On Fri, 30 Aug 2019 12:33:22 +0000 Parav Pandit <parav@xxxxxxxxxxxx> wrote: > > -----Original Message----- > > From: Cornelia Huck <cohuck@xxxxxxxxxx> > > Sent: Friday, August 30, 2019 2:47 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 v2 1/6] mdev: Introduce sha1 based mdev alias > > > > On Thu, 29 Aug 2019 06:18:59 -0500 > > Parav Pandit <parav@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> > > > > > > --- > > > 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.
