Re: [PATCH 01/34] VFS: Make clone_mnt() and copy_tree() return error codes

[Ram Pai <linuxram@us.ibm.com>]

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