RE: [PATCH v2 1/6] mdev: Introduce sha1 based mdev alias

[Parav Pandit <parav@mellanox.com>]

> -----Original Message-----
> From: Cornelia Huck <cohuck@redhat.com>
> Sent: Friday, August 30, 2019 7:32 PM
> To: Parav Pandit <parav@mellanox.com>
> Cc: alex.williamson@redhat.com; Jiri Pirko <jiri@mellanox.com>;
> kwankhede@nvidia.com; davem@davemloft.net; kvm@vger.kernel.org; linux-
> kernel@vger.kernel.org; netdev@vger.kernel.org
> Subject: Re: [PATCH v2 1/6] mdev: Introduce sha1 based mdev alias
> 
> On Fri, 30 Aug 2019 12:58:04 +0000
> Parav Pandit <parav@mellanox.com> wrote:
> 
> > > -----Original Message-----
> > > From: Cornelia Huck <cohuck@redhat.com>
> > > Sent: Friday, August 30, 2019 6:09 PM
> > > To: Parav Pandit <parav@mellanox.com>
> > > Cc: alex.williamson@redhat.com; Jiri Pirko <jiri@mellanox.com>;
> > > kwankhede@nvidia.com; davem@davemloft.net; kvm@vger.kernel.org;
> > > linux- kernel@vger.kernel.org; netdev@vger.kernel.org
> > > Subject: Re: [PATCH v2 1/6] mdev: Introduce sha1 based mdev alias
> > >
> > > On Fri, 30 Aug 2019 12:33:22 +0000
> > > Parav Pandit <parav@mellanox.com> wrote:
> > >
> > > > > -----Original Message-----
> > > > > From: Cornelia Huck <cohuck@redhat.com>
> > > > > Sent: Friday, August 30, 2019 2:47 PM
> > > > > To: Parav Pandit <parav@mellanox.com>
> > > > > Cc: alex.williamson@redhat.com; Jiri Pirko <jiri@mellanox.com>;
> > > > > kwankhede@nvidia.com; davem@davemloft.net; kvm@vger.kernel.org;
> > > > > linux- kernel@vger.kernel.org; netdev@vger.kernel.org
> > > > > Subject: Re: [PATCH v2 1/6] mdev: Introduce sha1 based mdev
> > > > > alias
> > > > >
> > > > > On Thu, 29 Aug 2019 06:18:59 -0500 Parav Pandit
> > > > > <parav@mellanox.com> wrote:
> > > > >
> > > > > > Some vendor drivers want an identifier for an mdev device that
> > > > > > is shorter than the UUID, due to length restrictions in the
> > > > > > consumers of that identifier.
> > > > > >
> > > > > > Add a callback that allows a vendor driver to request an alias
> > > > > > of a specified length to be generated for an mdev device. If
> > > > > > generated, that alias is checked for collisions.
> > > > > >
> > > > > > It is an optional attribute.
> > > > > > mdev alias is generated using sha1 from the mdev name.
> > > > > >
> > > > > > Signed-off-by: Parav Pandit <parav@mellanox.com>
> > > > > >
> > > > > > ---
> > > > > > Changelog:
> > > > > > v1->v2:
> > > > > >  - Kept mdev_device naturally aligned
> > > > > >  - Added error checking for crypt_*() calls
> > > > > >  - Corrected a typo from 'and' to 'an'
> > > > > >  - Changed return type of generate_alias() from int to char*
> > > > > > v0->v1:
> > > > > >  - Moved alias length check outside of the parent lock
> > > > > >  - Moved alias and digest allocation from kvzalloc to kzalloc
> > > > > >  - &alias[0] changed to alias
> > > > > >  - alias_length check is nested under get_alias_length
> > > > > > callback check
> > > > > >  - Changed comments to start with an empty line
> > > > > >  - Fixed cleaunup of hash if mdev_bus_register() fails
> > > > > >  - Added comment where alias memory ownership is handed over
> > > > > > to mdev device
> > > > > >  - Updated commit log to indicate motivation for this feature
> > > > > > ---
> > > > > >  drivers/vfio/mdev/mdev_core.c    | 123
> > > > > ++++++++++++++++++++++++++++++-
> > > > > >  drivers/vfio/mdev/mdev_private.h |   5 +-
> > > > > >  drivers/vfio/mdev/mdev_sysfs.c   |  13 ++--
> > > > > >  include/linux/mdev.h             |   4 +
> > > > > >  4 files changed, 135 insertions(+), 10 deletions(-)
> > >
> > > > > ...and detached from the local variable here. Who is freeing it?
> > > > > The comment states that it is done by the mdev, but I don't see it?
> > > > >
> > > > mdev_device_free() frees it.
> > >
> > > Ah yes, I overlooked the kfree().
> > >
> > > > once its assigned to mdev, mdev is the owner of it.
> > > >
> > > > > This detour via the local variable looks weird to me. Can you
> > > > > either create the alias directly in the mdev (would need to
> > > > > happen later in the function, but I'm not sure why you generate
> > > > > the alias before checking for duplicates anyway), or do an explicit copy?
> > > > Alias duplicate check is done after generating it, because
> > > > duplicate alias are
> > > not allowed.
> > > > The probability of collision is rare.
> > > > So it is speculatively generated without hold the lock, because
> > > > there is no
> > > need to hold the lock.
> > > > It is compared along with guid while mutex lock is held in single loop.
> > > > And if it is duplicate, there is no need to allocate mdev.
> > > >
> > > > It will be sub optimal to run through the mdev list 2nd time after
> > > > mdev
> > > creation and after generating alias for duplicate check.
> > >
> > > Ok, but what about copying it? I find this "set local variable to
> > > NULL after ownership is transferred" pattern a bit unintuitive.
> > > Copying it to the mdev (and then unconditionally freeing it) looks more
> obvious to me.
> > Its not unconditionally freed.
> 
> That's not what I have been saying :(
> 
Ah I see. You want to allocate alias memory twice; once inside mdev device and another one in _create() function.
_create() one you want to free unconditionally.

Well, passing pointer is fine.
mdev_register_device() has similar little tricky pattern that makes parent = NULL on __find_parent_device() finds duplicate one.

Ownership transfer is more straight forward code.

It is similar to device_initialize(), device init sequence code, where once device_initialize is done, freeing the device memory will be left to the put_device(), we don't call kfree() on mdev device.

> > Its freed in the error unwinding path.
> > I think its ok along with the comment that describes this error path area.
> 
> It is not wrong, but I'm not sure I like it.
Ok.