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


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