linux-kernel - Re: [PATCH 0/4] Remove un-needed 'major' registration when alloc

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Message-ID: <56E8A6BF.7060304@fb.com>
Date:	Tue, 15 Mar 2016 17:20:15 -0700
From:	Jens Axboe <axboe@...com>
To:	NeilBrown <neilb@...e.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>
CC:	Keith Busch <keith.busch@...el.com>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	<linux-nvme@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
	<linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH 0/4] Remove un-needed 'major' registration when
 alloc_disk(0) is used.

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