Re: [PATCH] virtiofs: Export filesystem tags through sysfs

[Alyssa Ross <hi@alyssa.is>]

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Vivek Goyal <vgoyal@redhat.com> writes:

> On Sat, Oct 21, 2023 at 04:10:21PM +0000, Alyssa Ross wrote:
>> Are you still thinking about exposing this in the uevent as well?
>> That would be much more convenient for me, because with this approach
>> by the time the "remove" uevent arrives, it's no longer possible to
>> check what tag was associated with the device — you have to store it
>> somewhere when the device appears, so you can look it up again when the
>> device is removed.  (Not everybody uses udev.)
>
> Looks like systemd + udev combination can already take care of it. I just
> had to specify "StopWhenUnneeded=true" in my systemd .mount unit file. And
> that made sure that when device goes away, virtiofs is unmounted and
> service is deactivated.

My point is that, if it's not exposed in the uevent, the tag information
has to be tracked somehow.  systemd/udev may do that already, but every
other system people might be using (mine uses mdevd) also has to track
that state.  Whereas if the uevent did contain that information,
userspace would be able to do the unmount directly, without needing to
look up some information it has previously saved.

Relying on tracking state from previous events also introduces potential
reliability problems — it's possible to miss uevents if the netlink
queue fills up.  Suppose I have a system where virtiofs filesystems
should always be unmounted when the device goes away.  I miss the uevent
for the device being added, the user mounts the filesystem anyway, and
then when the device removal uevent comes in, unless that uevent
contains the filesystem tag, I'm not going to know which filesystem to
unmount.

> Following is my mount unit file.
>
> $ cat /etc/systemd/system/mnt-virtiofs.mount
>
> [Unit]
> Description=Virtiofs mount myfs
> DefaultDependencies=no
> ConditionPathExists=/mnt/virtiofs
> ConditionCapability=CAP_SYS_ADMIN
> Before=sysinit.target
> StopWhenUnneeded=true 
>
> [Mount]
> What=myfs
> Where=/mnt/virtiofs
> Type=virtiofs
>
> And following is the udev rule I used.
>
> $ cat /etc/udev/rules.d/80-local.rules
> SUBSYSTEM=="virtio", DRIVER=="virtiofs", ATTR{tag}=="myfs", TAG+="systemd", ENV{SYSTEMD_WANTS}+="mnt-virtiofs.mount"
>
> And a combination of above two seems to work. virtiofs is automatically
> mounted when device is hotplugged and it is unmounted when device is
> hot unplugged.

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