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

[Alex Williamson]

On Tue, 27 Aug 2019 16:13:27 +0000
Parav Pandit <parav@xxxxxxxxxxxx> wrote:

> > -----Original Message-----
> > From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > Sent: Tuesday, August 27, 2019 8:59 PM
> > To: Cornelia Huck <cohuck@xxxxxxxxxx>
> > Cc: Parav Pandit <parav@xxxxxxxxxxxx>; Jiri Pirko <jiri@xxxxxxxxxxxx>;
> > kwankhede@xxxxxxxxxx; davem@xxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx
> > Subject: Re: [PATCH 2/4] mdev: Make mdev alias unique among all mdevs
> > 
> > On Tue, 27 Aug 2019 13:29:46 +0200
> > Cornelia Huck <cohuck@xxxxxxxxxx> wrote:
> >   
> > > On Tue, 27 Aug 2019 11:08:59 +0000
> > > Parav Pandit <parav@xxxxxxxxxxxx> wrote:
> > >  
> > > > > -----Original Message-----
> > > > > From: Cornelia Huck <cohuck@xxxxxxxxxx>
> > > > > Sent: Tuesday, August 27, 2019 3:59 PM
> > > > > To: Parav Pandit <parav@xxxxxxxxxxxx>
> > > > > Cc: alex.williamson@xxxxxxxxxx; Jiri Pirko <jiri@xxxxxxxxxxxx>;
> > > > > kwankhede@xxxxxxxxxx; davem@xxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
> > > > > linux- kernel@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx
> > > > > 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@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > > > > > ---
> > > > > >  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.  
> > 
> > It's a rare event, maybe just dev_dbg(dev, "Hash collision creating alias \"%s\"
> > for mdev device %pUl\n",...
> >   
> Ok.
> dev_dbg_once() to avoid message flood.

I'd suggest a rate-limit rather than a once.  The fact that the kernel
may have experienced a collision at some time in the past does not help
someone debug why they can't create a device now.  The only way we're
going to get a flood is if a user sufficiently privileged to create
mdev devices stumbles onto a collision and continues to repeat the same
operation.  That falls into shoot-yourself-in-the-foot behavior imo.
Thanks,

Alex