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

[Rafael Aquini <aquini@redhat.com>]

On Thu, Aug 23, 2012 at 09:38:48PM -0300, Rafael Aquini wrote:
> On Thu, Aug 23, 2012 at 09:33:53PM -0300, Rafael Aquini wrote:
> > On Fri, Aug 24, 2012 at 02:36:16AM +0300, Michael S. Tsirkin wrote:
> > > I would wake it each time after adding a page, then it
> > > can stop waiting when it leaks enough.
> > > But again, it's cleaner to just keep tracking all
> > > pages, let mm hang on to them by keeping a reference.
> > > 
> 
> Btw, it's also late here, and there still some work to be done around those
> bits, but I guess that has potential to get this issue nailed.
> 
> > Here is a rough idea on how it's getting:
> > 
> > Basically, I'm have introducing an atomic counter to track isolated pages, I
> > also have changed vb->num_pages into an atomic conter. All inc/dec operations
> > take place under pages_lock spinlock, and we only perform work under page lock.
> > 
> > It's still missing the wait-part (I'll write it during the weekend) and your
> > concerns (and mine) will be addressed, IMHO.
> > 
> > ---8<---
> > +/*
> > + *
> > + */
> > +static inline void __wait_on_isolated_pages(struct virtio_balloon *vb,
> > +                                           size_t num)
> > +{
> > +       /* There are no isolated pages for this balloon device */
> > +       if (!atomic_read(&vb->num_isolated_pages))
> > +               return;
> > +
> > +       /* the leak target is smaller than # of pages on vb->pages list */
> > +       if (num < (atomic_read(&vb->num_pages) -
> > +           atomic_read(&vb->num_isolated_pages)))
> > +               return;
> > +       else {
> > +               spin_unlock(&vb->pages_lock);
> > +               /* wait stuff goes here */
> > +               spin_lock(&vb->pages_lock);
> > +       }
> > +}
> > +
> >  static void leak_balloon(struct virtio_balloon *vb, size_t num)
> >  {
> > -       struct page *page;
> > +       /* The array of pfns we tell the Host about. */
> > +       unsigned int num_pfns;
> > +       u32 pfns[VIRTIO_BALLOON_ARRAY_PFNS_MAX];
> > 
> >         /* We can only do one array worth at a time. */
> > -       num = min(num, ARRAY_SIZE(vb->pfns));
> > +       num = min(num, ARRAY_SIZE(pfns));
> > 
> > -       for (vb->num_pfns = 0; vb->num_pfns < num;
> > -            vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
> > -               page = list_first_entry(&vb->pages, struct page, lru);
> > -               list_del(&page->lru);
> > -               set_page_pfns(vb->pfns + vb->num_pfns, page);
> > -               vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
> > +       for (num_pfns = 0; num_pfns < num;
> > +            num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {

Doh, here I need a while(num_pfns < num;)


> > +               struct page *page = NULL;
> > +               spin_lock(&vb->pages_lock);
> > +               __wait_on_isolated_pages(vb, num);
> > +
> > +               if (!list_empty(&vb->pages))
> > +                       page = list_first_entry(&vb->pages, struct page, lru);
> > +               /*
> > +                * Grab the page lock to avoid racing against threads isolating
> > +                * pages from, or migrating pages back to vb->pages list.
> > +                * (both tasks are done under page lock protection)
> > +                *
> > +                * Failing to grab the page lock here means this page is being
> > +                * isolated already, or its migration has not finished yet.
> > +                */
> > +               if (page && trylock_page(page)) {
> > +                       clear_balloon_mapping(page);
> > +                       list_del(&page->lru);
> > +                       set_page_pfns(pfns + num_pfns, page);
> > +                       atomic_sub(VIRTIO_BALLOON_PAGES_PER_PAGE,
> > +                                  &vb->num_pages);

and here: num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE;		

> > +                       unlock_page(page);
> > +               }
> > +               spin_unlock(&vb->pages_lock);
> >         }
> > 
> >         /*
> > @@ -182,8 +251,10 @@ static void leak_balloon(struct virtio_balloon *vb, size_t
> > num)
> >          * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
> >          * is true, we *have* to do it in this order
> >          */
> > +       mutex_lock(&vb->balloon_lock);
> >         tell_host(vb, vb->deflate_vq);
> > -       release_pages_by_pfn(vb->pfns, vb->num_pfns);
> > +       mutex_unlock(&vb->balloon_lock);
> > +       release_pages_by_pfn(pfns, num_pfns);
> >  }
> > ---8<---

Well, I'll assume that's because I'm working for 14h already and running out of
caffeine.


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