Re: [PATCH v11 5/7] virtio_balloon: introduce migration primitivesto balloon pages

[Andrew Morton]

On Thu, 08 Nov 2012 09:32:18 +1030
Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote:

> Rafael Aquini <aquini@xxxxxxxxxx> writes:
> > + * virtballoon_migratepage - perform the balloon page migration on behalf of
> > + *			     a compation thread.     (called under page lock)
> 
> > +	if (!mutex_trylock(&vb->balloon_lock))
> > +		return -EAGAIN;
> 
> Erk, OK...

Not really.  As is almost always the case with a trylock, it needs a
comment explaining why we couldn't use the far superior mutex_lock(). 
Data: this reader doesn't know!

> > +	/* balloon's page migration 1st step  -- inflate "newpage" */
> > +	spin_lock_irqsave(&vb_dev_info->pages_lock, flags);
> > +	balloon_page_insert(newpage, mapping, &vb_dev_info->pages);
> > +	vb_dev_info->isolated_pages--;
> > +	spin_unlock_irqrestore(&vb_dev_info->pages_lock, flags);
> > +	vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> > +	set_page_pfns(vb->pfns, newpage);
> > +	tell_host(vb, vb->inflate_vq);
> 
> tell_host does wait_event(), so you can't call it under the page_lock.
> Right?

Sleeping inside lock_page() is OK.  More problematic is that GFP_KERNEL
allocation.  iirc it _should_ be OK.  Core VM uses trylock_page() and
the filesystems shouldn't be doing a synchronous lock_page() in the
pageout path.  But I suspect it isn't a well-tested area.

> You probably get away with it because current qemu will service you
> immediately.  You could spin here in this case for the moment.
> 
> There's a second call to tell_host():
> 
> > +	/*
> > +	 * balloon's page migration 2nd step -- deflate "page"
> > +	 *
> > +	 * It's safe to delete page->lru here because this page is at
> > +	 * an isolated migration list, and this step is expected to happen here
> > +	 */
> > +	balloon_page_delete(page);
> > +	vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> > +	set_page_pfns(vb->pfns, page);
> > +	tell_host(vb, vb->deflate_vq);
> 
> The first one can be delayed, the second one can be delayed if the host
> didn't ask for VIRTIO_BALLOON_F_MUST_TELL_HOST (qemu doesn't).
> 
> We could implement a proper request queue for these, and return -EAGAIN
> if the queue fills.  Though in practice, it's not important (it might
> help performance).

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