Re: [PATCH 4/4] nbd: add a nbd-control interface

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Jan 23, 2017 at 09:42:08AM -0500, Josef Bacik wrote:
> 
> 
> On Sat, Jan 21, 2017 at 4:05 AM, Greg KH <gregkh@linuxfoundation.org> wrote:
> > On Fri, Jan 20, 2017 at 04:56:52PM -0500, Josef Bacik wrote:
> > >  This patch mirrors the loop back device behavior with a few
> > > changes.  First
> > >  there is no DEL operation as NBD doesn't get as much churn as loop
> > > devices do.
> > >  Secondly the GET_NEXT operation can optionally create a new NBD
> > > device or not.
> > >  Our infrastructure people want to not allow NBD to create new
> > > devices as it
> > >  causes problems for them in containers.  However allow this to be
> > > optional as
> > >  things like the OSS NBD client probably doesn't care and would like
> > > to just be
> > >  given a device to use.
> > > 
> > >  Signed-off-by: Josef Bacik <jbacik@fb.com>
> > 
> > A random char device with odd ioctls?  Why?  There's no other
> > configuration choice you could possibly use?  Where is the userspace
> > tool that uses this new kernel api?
> > 
> > You aren't passing in structures to the ioctl, so why does this HAVE to
> > be an ioctl?
> 
> Again, this is how loop does it so I assumed a known, regularly used API was
> the best bet.  I can do literally anything, but these interfaces have to be
> used by other people, including internal people.  The /dev/whatever-control
> is a well established way for interacting with dynamic device drivers (loop,
> DM, btrfs), so that's what I went with.  Thanks,

Again, please don't duplicate what loop did, we must _learn_ from
history, not repeat it :(