linux-kernel - Re: [PATCH -V14 0/11] Generic name to handle and open by handle syscalls

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Date:	Wed, 7 Jul 2010 09:17:21 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	neilb@...e.de, david@...morbit.com,
	aneesh.kumar@...ux.vnet.ibm.com, hch@...radead.org,
	viro@...iv.linux.org.uk, adilger@....com, corbet@....net,
	serue@...ibm.com, hooanon05@...oo.co.jp,
	linux-fsdevel@...r.kernel.org, sfrench@...ibm.com,
	philippe.deniel@....FR, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -V14 0/11] Generic name to handle and open by handle
 syscalls

On Wed, Jul 07, 2010 at 03:10:21PM +0200, Miklos Szeredi wrote:
> On Wed, 7 Jul 2010, J. Bruce Fields wrote:
> > On Wed, Jul 07, 2010 at 12:57:26PM +1000, Neil Brown wrote:
> > > The trouble with /proc/mounts is that it is somewhat clumsy to parse
> > > (remember to handle \0ctal escapes) and doesn't include major/minor number
> > > which is the primary key for identifying filesystems in Linux
> > > (see /sys/class/bdi/MAJOR:MINOR which is e.g. the best place to configure
> > > read-ahead for a filesystem).
> > > 
> > > So /proc/mounts could work (and would probably be better than a new syscall)
> > > but I would really rather see something sane in /sys for
> > > inspecting/configuring filesystems (rather than each filesystem doing their
> > > own independent thing in /sys/fs).
> > 
> > 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?)  And it must also have the same
problems as libblkid (e.g. btrfs subvolumes share the same st_dev).

--b.

> then look for the major:minor calculated from
> st_dev in /proc/self/mountinfo.
> 
> Thanks,
> Miklos
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