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@xxxxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
