Re: [PATCH] pmem: don't allocate unused major device number
From: Ross Zwisler
Date: Wed Mar 09 2016 - 13:58:14 EST
On Tue, Mar 08, 2016 at 02:29:58PM -0800, Dan Williams wrote:
> On Tue, Mar 8, 2016 at 2:21 PM, NeilBrown <neilb@xxxxxxxx> 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@xxxxxxxx>
> > ---
> > 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.
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!