On Tue, Aug 21, 2012 at 10:30:31PM +0300, Michael S. Tsirkin wrote:
> On Tue, Aug 21, 2012 at 04:23:58PM -0300, Rafael Aquini wrote:
> > On Tue, Aug 21, 2012 at 10:13:30PM +0300, Michael S. Tsirkin wrote:
> > > >
> > > > I believe rcu_dereference_protected() is what I want/need here, since this code
> > > > is always called for pages which we hold locked (PG_locked bit).
> > >
> > > It would only help if we locked the page while updating the mapping,
> > > as far as I can see we don't.
> > >
> >
> > But we can do it. In fact, by doing it (locking the page) we can easily avoid
> > the nasty race balloon_isolate_page / leak_balloon, in a much simpler way, IMHO.
>
> Absolutely. Further, we should look hard at whether most RCU uses
> in this patchset can be replaced with page lock.
>
Yeah, In fact, by testing/grabbing the page lock at leak_balloon() even the
module unload X migration / putback race seems to fade away, since migration
code holds the page locked all the way.
And that seems a quite easy task to be accomplished:
....
@@ -169,21 +197,61 @@ static void leak_balloon(struct virtio_balloon *vb, size_t
num)
/* We can only do one array worth at a time. */
num = min(num, ARRAY_SIZE(vb->pfns));
+ mutex_lock(&vb->balloon_lock);
for (vb->num_pfns = 0; vb->num_pfns < num;
vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
+ spin_lock(&vb->pages_lock);
+ /*
+ * 'virtballoon_isolatepage()' can drain vb->pages list
+ * making us to stumble across a _temporarily_ empty list.
+ *
+ * Release the spinlock and resume from here in order to
+ * give page migration a shot to refill vb->pages list.
+ */
+ if (unlikely(list_empty(&vb->pages))) {
+ spin_unlock(&vb->pages_lock);
+ break;
+ }
+
page = list_first_entry(&vb->pages, struct page, lru);
+
+ /*
+ * Grab the page lock to avoid racing against threads isolating
+ * pages from vb->pages list (it's done under page lock).
+ *
+ * Failing to grab the page lock here means this page has been
+ * selected for isolation already.
+ */
+ if (!trylock_page(page)) {
+ spin_unlock(&vb->pages_lock);
+ break;
+ }
+
+ clear_balloon_mapping(page);
list_del(&page->lru);
set_page_pfns(vb->pfns + vb->num_pfns, page);
vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
+ unlock_page(page);
+ spin_unlock(&vb->pages_lock);
}
.....
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