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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <e546e411-9fd9-473f-ac58-66e4b5183e33@arm.com>
Date: Fri, 2 Aug 2024 18:28:35 +0100
From: James Morse <james.morse@....com>
To: Reinette Chatre <reinette.chatre@...el.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 Reinette,

On 08/07/2024 18:47, Reinette Chatre wrote:
> On 7/4/24 9:41 AM, James Morse wrote:
>> 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()?

There may be, but when a filesystem is mounted the code in super.c holds a reference to
the filesystem - which translates to a reference on the module/filesystem->owner.

My point was only that its possible to unregister a filesystem while its mounted. The
reference counting takes care of this - and is unnecessary in our case.


>> 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?

I think we are rat-holing on something that doesn't matter:
 * resctrl can't be built as a module - it is always built in.
 * rdt_fs_type is therefore part of the kernel data section - it can't be freed.
 * likewise the code that is part of resctrl can't be freed either.


Thanks,

James