Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751450AbcCHWaF (ORCPT ); Tue, 8 Mar 2016 17:30:05 -0500 Received: from mail-ob0-f177.google.com ([209.85.214.177]:32994 "EHLO mail-ob0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750802AbcCHW37 (ORCPT ); Tue, 8 Mar 2016 17:29:59 -0500 MIME-Version: 1.0 In-Reply-To: <87k2lclif1.fsf@notabene.neil.brown.name> References: <87k2lclif1.fsf@notabene.neil.brown.name> Date: Tue, 8 Mar 2016 14:29:58 -0800 Message-ID: Subject: Re: [PATCH] pmem: don't allocate unused major device number From: Dan Williams To: NeilBrown Cc: Ross Zwisler , "linux-nvdimm@lists.01.org" , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1658 Lines: 42 On Tue, Mar 8, 2016 at 2:21 PM, NeilBrown 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 > --- > 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. > 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!