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    SubjectRe: [PATCH 01/34] VFS: Make clone_mnt() and copy_tree() return error codes
    On Wed, Oct 06, 2010 at 02:24:50PM -0400, Valerie Aurora wrote:
    > On Fri, Oct 01, 2010 at 11:32:43AM -0700, Ram Pai wrote:
    > > 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()
    >
    > Actually, it fails in do_move_mount(), as Miklos theorized. I tested
    > it with the above in an attempt to trigger it in practice in case the
    > code review was wrong, but failed.

    Well, yes there is a check in do_move_mount() for this case.
    I was incorrect.

    >
    > > > 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().

    Since the check is already in there in do_move_mount(), I now agree with Miklos.
    The check in copy_tree() does nothing but chews up a few cycles unnecessarily.
    However just to be safe we can make it a WARN_ON.

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

    the code is already skipping unbindable source mounts in propagate_mnt().
    Miklos: did you have something else in mind here?

    > >
    > > No. If we do that, I am afraid, we will end up with inconsistent peer-mount trees
    > > which will not resemble each other.
    >
    > Any chance you have the time to do a little documentation on where
    > checks should be done and what flags each function expects? Right now
    > the distribution and location of the checks tend to send the reader
    > off on false trails...

    Yes. some additional documentation is needed, given that I myself trailed on wrong
    paths after having not looked at this code for more than 4years.

    RP


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