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Message-ID: <20120814195139.GA28870@redhat.com>
Date: Tue, 14 Aug 2012 22:51:39 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Rafael Aquini <aquini@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
virtualization@...ts.linux-foundation.org,
Rusty Russell <rusty@...tcorp.com.au>,
Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
Andi Kleen <andi@...stfloor.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to
balloon pages
On Tue, Aug 14, 2012 at 03:22:45PM -0300, Rafael Aquini wrote:
> 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.
>
What I think you should do is use rcu for access.
And here sync rcu before freeing.
Maybe an overkill but at least a documented synchronization
primitive, and it is very light weight.
> > > 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.
>
So add a comment explaining why it is safe pls.
> > > +/*
> > > + * '*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?
>
Second.
It is easy to create several balloons they are just
pci devices.
> > > + /* 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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