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

[Reinette Chatre]

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