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

["J. Bruce Fields" <bfields@fieldses.org>]

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" <bfields@fieldses.org> 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