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Message-ID: <b87653fa-34e3-4124-a96f-f5d2b9730f10@marvell.com>
Date: Tue, 30 Apr 2024 12:43:05 +0530
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v1 28/31] x86/resctrl: Drop __init/__exit on assorted
 symbols
To: Dave Martin <Dave.Martin@arm.com>
Cc: Reinette Chatre <reinette.chatre@intel.com>,
        James Morse <james.morse@arm.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Fenghua Yu <fenghua.yu@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Borislav Petkov <bp@alien8.de>, H Peter Anvin <hpa@zytor.com>,
        Babu Moger <Babu.Moger@amd.com>, shameerali.kolothum.thodi@huawei.com,
        D Scott Phillips OS <scott@os.amperecomputing.com>,
        carl@os.amperecomputing.com, lcherian@marvell.com,
        bobo.shaobowang@huawei.com, tan.shaopeng@fujitsu.com,
        baolin.wang@linux.alibaba.com, Jamie Iles <quic_jiles@quicinc.com>,
        Xin Hao <xhao@linux.alibaba.com>, peternewman@google.com,
        dfustini@baylibre.com, David Hildenbrand <david@redhat.com>,
        Rex Nie <rex.nie@jaguarmicro.com>
References: <20240321165106.31602-1-james.morse@arm.com>
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From: Amit Singh Tomar <amitsinght@marvell.com>
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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@localhost:~# 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@localhost:~# 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@localhost:~# devmem msc_addr 32 0x9999
[  687.096276] mpam: error irq from msc:1 'PARTID_SEL_Range', 
partid:39321, pmg: 0, ris: 0

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

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

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