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    SubjectRe: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to balloon pages
    On Tue, Aug 14, 2012 at 09:29:49AM +0930, Rusty Russell wrote:
    > On Mon, 13 Aug 2012 11:41:23 +0300, "Michael S. Tsirkin" <mst@redhat.com> wrote:
    > > On Fri, Aug 10, 2012 at 02:55:15PM -0300, Rafael Aquini wrote:
    > > > +/*
    > > > + * Populate balloon_mapping->a_ops->freepage method to help compaction on
    > > > + * re-inserting an isolated page into the balloon page list.
    > > > + */
    > > > +void virtballoon_putbackpage(struct page *page)
    > > > +{
    > > > + spin_lock(&pages_lock);
    > > > + list_add(&page->lru, &vb_ptr->pages);
    > > > + spin_unlock(&pages_lock);
    > >
    > > Could the following race trigger:
    > > migration happens while module unloading is in progress,
    > > module goes away between here and when the function
    > > returns, then code for this function gets overwritten?
    > > If yes we need locking external to module to prevent this.
    > > Maybe add a spinlock to struct address_space?
    >
    > The balloon module cannot be unloaded until it has leaked all its pages,
    > so I think this is safe:
    >
    > static void remove_common(struct virtio_balloon *vb)
    > {
    > /* There might be pages left in the balloon: free them. */
    > while (vb->num_pages)
    > leak_balloon(vb, vb->num_pages);
    >
    > Cheers,
    > Rusty.

    I know I meant something else.
    Let me lay this out:

    CPU1 executes:
    void virtballoon_putbackpage(struct page *page)
    {
    spin_lock(&pages_lock);
    list_add(&page->lru, &vb_ptr->pages);
    spin_unlock(&pages_lock);


    at this point CPU2 unloads module:
    leak_balloon
    ......

    next CPU2 loads another module so code memory gets overwritten

    now CPU1 executes the next instruction:

    }

    which would normally return to function's caller,
    but it has been overwritten by CPU2 so we get corruption.

    No?

    --
    MST


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