Re: transparent huge pages breaks KVM on AMD.

["Dr. David Alan Gilbert" <dave@treblig.org>]

* Marc Haber (mh+linux-kernel@zugschlus.de) wrote:
> Hi David,
> 
> On Sat, Apr 23, 2016 at 07:52:46PM +0100, Dr. David Alan Gilbert wrote:
> > Hmm, your problem does sound like bad hardware, but....
> > If you've got a nice reliable crash, can you try turning transparent huge pages
> > off on the host;
> >    echo never > /sys/kernel/mm/transparent_hugepage/enabled
> 
> I must have missed this hint in the middle of the "your hardware is
> bad" avalance that came over me.
> 
> I spent two weeks bisecting "good" kernels since during the repeated
> reconfigurations, transparent huge pages got turned off in kernel
> configuration. After running each kernel for 24 hours, I eventually
> ended up with a working 4.5 kernel. The configuration diff was short,
> showing transparent huge pages, and - finally - upon re-reading the
> thread I found your hint.

OK, good.  When I sent that mail I'd hit a THP bug but in a
corner of migration and at the time we didn't know why and there was
no reason to think it would cause any other symptoms, but since it was
also between 4.4 and 4.5 it did seem worth mentioning as a long shot,
but it was no more than a long shot.

> I have now the result that 4.5, 4.5.1 and 4.5.4 corrupt KVM guest
> memory reliably in the first hour of running under disk load, causing
> the VM to either drop dead in the water, or to read randomness from
> disk. Rebooting fixes the VM. This happens as soon as transparent huge
> pages are turned on in the host.
> 
> Turning off transparent huge pages by echo never >
> /sys/kernel/mm/transparent_hugepage/enabled fixes the issue even
> without rebooting the host. Start up the VM again and it works just
> fine.
> 
> Is this an issue in (a) transparent huge pages, (b) KVM or (c) qemu?
> Where should this issue be forwarded? Or do we just accept it and turn
> transparent huge pages off?

Try Andrea's fix for (a).

Dave

> 
> Greetings
> Marc
> 
> -- 
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