Re: [PATCH] kvm: add oom notifier for virtio balloon

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* Dave Young <hidave.darkstar@gmail.com> [2010-10-08 21:33:02]:

> On Fri, Oct 8, 2010 at 9:09 PM, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> > * Dave Young <hidave.darkstar@gmail.com> [2010-10-05 20:45:21]:
> >
> >> Balloon could cause guest memory oom killing and panic.
> >>
> >> Add oom notify to leak some memory and retry fill balloon after 5 minutes.
> >>
> >> At the same time add a mutex to protect balloon operations
> >> because we need leak balloon in oom notifier and give back freed value.
> >>
> >> Thanks Anthony Liguori for his sugestion about inflate retrying.
> >> Sometimes it will cause endless inflate/oom/delay loop,
> >> so I think next step is to add an option to do noretry-when-oom balloon.
> >>
> >> Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
> >
> > Won't __GFP_NORETRY prevent OOM? Could you please describe how you
> > tested the patch?
> 
> I have not tried __GFP_NORETRY, it should work, but balloon thread
> will keep wasting cpu resource to allocating.
> 
> 
> To test the patch, just balloon to small than minimal memory.
> 
> I use "balloon 30" in qemu monitor to limit slackware guest memory
> usage. The normal memory used is ~40M.
> 
> Actually we need to differentiate the process which caused oom. If it
> is balloon thread we should just stop ballooning, if it is others we
> can do something like this patch, e.g. retry ballooning after 5
> minutes.

Ideally the balloon thread should never OOM with __GFP_NORETRY (IIRC).
The other situation should be dealt with, we should free up any pages
we have. I wonder if the timeout should be a sysctl tunable.

-- 
	Three Cheers,
	Balbir