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

["Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com>]

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.

diff --git a/fs/namespace.c b/fs/namespace.c
index 88058de..498bd9a 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -871,6 +871,9 @@ static int show_mountinfo(struct seq_file *m, void *v)
 	if (IS_MNT_UNBINDABLE(mnt))
 		seq_puts(m, " unbindable");
 
+	/* print the uuid */
+	seq_printf(m, ",uuid:%pU", mnt->mnt_sb->s_uuid);
+
 	/* Filesystem specific data */
 	seq_puts(m, " - ");
 	show_type(m, sb);
diff --git a/fs/open.c b/fs/open.c
index 23d05d3..13d426e 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1092,6 +1092,8 @@ static long do_sys_name_to_handle(struct path *path,
 	handle_size *= sizeof(u32);
 	handle->handle_type = retval;
 	handle->handle_size = handle_size;
+	/* copy the mount id */
+	handle->mnt_id = path->mnt->mnt_id;
 	if (handle_size > f_handle.handle_size) {
 		/*
 		 * set the handle_size to zero so we copy only
diff --git a/include/linux/fs.h b/include/linux/fs.h
index ffcb9bf..5f43472 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -952,6 +952,7 @@ struct file {
 };
 
 struct file_handle {
+	int mnt_id;
 	int handle_size;
 	int handle_type;
 	/* file identifier */

-aneesh