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

From: Dan Williams
Date: Tue Mar 08 2016 - 17:30:09 EST

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
> 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.

> 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!