lkml.org 
[lkml]   [2012]   [Aug]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to balloon pages
    On Tue, Aug 14, 2012 at 03:44:09PM -0300, Rafael Aquini wrote:
    > On Tue, Aug 14, 2012 at 11:33:20AM +0300, Michael S. Tsirkin wrote:
    > > 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?
    >
    > At the point CPU2 is unloading the module, it will be kept looping at the
    > snippet Rusty pointed out because the isolation / migration steps do not mess
    > with 'vb->num_pages'. The driver will only unload after leaking the total amount
    > of balloon's inflated pages, which means (for this hypothetical case) CPU2 will
    > wait until CPU1 finishes the putaback procedure.
    >

    Yes but only until unlock finishes. The last return from function
    is not guarded and can be overwritten.


    \
     
     \ /
      Last update: 2012-08-14 22:02    [W:0.028 / U:3.856 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site