On Thu, 08 Jul 2010 16:10:09 +0530 "Aneesh Kumar K. V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> wrote: > On Thu, 8 Jul 2010 08:21:43 +1000, Neil Brown <neilb@xxxxxxx> 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: > > > > 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 -- 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
