Re: [PATCH v8 1/5] mm: introduce a common interface for balloon pages mobility

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Aug 21, 2012 at 05:45:56PM -0300, Rafael Aquini wrote:
> 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.

This still worries me. If this happens we do not
lock the page so module can go away?
if not need to document why.

> +                *
> +                * 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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