From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754666Ab0CZWyo (ORCPT ); Fri, 26 Mar 2010 18:54:44 -0400 Received: from smtp-out.google.com ([216.239.44.51]:28912 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754482Ab0CZWym convert rfc822-to-8bit (ORCPT ); Fri, 26 Mar 2010 18:54:42 -0400 DomainKey-Signature: a=rsa-sha1; s=beta; d=google.com; c=nofws; q=dns; h=mime-version:in-reply-to:references:date:message-id:subject:from:to: cc:content-type:content-transfer-encoding:x-system-of-record; b=RA4jdi5FcSreNmnRuhVpNB5V3KoM+UaECDOy4vBHPdoBFkdCtCr/RONVXHV//I+C6 oSTs2SPTXnMWE2rKf/vAQ== MIME-Version: 1.0 In-Reply-To: <20100326152012.GB18128@redhat.com> References: <20100325180310.25299.64877.stgit@meat.mtv.corp.google.com> <4BAC0E7D.9080305@cn.fujitsu.com> <20100326152012.GB18128@redhat.com> Date: Fri, 26 Mar 2010 15:54:40 -0700 Message-ID: <1786ab031003261554y40c21bc8sb4ea2f347c2c8bf8@mail.gmail.com> Subject: Re: [PATCH 0/4] io-controller: Use names rather than major:minor From: Chad Talbott To: Vivek Goyal Cc: Gui Jianfeng , jens.axboe@oracle.com, mrubin@google.com, Li Zefan , linux-kernel@vger.kernel.org, dpshah@google.com, Nauman Rafique Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 26, 2010 at 8:20 AM, Vivek Goyal wrote: > On Fri, Mar 26, 2010 at 09:31:41AM +0800, Gui Jianfeng wrote: >> +int blk_lookup_devname(dev_t devt, char *name) >> +{ [ snip... loop through all block devices for devt ...snip ] >> So we can keep dev_t in blkio layer, and export to user a device name by calling >> this function. Also, we retrive device number by calling blk_lookup_devt(). >> This change might keep things much simple. Jens, do you have any thoughts? >> > I agree with Gui that lets keep the dev_t the core in blkio layer. Keeping > a pointer to gendisk in request queue is becoming little messy. Agreed on leaving gendisk pointer out of request_queue. In doing further investigation, I've found that it's up to the driver to maintain the association between gendisk and request_queue, and some drivers put multiple gendisk behind a single request_queue, so the back pointer would be ill-specified. > But if that does not work for you, then I would also like to keep things > simple and translate dev_t to diskname during read routine. Similiarly, > while somebody is putting policy, use blk_lookup_devt(). I like the simplicity of blk_lookup_devt(), but I don't like the idea of iterating through all block devices on every lookup of the name. Perhaps we could cache the name somewhere? Actually, the name is the name of the *queue* (or the key in blk-cgroup), because as I mentioned above there can be a many to one relationship between disks and queues in general. The more I think about it, the more it seems to make sense to extend blkio_policy_ops to include a function to get the name of the key. blk-cgroup makes no current use of the dev, except to invent a name for the request_queue whose policy is being set or printed. It could be argued that the thing being scheduled has a better idea of the name of that thing. > But this will lead to issue of how do you now display both device number > and disk name in the output. May be following. > > major:minor  diskname  data > > I am not sure if people are fond of multiple values in a single file. At > the same time for setting the rules or deleting the rules, it will make > syntax complicated/confusing. Also will require breaking ABI for existing > blkio.time, blkio.sectors, blkio.dequeue files. I don't like this, either. It breaks ABI and is more confusing for users. > So I would prefer to keep the major/minor number based interface for > follwing reasons. > > - Chaning it now breaks ABI. > - Other cgroup controller "device" is also using major/minor number based >  interface for device access policy. So it is consistent with other >  controller. Which controllers are these? > - Displaying both device major/minor and diskname is an option but that >  makes the file format syntax little complicated and new rule setting >  or removoal confusing. A few messages back you mentioned that you preferred device names because they would be better for users of the system. If there was a simple implementation, would you still be behind a new name-based interface? We could go that direction and maintain ABI by deprecating current interface and making a new interface with names. If you can't tell, I'm a big fan of using the name! :) It's *much* more consistent with the interfaces in /sys. Chad