Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756033Ab0GHKkT (ORCPT ); Thu, 8 Jul 2010 06:40:19 -0400 Received: from e28smtp02.in.ibm.com ([122.248.162.2]:35881 "EHLO e28smtp02.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755571Ab0GHKkR (ORCPT ); Thu, 8 Jul 2010 06:40:17 -0400 From: "Aneesh Kumar K. V" To: Neil Brown , "J. Bruce Fields" Cc: 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 In-Reply-To: <20100708082143.3701bfc7@notabene.brown> 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> User-Agent: Notmuch/ (http://notmuchmail.org) Emacs/24.0.50.1 (i686-pc-linux-gnu) Date: Thu, 08 Jul 2010 16:10:09 +0530 Message-ID: <87wrt6dzp2.fsf@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4442 Lines: 117 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. diff --git a/fs/namespace.c b/fs/namespace.c index 88058de..498bd9a 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -871,6 +871,9 @@ static int show_mountinfo(struct seq_file *m, void *v) if (IS_MNT_UNBINDABLE(mnt)) seq_puts(m, " unbindable"); + /* print the uuid */ + seq_printf(m, ",uuid:%pU", mnt->mnt_sb->s_uuid); + /* Filesystem specific data */ seq_puts(m, " - "); show_type(m, sb); diff --git a/fs/open.c b/fs/open.c index 23d05d3..13d426e 100644 --- a/fs/open.c +++ b/fs/open.c @@ -1092,6 +1092,8 @@ static long do_sys_name_to_handle(struct path *path, handle_size *= sizeof(u32); handle->handle_type = retval; handle->handle_size = handle_size; + /* copy the mount id */ + handle->mnt_id = path->mnt->mnt_id; if (handle_size > f_handle.handle_size) { /* * set the handle_size to zero so we copy only diff --git a/include/linux/fs.h b/include/linux/fs.h index ffcb9bf..5f43472 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -952,6 +952,7 @@ struct file { }; struct file_handle { + int mnt_id; int handle_size; int handle_type; /* file identifier */ -aneesh -- 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/