linux-kernel - Re: [RFC PATCH v1 3/5] KVM: Add paravirt kvm_flush_tlb

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Date:	Tue, 01 May 2012 18:16:46 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>, mingo@...e.hu,
	jeremy@...p.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	x86@...nel.org, vatsa@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, hpa@...or.com,
	paulmck <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH v1 3/5] KVM: Add paravirt kvm_flush_tlb_others

On Tue, 2012-05-01 at 18:36 +0300, Avi Kivity wrote:

> > > What bounds the amount of memory waiting to be freed during an rcu grace
> > > period?
> >
> > Most RCU implementations don't have limits, so that could be quite a
> > lot. I think preemptible RCU has a batch limit at which point it tries
> > rather hard to force a grace period, but I'm not sure if even that
> > provides a hard limit.
> >
> > Practically though, I haven't had reports of PPC/Sparc going funny
> > because of this.
> 
> It could be considered a DoS if a user is able to free page tables
> faster than rcu is able to recycle them, possibly triggering the oom
> killer (should that force a grace period before firing from the hip?)

One would think that would be a good thing, yes. However I cannot seem
to find anything like that in the current OOM killer. David, Paul, I
seem to have vague recollections of a discussion about RCU vs OOM, what
was the resolution (if anything) and would something like the below make
sense?

---
 mm/oom_kill.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 46bf2ed5..244a371 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -607,6 +607,9 @@ int try_set_zonelist_oom(struct zonelist *zonelist, gfp_t gfp_mask)
 	struct zone *zone;
 	int ret = 1;
 
+	synchronize_sched();
+	synchronize_rcu();
+
 	spin_lock(&zone_scan_lock);
 	for_each_zone_zonelist(zone, z, zonelist, gfp_zone(gfp_mask)) {
 		if (zone_is_oom_locked(zone)) {

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