linux-kernel - RE: [PATCH 2/4] mdev: Make mdev alias unique among all mdevs

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Date:   Tue, 27 Aug 2019 18:54:07 +0000
From:   Parav Pandit <parav@...lanox.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     Cornelia Huck <cohuck@...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



> -----Original Message-----
> From: Alex Williamson <alex.williamson@...hat.com>
> Sent: Tuesday, August 27, 2019 9:55 PM
> To: Parav Pandit <parav@...lanox.com>
> Cc: Cornelia Huck <cohuck@...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 Tue, 27 Aug 2019 16:13:27 +0000
> Parav Pandit <parav@...lanox.com> wrote:
> 
> > > -----Original Message-----
> > > From: Alex Williamson <alex.williamson@...hat.com>
> > > Sent: Tuesday, August 27, 2019 8:59 PM
> > > To: Cornelia Huck <cohuck@...hat.com>
> > > Cc: Parav Pandit <parav@...lanox.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 Tue, 27 Aug 2019 13:29:46 +0200
> > > Cornelia Huck <cohuck@...hat.com> wrote:
> > >
> > > > 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.
> > >
> > > 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,
> 
Ok. Will do.