how about something like:

if err := unmount(shmPath); err != nil && !os.IsNotExist(err) {
    mounted, _ := mount.Mounted(shmPath); mounted {
        // append warning
    }
}

On Linux, EINVAL may mean that umount2() was called with an invalid flag value in flags.
We need to use Mounted.

Sorry we may want to go ahead and read this error like

if mounted, mErr := ...; mounted || mErr != nil {
    // append err
}

@wenjianhn can you address this comment?

@cpuguy83 @thaJeztah Done. Thanks.

@cpuguy83 looks like this is now doing the same as mount.Unmount(), only in reverse order (first try syscall.Unmount(), then check if it was mounted - I assume that's a performance optimisation) - should we move that optimisation in mount.Unmount() and use it here?

It looks like the unmount argument for UnmountIpcMounts() is not really needed anymore (it looks like it was was added in 78bd17e to allow different unmount methods in different parts of the code)

It's really different since we're only checking if it's mounted if the unmount failed, vs only trying to unmount if it is mounted. Checking if it's mounted first is racey... and I wouldn't be surprised if it's technically slower to do it that way.