netdev - Re: [PATCH 2/4] mdev: Make mdev alias unique among all mdevs

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Date:   Tue, 27 Aug 2019 13:29:46 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Parav Pandit <parav@...lanox.com>
Cc:     "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        Jiri Pirko <jiri@...lanox.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH 2/4] mdev: Make mdev alias unique among all mdevs

On Tue, 27 Aug 2019 11:08:59 +0000
Parav Pandit <parav@...lanox.com> wrote:

> > -----Original Message-----
> > From: Cornelia Huck <cohuck@...hat.com>
> > Sent: Tuesday, August 27, 2019 3:59 PM
> > To: Parav Pandit <parav@...lanox.com>
> > Cc: alex.williamson@...hat.com; Jiri Pirko <jiri@...lanox.com>;
> > kwankhede@...dia.com; davem@...emloft.net; kvm@...r.kernel.org; linux-
> > kernel@...r.kernel.org; netdev@...r.kernel.org
> > Subject: Re: [PATCH 2/4] mdev: Make mdev alias unique among all mdevs
> > 
> > On Mon, 26 Aug 2019 15:41:17 -0500
> > Parav Pandit <parav@...lanox.com> wrote:
> >   
> > > Mdev alias should be unique among all the mdevs, so that when such
> > > alias is used by the mdev users to derive other objects, there is no
> > > collision in a given system.
> > >
> > > Signed-off-by: Parav Pandit <parav@...lanox.com>
> > > ---
> > >  drivers/vfio/mdev/mdev_core.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > >
> > > diff --git a/drivers/vfio/mdev/mdev_core.c
> > > b/drivers/vfio/mdev/mdev_core.c index e825ff38b037..6eb37f0c6369
> > > 100644
> > > --- a/drivers/vfio/mdev/mdev_core.c
> > > +++ b/drivers/vfio/mdev/mdev_core.c
> > > @@ -375,6 +375,11 @@ int mdev_device_create(struct kobject *kobj, struct  
> > device *dev,  
> > >  			ret = -EEXIST;
> > >  			goto mdev_fail;
> > >  		}
> > > +		if (tmp->alias && strcmp(tmp->alias, alias) == 0) {  
> > 
> > Any way we can relay to the caller that the uuid was fine, but that we had a
> > hash collision? Duplicate uuids are much more obvious than a collision here.
> >   
> How do you want to relay this rare event?
> Netlink interface has way to return the error message back, but sysfs is limited due to its error code based interface.

I don't know, that's why I asked :)

The problem is that "uuid already used" and "hash collision" are
indistinguishable. While "use a different uuid" will probably work in
both cases, "increase alias length" might be a good alternative in some
cases.

But if there is no good way to relay the problem, we can live with it.

> 
> > > +			mutex_unlock(&mdev_list_lock);
> > > +			ret = -EEXIST;
> > > +			goto mdev_fail;
> > > +		}
> > >  	}
> > >
> > >  	mdev = kzalloc(sizeof(*mdev), GFP_KERNEL);  
>