Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to balloon pages

[Rafael Aquini <aquini@redhat.com>]

On Mon, Aug 13, 2012 at 11:41:23AM +0300, Michael S. Tsirkin wrote:
> > @@ -141,7 +151,10 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
> >  		set_page_pfns(vb->pfns + vb->num_pfns, page);
> >  		vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
> >  		totalram_pages--;
> > +		spin_lock(&pages_lock);
> >  		list_add(&page->lru, &vb->pages);
> 
> If list_add above is reordered with mapping assignment below,
> then nothing bad happens because balloon_mapping takes
> pages_lock.
> 
> > +		page->mapping = balloon_mapping;
> > +		spin_unlock(&pages_lock);
> >  	}
> >  
> >  	/* Didn't get any?  Oh well. */
> > @@ -149,6 +162,7 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
> >  		return;
> >  
> >  	tell_host(vb, vb->inflate_vq);
> > +	mutex_unlock(&balloon_lock);
> >  }
> >  
> >  static void release_pages_by_pfn(const u32 pfns[], unsigned int num)
> > @@ -169,10 +183,22 @@ 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(&balloon_lock);
> >  	for (vb->num_pfns = 0; vb->num_pfns < num;
> >  	     vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
> > +		/*
> > +		 * We can race against virtballoon_isolatepage() and end up
> > +		 * stumbling across a _temporarily_ empty 'pages' list.
> > +		 */
> > +		spin_lock(&pages_lock);
> > +		if (unlikely(list_empty(&vb->pages))) {
> > +			spin_unlock(&pages_lock);
> > +			break;
> > +		}
> >  		page = list_first_entry(&vb->pages, struct page, lru);
> > +		page->mapping = NULL;
> 
> Unlike the case above, here
> if = NULL write above is reordered with list_del below,
> then isolate_page can run on a page that is not
> on lru.
> 
> So I think this needs a wmb().
> And maybe a comment above explaining why it is safe?

Good point. Presumably, this nit has potential to explain your guessing on the
read barrier stuff at movable_balloon_page() on the other patch.

 
> >  		list_del(&page->lru);
> 
> I wonder why changing page->lru here is safe against
> races with unmap_and_move in the previous patch.

leak_balloon() doesn't race against unmap_and_move() because the later works on
an isolated page set. So, theoretically, pages being dequeued from balloon page
list here are either migrated (already) or they were not isolated yet.


> > +/*
> > + * '*vb_ptr' allows virtballoon_migratepage() & virtballoon_putbackpage() to
> > + * access pertinent elements from struct virtio_balloon
> > + */
> 
> What if there is more than one balloon device?

Is it possible to load this driver twice, or are you foreseeing a future case
where this driver will be able to manage several distinct memory balloons for
the same guest?


> > +	/* Init the ballooned page->mapping special balloon_mapping */
> > +	balloon_mapping = kmalloc(sizeof(*balloon_mapping), GFP_KERNEL);
> > +	if (!balloon_mapping) {
> > +		err = -ENOMEM;
> > +		goto out_free_vb;
> > +	}
> 
> Can balloon_mapping be dereferenced at this point?
> Then what happens?

Since there's no balloon page enqueued for this balloon driver yet, there's no
chance of balloon_mapping being dereferenced at this point.

 
> > +
> > +	INIT_RADIX_TREE(&balloon_mapping->page_tree, GFP_ATOMIC | __GFP_NOWARN);
> > +	INIT_LIST_HEAD(&balloon_mapping->i_mmap_nonlinear);
> > +	spin_lock_init(&balloon_mapping->tree_lock);
> > +	balloon_mapping->a_ops = &virtio_balloon_aops;
> >  
> >  	err = init_vqs(vb);
> >  	if (err)
> > @@ -373,6 +493,7 @@ out_del_vqs:
> >  	vdev->config->del_vqs(vdev);
> >  out_free_vb:
> >  	kfree(vb);
> > +	kfree(balloon_mapping);
> 
> No need to set it to NULL? It seems if someone else allocates a mapping
> and gets this chunk of memory by chance, the logic in mm will get
> confused.

Good point. It surely doesn't hurt be asured of this sort of safety.

> >  out:
> >  	return err;
> >  }
> > @@ -397,6 +518,7 @@ static void __devexit virtballoon_remove(struct virtio_device *vdev)
> >  	kthread_stop(vb->thread);
> >  	remove_common(vb);
> >  	kfree(vb);
> > +	kfree(balloon_mapping);
> 
> Neither here?

ditto.

 
> >  }
> >  
> >  #ifdef CONFIG_PM
> > diff --git a/include/linux/virtio_balloon.h b/include/linux/virtio_balloon.h
> > index 652dc8b..930f1b7 100644
> > --- a/include/linux/virtio_balloon.h
> > +++ b/include/linux/virtio_balloon.h
> > @@ -56,4 +56,8 @@ struct virtio_balloon_stat {
> >  	u64 val;
> >  } __attribute__((packed));
> >  
> > +#if !defined(CONFIG_COMPACTION)
> > +struct address_space *balloon_mapping;
> > +#endif
> > +
> 
> Anyone including this header will get a different copy of
> balloon_mapping. Besides, need to be ifdef KERNEL.

Good point. I'll move this hunk to the balloon driver itself.


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