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

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Date:	Tue, 01 May 2012 18:36:20 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
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
Subject: Re: [RFC PATCH v1 3/5] KVM: Add paravirt kvm_flush_tlb_others

On 05/01/2012 06:14 PM, Peter Zijlstra wrote:
> On Tue, 2012-05-01 at 15:12 +0300, Avi Kivity wrote:
> > 
> > What's changed is not gup_fast() but the performance of munmap(),
> > exit(), and exec(), no?
>
> If it is indeed cache related like you suggested earlier, it would be
> the allocation side of things, like fork()/mmap() that suffer since
> there's fewer hot pages about, but yes, anything creating/destroying
> page-tables.

Right.

>
> > 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?)

-- 
error compiling committee.c: too many arguments to function

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