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

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

On Mon, Mar 02, 2015 at 01:11:02PM +0100, Cornelia Huck wrote:
> On Mon, 2 Mar 2015 12:46:57 +0100
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > 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?
> 
> We'd need to make sure that vcdev->config is allocated with GFP_DMA, as
> we need it to be under 2G. And we need to be more careful wrt
> serialization, especially if we want to reuse the ccw structure as
> well, for example. Nothing complicated, I'd just need some free time to
> do it :)
> 
> The more likely reason for get_config to fail is a device hotunplug,
> however. We'll get a seperate notification about that (via machine
> check + channel report), but it would be nice if we could stop poking
> the device immediately, as there's no use trying to do something with
> it anymore.

Normally, hotunplug requires guest cooperation.
IOW unplug request should send guest interrupt,
then block until guest confirms it's not using the
device anymore.
virtio pci already handles that fine, can't ccw
do something similar?

-- 
MST