linux-kernel - Re: [PATCH v1 28/31] x86/resctrl: Drop __init/_

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Date: Tue, 30 Apr 2024 12:43:05 +0530
From: Amit Singh Tomar <amitsinght@...vell.com>
To: Dave Martin <Dave.Martin@....com>
Cc: Reinette Chatre <reinette.chatre@...el.com>,
        James Morse <james.morse@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, 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, David Hildenbrand <david@...hat.com>,
        Rex Nie <rex.nie@...uarmicro.com>
Subject: Re: [PATCH v1 28/31] x86/resctrl: Drop __init/__exit on assorted
 symbols

Hi Dave,

> Is it possible to unmount resctrl once the system is in this state?
No, it can't be unmounted, as there is no mount exist.

>> # mount -t resctrl resctrl /sys/fs/resctrl
>> mount: /sys/fs/resctrl: mount point does not exist.
> 
> What if you now try to mount resctrl somewhere else, e.g.:
> 
> # mount -t resctrl resctrl /mnt
root@...alhost:~# mount -t resctrl resctrl /test
mount: /test: unknown filesystem type 'resctrl'.

> 
> I'm guessing this _should_ fail if you weren't able to unmount resctrl,
> since resctrl seems to forbid multiple mount instances.
> 
> I'm not sure what the best behaviour is here.  Leaving resctrl "half-
> mounted" might be a good thing: at this point the system is in a semi-
> bad state we want to make sure it can't be remounted.  Unregistering the
> resctrl filesystem from the fs core feels cleaner if feasible though.
> 
> Leaving an impossible unmount operation for init to do during reboot/
> shutdown feels unfortunate.
> 
> We might have to look at what other filesystems do in this area.
> 
> The mount machinery does provide other ways of getting into broken,
> impossible situations from userspace, so this doesn't feel like an
> entirely new problem.
> 
>>
>> Additionally, a question regarding this, Is a complete system restart
>> necessary to regain the mount?
>>
>> Thanks
>> -Amit
> 
> I think James will need to comment on this, but I think that yes, it
> is probably appropriate to require a reboot.  I think an MPAM error
> interrupt should only happen if the software did something wrong, so
> it's a bit like hitting a BUG(): we don't promise that everything works
> 100% properly until the system is restarted.  Misbehaviour should be
> contained to MPAM though.
> 
if "resctrl" is nonfunctional in this state, then this comment[1] here 
does *not* make sense.

"restore any modified controls to their reset values."

Thanks
-Amit

[1]: 
https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git/tree/drivers/platform/mpam/mpam_devices.c?h=mpam/snapshot/v6.7-rc2#n2228

root@...alhost:~# mount
tmpfs on /run/user/0 type tmpfs 
(rw,nosuid,nodev,relatime,size=32923772k,nr_inodes=8230943,mode=700)
resctrl on /sys/fs/resctrl type resctrl (rw,relatime)

root@...alhost:~# devmem msc_addr 32 0x9999
[  687.096276] mpam: error irq from msc:1 'PARTID_SEL_Range', 
partid:39321, pmg: 0, ris: 0

root@...alhost:~# mount
tmpfs on /run/user/0 type tmpfs 
(rw,nosuid,nodev,relatime,size=32923772k,nr_inodes=8230943,mode=700)
resctrl on /sys/fs/resctrl type resctrl (rw,relatime)

root@...alhost:~# umount resctrl
umount: /sys/fs/resctrl: no mount point specified.

root@...alhost:~# mount
tmpfs on /run/user/0 type tmpfs 
(rw,nosuid,nodev,relatime,size=32923772k,nr_inodes=8230943,mode=700)

root@...alhost:~# mount -t resctrl resctrl /test
mount: /test: unknown filesystem type 'resctrl'.