From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Dave Young <hidave.darkstar@gmail.com>
Subject: Re: [PATCH 1/2] virtio_balloon: disable oom killer when fill balloon
Cc: kosaki.motohiro@jp.fujitsu.com, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
In-Reply-To: <BANLkTikXR-A-za7dd6FDqc6B0fLhi4LbJg@mail.gmail.com>
References: <20110426154037.F38B.A69D9226@jp.fujitsu.com> <BANLkTikXR-A-za7dd6FDqc6B0fLhi4LbJg@mail.gmail.com>
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Date: Tue, 26 Apr 2011 16:22:12 +0900 (JST)
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> On Tue, Apr 26, 2011 at 2:38 PM, KOSAKI Motohiro
> <kosaki.motohiro@jp.fujitsu.com> wrote:
> >> When memory pressure is high, virtio ballooning will probably cause oom killing.
> >> Even if alloc_page with GFP_NORETRY itself does not directly trigger oom it
> >> will make memory becoming low then memory alloc of other processes will trigger
> >> oom killing. It is not desired behaviour.
> >>
> >> Here disable oom killer in fill_balloon to address this issue.
> >>
> >> Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
> >> ---
> >>  drivers/virtio/virtio_balloon.c |    3 +++
> >>  1 file changed, 3 insertions(+)
> >>
> >> --- linux-2.6.orig/drivers/virtio/virtio_balloon.c    2010-10-13 10:14:38.000000000 +0800
> >> +++ linux-2.6/drivers/virtio/virtio_balloon.c 2011-04-26 11:38:43.979785141 +0800
> >> @@ -25,6 +25,7 @@
> >>  #include <linux/freezer.h>
> >>  #include <linux/delay.h>
> >>  #include <linux/slab.h>
> >> +#include <linux/oom.h>
> >>
> >>  struct virtio_balloon
> >>  {
> >> @@ -102,6 +103,7 @@ static void fill_balloon(struct virtio_b
> >>       /* We can only do one array worth at a time. */
> >>       num = min(num, ARRAY_SIZE(vb->pfns));
> >>
> >> +     oom_killer_disable();
> >
> > I think this patch need proper comment at least. My first impression
> > is, "Hm, __GFP_NORETRY should prevent oom, why is this necessary?".
> > So, this actually prevent _another_ thread call out_of_memory().
> 
> Thanks, will fix.
> 
> > Also, Here doesn't have any exclusion against hibernation (ie another
> > oom_killer_disable() callsite). It should be described why lock is
> > unnecessary.
> 
> Good catch, but lock should better be handled in oom_killer_disable
> function itself,
> What do you think?
> 
> For oom killer multi user there's more problem, if process A disable
> oom killer then Process B enable oom killer, it is not A want to see,
> Any thoughts?

If baloon and hibernation don't have any implicit exclusion, you are
right.

Sorry, I don't virtio internal. please don't ask me.


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