Re: virtio balloon: do not call blocking ops when !TASK_RUNNING

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Mar 02, 2015 at 12:31:06PM +0100, Cornelia Huck wrote:
> On Mon, 2 Mar 2015 12:13:58 +0100
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Mon, Mar 02, 2015 at 10:37:26AM +1030, Rusty Russell wrote:
> > > Thomas Huth <thuth@linux.vnet.ibm.com> writes:
> > > > On Thu, 26 Feb 2015 11:50:42 +1030
> > > > Rusty Russell <rusty@rustcorp.com.au> wrote:
> > > >
> > > >> Thomas Huth <thuth@linux.vnet.ibm.com> writes:
> > > >> >  Hi all,
> > > >> >
> > > >> > with the recent kernel 3.19, I get a kernel warning when I start my
> > > >> > KVM guest on s390 with virtio balloon enabled:
> > > >> 
> > > >> The deeper problem is that virtio_ccw_get_config just silently fails on
> > > >> OOM.
> > > >> 
> > > >> Neither get_config nor set_config are expected to fail.
> > > >
> > > > AFAIK this is currently not a problem. According to
> > > > http://lwn.net/Articles/627419/ these kmalloc calls never
> > > > fail because they allocate less than a page.
> > > 
> > > I strongly suggest you unlearn that fact.
> > > The fix for this is in two parts:
> > > 
> > > 1) Annotate using sched_annotate_sleep() and add a comment: we may spin
> > >    a few times in low memory situations, but this isn't a high
> > >    performance path.
> > > 
> > > 2) Handle get_config (and other) failure in some more elegant way.
> 
> Do you mean we need to enable the caller to deal with get_config
> failures (and the transport to relay those failures)? I agree with that.

We can certainly tweak code to bypass need to kmalloc
on get_config.

Why is it doing these allocs? What's wrong with using
vcdev->config directly?


> > > 
> > > Cheers,
> > > Rusty.
> > 
> > I agree, but I'd like to point out that even without kmalloc,
> > on s390 get_config is blocking - it's waiting
> > for a hardware interrupt.
> > 
> > And it makes sense: config is not data path, I don't think
> > we should spin there.
> > 
> > So I think besides these two parts, we still need my two patches:
> >     virtio-balloon: do not call blocking ops when !TASK_RUNNING
> >     virtio_console: avoid config access from irq
> > in 4.0.
> > 
> > agree?
> 
> I agree that we need those fixes as well.