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    SubjectRe: [PATCH 01/34] VFS: Make clone_mnt() and copy_tree() return error codes
    On Fri, Oct 01, 2010 at 11:12:48AM +0200, Szeredi Miklos wrote:
    > On Fri, Oct 1, 2010 at 3:58 AM, Ram Pai <linuxram@us.ibm.com> wrote:
    > > > > > > @@ -1212,11 +1216,12 @@ struct vfsmount *copy_tree(struct vfsmount *mnt, struct dentry *dentry,
    > > > > > >         struct path path;
    > > > > > >
    > > > > > >         if (!(flag & CL_COPY_ALL) && IS_MNT_UNBINDABLE(mnt))
    > > > > > > -               return NULL;
    > > > > > > +               return ERR_PTR(-EINVAL);
    > > > >
    > > > > Ram, do you remember how this worked?
    > > >
    > > > Oops. That should be a OR condition. there is one other check in that
    > > > function that should also be a OR condition.
    > >
    > > I may be wrong here. Can't exactly recollect what CL_COPY_ALL flag means. Al Viro
    > > might remember?  If CL_COPY_ALL means, to clone everything irrespective of any other
    > > flags, then the above code seems right.
    >
    > CL_COPY_ALL means clone the mount despite MNT_UNBINDABLE. It is used
    > for cloning the whole namespace and for collect_mounts(), both of
    > which ignore MNT_UNBINDABLE.

    Ok. That reminds me when the above piece of code in copy_tree() is triggered.
    It triggered when a mount tree with a unbindable mount at its head
    is moved on a shared mount with atleast one peer.

    something like this should trigger the code.

    # create a unbindable mount
    mkdir -p /mnt2/m1
    mount --bind /mnt2/m1 /mnt2/m1
    mount --make-unbindable /mnt2/m1

    #create a shared mount with one peer.
    mkdir -p /mnt2/s1
    mkdir -p /mnt2/s2
    mount --bind /mnt2/s1 /mnt2/s1
    mount --make-shared /mnt2/s1
    mount --bind /mnt2/s1 /mnt2/s2

    #move the unbindable mount to one of the shared peer
    mkdir -p /mnt2/s1/movemount
    mount --move /mnt2/m1 /mnt2/s1/movemount

    the last step will fail and that is because of the above check in copy_tree()

    >
    > Of the two remaining callers of copy_tree() do_loopback already checks
    > MNT_UNBINDABLE on the root of the tree to be copied.
    >
    > So that leaves the one in pnode.c. That one will be called when
    > attaching a new mount or mount tree. If the root of that tree is
    > unbindable then the propagation will fail with -ENOMEM which is wrong,
    > it should simply skip the whole tree and not try to propagate.

    Yes. the propagation_mnt() should fail if it is unable to create clones
    of the source mount due to any reason. However -ENOMEM may not be
    the right return code.


    > Calls
    > which result in propagation are do_loopback, do_move_mount and
    > do_add_mount. Of this do_loopback and do_move_mount already check for
    > MNT_UNBINDABLE, do_add_mount doesn't check, but should probably just
    > mask out MNT_UNBINDABLE.
    >
    > So in the end that check in copy_tree() should never actually trigger
    > and can be turned into a WARN_ON

    You can do that. But then we have to catch for the cases where a unbindable
    mount is moved on a shared mounts. I suppose we can put in a check in do_move_mount().

    >
    > Additionally the propagation code should perhaps be more defensive and
    > skip MNT_UNBINDABLE source mounts.

    No. If we do that, I am afraid, we will end up with inconsistent peer-mount trees
    which will not resemble each other.

    RP

    >
    > Thanks,
    > Miklos
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