Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757967Ab0GGW0R (ORCPT ); Wed, 7 Jul 2010 18:26:17 -0400 Received: from fieldses.org ([174.143.236.118]:48114 "EHLO fieldses.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757144Ab0GGW0P (ORCPT ); Wed, 7 Jul 2010 18:26:15 -0400 Date: Wed, 7 Jul 2010 18:25:51 -0400 From: "J. Bruce Fields" To: Neil Brown Cc: Miklos Szeredi , david@fromorbit.com, aneesh.kumar@linux.vnet.ibm.com, hch@infradead.org, viro@zeniv.linux.org.uk, adilger@sun.com, corbet@lwn.net, serue@us.ibm.com, hooanon05@yahoo.co.jp, linux-fsdevel@vger.kernel.org, sfrench@us.ibm.com, philippe.deniel@CEA.FR, linux-kernel@vger.kernel.org Subject: Re: [PATCH -V14 0/11] Generic name to handle and open by handle syscalls Message-ID: <20100707222551.GA9878@fieldses.org> References: <20100706232351.GD25018@dastard> <20100707093629.10c2feab@notabene.brown> <20100707021150.GF25018@dastard> <20100707125726.3695587a@notabene.brown> <20100707125701.GA19872@fieldses.org> <20100707131721.GB19872@fieldses.org> <20100707144511.GA24360@fieldses.org> <20100708082143.3701bfc7@notabene.brown> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100708082143.3701bfc7@notabene.brown> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2389 Lines: 58 On Thu, Jul 08, 2010 at 08:21:43AM +1000, Neil Brown wrote: > On Wed, 7 Jul 2010 10:45:11 -0400 > "J. Bruce Fields" wrote: > > > On Wed, Jul 07, 2010 at 03:35:50PM +0200, Miklos Szeredi wrote: > > > It's unique at a single point in time. But if you have a reference > > > (e.g. open file descriptor) on the mount then that's not a problem. > > > > > > fd = open(path, ...); > > > fstat(fd, &st); > > > search st.st_dev in mountinfo > > > close(fd) > > > > > > is effectively the same as an getuuid(path) syscall (lazy unmounted > > > filesystems will not be found in mountinfo, but the reference is still > > > there so st_dev will not be reused for other filesystems). > > > > OK, cool. > > > > That still leaves the problem that there isn't always an underlying > > block device, and/or when there is it doesn't always uniquely specify > > the filesystem. > > It doesn't matter if there is an underlying block device, or if it is shared > among subvolmes. > st_dev is *the* primary key for filesystems. Every "struct super_block" has a > unquie s_dev and that is returned in st_dev. > > For "traditional" filesystem, this is the major/minor number of the block > device. > For NFS and btrfs and other filesystems which don't have exclusive use of a > block device, 'set_anon_super' is used to get a unique s_dev based on a major > number of '0'. Whoops, OK, thanks for the explanation. --b. > So you can *always* use st_dev as an identifier for the filesystem which is > stable and unique as long as you hold an active reference to the filesystem > (open file descriptor, cwd in fs, etc). > > If you poll(2) /proc/mounts to get notifications of changes to the mount > table, then it should be quite easy to cache st-dev -> uuid mappings in a > race-free way. > > There might be value in getting name_to_handle to return the st_dev of the > target file to ensure that you haven't unexepected crossed into a different > filesystem. I would prefer that to returning a uuid: st_dev is guaranteed > to be unique, a uuid is only supposed to be unique (i.e. that is not > enforced). > > NeilBrown -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/