> how /proc/mounts is able to print the filesystem name from struct file_system_type without
> taking the lock - it holds a reference to any module to prevent the structure from being

hmmm ... does this mean I am supposed to find calls to try_module_get() in the flow from
mounts_open_common()?

> freed. Because resctrl can't be built as a module, we can say there is always a reference
> held, and we can never free struct file_system_type.

unregister_filesystem() continues to be called and as I understand in new MPAM usages will be
called during runtime. unregister_filesystem() comments state "Once this function has returned
the &struct file_system_type structure may be freed or reused.". Could you please highlight to me
what gives the confidence of "we can say there is always a reference held"? Could you please
point to me where that reference is obtained that will prevent the structure from being
freed?

Thank you

Reinette