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

[Michael S. Tsirkin]

On Wed, Nov 07, 2012 at 04:11:46PM -0800, Andrew Morton wrote:
> 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.

Do you mean this one:
        if (virtqueue_add_buf(vq, &sg, 1, 0, vb, GFP_KERNEL) < 0)
 ?

In practice it never triggers an allocation, we can pass in
GFP_ATOMIC just as well.

-- 
MST
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