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

[Neil Brown <neilb@suse.de>]

On Thu, 08 Jul 2010 16:10:09 +0530
"Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com> wrote:

> On Thu, 8 Jul 2010 08:21:43 +1000, Neil Brown <neilb@suse.de> 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:
> > > > 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