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

From: Jens Axboe
Date: Tue Mar 15 2016 - 20:21:12 EST

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

Note that I am not in a position to test these beyond simple compile



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