linux-kernel - Re: [PATCH] pmem: don't allocate unused major device number

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Date:	Wed, 9 Mar 2016 11:57:41 -0700
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	NeilBrown <neilb@...e.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pmem: don't allocate unused major device number

On Tue, Mar 08, 2016 at 02:29:58PM -0800, Dan Williams wrote:
> On Tue, Mar 8, 2016 at 2:21 PM, NeilBrown <neilb@...e.com> wrote:
> >
> > When alloc_disk(0) or alloc_disk-node(0, XX) is used, the ->major
> > number is completely ignored:  all devices are allocated with a
> > major of BLOCK_EXT_MAJOR.
> >
> > So there is no point allocating pmem_major.
> >
> > Signed-off-by: NeilBrown <neilb@...e.com>
> > ---
> >  drivers/nvdimm/pmem.c | 19 +------------------
> >  1 file changed, 1 insertion(+), 18 deletions(-)
> >
> > Hi Dan et al,
> >  I was recently educating myself about the behavior of alloc_disk(0).
> >  As I understand it, the ->major is ignored and all device numbers for all
> >  partitions (including '0') are allocated on demand with major number of
> >  BLOCK_EXT_MAJOR.
> >
> >  So I was a little surprised to find that pmem.c allocated a major
> >  number which is never used - historical anomaly I suspect.
> >  I was a bit more surprised at the comment in:
> >
> >   Commit: 9f53f9fa4ad1 ("libnvdimm, pmem: add libnvdimm support to the pmem driver")
> >
> >  "The minor numbers are also more predictable by passing 0 to alloc_disk()."
> >
> >  How can they possibly be more predictable given that they are allocated
> >  on-demand?  Maybe discovery order is very predictable???
> 
> Ross, I remember you looked into this when Boaz pointed out something similar.

I think you're probably remembering a conversation we had about BRD.

https://lkml.org/lkml/2014/8/6/568

I honestly don't remember the details well enough to comment - I'd have to dig
into it again and test to have an informed opinion.  But, of course, if we can
get rid of some useless code, we should. :)

> >  In any case, I propose this patch but cannot test it (beyond compiling)
> >  as I don't have relevant hardware.  And maybe some user-space code greps
> >  /proc/devices for "pmem" to determine if "pmem" is compiled in (though
> >  I sincerely hope not).
> >  So I cannot be certain that this patch won't break anything, but am
> >  hoping that if you like it you might test it.
> 
> Will do.
> 
> Thanks Neil!