Linux Virtualization - Mar 2015 - virtio balloon: do not call blocking ops when !TASK_RUNNING

On Thu, 26 Feb 2015 11:50:42 +1030
Rusty Russell <rusty at rustcorp.com.au> wrote:
> Thomas Huth <thuth at 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.

 Thomas

Thomas Huth <thuth at linux.vnet.ibm.com> writes:
> On Thu, 26 Feb 2015 11:50:42 +1030
> Rusty Russell <rusty at rustcorp.com.au> wrote:
>
>> Thomas Huth <thuth at 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.

Cheers,
Rusty.

On Mon, Mar 02, 2015 at 10:37:26AM +1030, Rusty Russell
wrote:
> Thomas Huth <thuth at linux.vnet.ibm.com> writes:
> > On Thu, 26 Feb 2015 11:50:42 +1030
> > Rusty Russell <rusty at rustcorp.com.au> wrote:
> >
> >> Thomas Huth <thuth at 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.
> 
> 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?