Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756618Ab0GHMVm (ORCPT ); Thu, 8 Jul 2010 08:21:42 -0400 Received: from cantor.suse.de ([195.135.220.2]:46464 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755277Ab0GHMVl (ORCPT ); Thu, 8 Jul 2010 08:21:41 -0400 Date: Thu, 8 Jul 2010 22:21:21 +1000 From: Neil Brown To: "Aneesh Kumar K. V" Cc: "J. Bruce Fields" , Miklos Szeredi , david@fromorbit.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: <20100708222121.5c0612ef@notabene.brown> In-Reply-To: <87wrt6dzp2.fsf@linux.vnet.ibm.com> References: <20100706161002.GD7387@fieldses.org> <87eifgfsez.fsf@linux.vnet.ibm.com> <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> <87wrt6dzp2.fsf@linux.vnet.ibm.com> X-Mailer: Claws Mail 3.7.6 (GTK+ 2.20.1; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3739 Lines: 85 On Thu, 08 Jul 2010 16:10:09 +0530 "Aneesh Kumar K. V" wrote: > On Thu, 8 Jul 2010 08:21:43 +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: > > > > On Wed, 7 Jul 2010, J. Bruce Fields wrote: > > > > > > > If you use sys or proc, is it possible to get the uuid from a file > > > > > > > descriptor or pathname without races? > > > > > > > > > > > > You can do stat/fstat to find out the device number (which is unique, > > > > > > but not persistent) > > > > > > > > > > Is it really unique over time? (Can't a given st_dev value map to one > > > > > filesystem now, and another later?) > > > > > > > > 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'. > > > > 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). > > How about adding mnt_id to the handle ? Documentation file says it is > unique > > (1) mount ID: unique identifier of the mount (may be reused after umount) > > I also updated (/proc/self/mountinfo) to carry the optional uuid field > With the below patch i get in /proc/self/mountinfo > > 13 1 253:0 / / rw,relatime,uuid:9b5af62a-a34a-43f6-a5bb-1cc22d97e862 - ext3 /dev/root rw,errors=continue,barrier=0,data=writeback > > And the handle returns the value 13 in mnt_id field. We should able to > lookup mountinfo with mnt_id and find the corresponding uuid. > That sounds good. mnt_id will even let you know if you have crossed a --bind mount, which st_dev wouldn't. That may not always be useful, but it is good to have it. 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/