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

From: NeilBrown <neilb@suse.com>
To: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Keith Busch <keith.busch@intel.com>,
	Maxim Levitsky <maximlevitsky@gmail.com>,
	Vishal Verma <vishal.l.verma@intel.com>,
	Jens Axboe <axboe@fb.com>,
	Dan Williams <dan.j.williams@intel.com>,
	linux-nvme@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-nvdimm@lists.01.org
Subject: Re: [PATCH 0/4] Remove un-needed 'major' registration when alloc_disk(0) is used.
Date: Wed, 16 Mar 2016 09:15:25 +1100	[thread overview]
Message-ID: <878u1jgzgi.fsf@notabene.neil.brown.name> (raw)
In-Reply-To: <20160314225017.GC19926@linux.intel.com>

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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.

>
> It seems like these could easily be converted in the same way so they'd use
> BLOCK_EXT_MAJOR for their major and have a bunch of dynamically assigned
> minors.
>
> Does this break something I'm not seeing?
>
> Yay for this series, by the way. :) 

Thanks... two are in -next now (thank Dan) - I might poke the other two
in a week or two if nothing happens.

Thanks,
NeilBrown

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