Re: [PATCH 0/4] Remove un-needed 'major' registration when alloc_disk(0) is used.

[Jens Axboe]

On 03/15/2016 03:15 PM, NeilBrown wrote:
> On Tue, Mar 15 2016, Ross Zwisler wrote:
>
> > On Thu, Mar 10, 2016 at 08:59:28AM +1100, NeilBrown wrote:
> > > When alloc_disk(0) is used, the ->major number is ignored and
> > > irrelevant.  Yet several drivers register a major number anyway.
> > >
> > > This series of patches removes the pointless registrations.  The pmem
> > > driver also does this, but a patch has already been sent for that
> > > driver.
> > >
> > > Note that I am not in a position to test these beyond simple compile
> > > testing.
> > >
> > > Thanks,
> > > NeilBrown
> > >
> > >
> > > ---
> > >
> > > NeilBrown (4):
> > >        nvdimm/blk: don't allocate unused major device number
> > >        nvdimm/btt: don't allocate unused major device number
> > >        memstick: don't allocate unused major for ms_block
> > >        NVMe: don't allocate unused nvme_major
> > >
> > >
> > >   drivers/memstick/core/ms_block.c |   17 ++---------------
> > >   drivers/nvdimm/blk.c             |   18 +-----------------
> > >   drivers/nvdimm/btt.c             |   19 ++-----------------
> > >   drivers/nvme/host/core.c         |   16 +---------------
> > >   4 files changed, 6 insertions(+), 64 deletions(-)
> >
> > There are several other drivers that allocate a major, but then use it for
> > some small number of minors (1 for null_blk.c and 16 for virtio_blk.c).  They
> > both have GENHD_FL_EXT_DEVT set, so I think what happens is that after we
> > exhaust the allocated minors they hop over to having BLOCK_EXT_MAJOR as a
> > major and a dynamically assigned minor.
>
> null_blk looks like it would be safe to convert - it is just used for
> testing.  Jens Axboe would probably know for sure.
>
> virtio_blk is a much older and there may will be code which has some
> sort of expectations about minor numbers.  I think it would not be worth
> the risks to change it.

Agree on both - null_blk can be trivially converted, and I too would be 
worried about virt_blkio changes breaking existing assumptions.

--
Jens Axboe