linux-kernel - Re: [PATCH v2 12/15] KVM: MMU: allow locklessly access shadow page table out of vcpu thread

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Message-ID: <20131012055356.GC14789@redhat.com>
Date:	Sat, 12 Oct 2013 08:53:56 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Xiao Guangrong <xiaoguangrong.eric@...il.com>,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	avi.kivity@...il.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 12/15] KVM: MMU: allow locklessly access shadow page
 table out of vcpu thread

On Fri, Oct 11, 2013 at 05:30:17PM -0300, Marcelo Tosatti wrote:
> On Fri, Oct 11, 2013 at 08:38:31AM +0300, Gleb Natapov wrote:
> > > n_max_mmu_pages is not a suitable limit to throttle freeing of pages via
> > > RCU (its too large). If the free memory watermarks are smaller than 
> > > n_max_mmu_pages for all guests, OOM is possible.
> > > 
> > Ah, yes. I am not saying n_max_mmu_pages will throttle RCU, just saying
> > that slab size will be bound, so hopefully shrinker will touch it
> > rarely.
> > 
> > > > > > and, in addition, page released to slab is immediately
> > > > > > available for allocation, no need to wait for grace period. 
> > > > > 
> > > > > See SLAB_DESTROY_BY_RCU comment at include/linux/slab.h.
> > > > > 
> > > > This comment is exactly what I was referring to in the code you quoted. Do
> > > > you see anything problematic in what comment describes?
> > > 
> > > "This delays freeing the SLAB page by a grace period, it does _NOT_
> > > delay object freeing." The page is not available for allocation.
> > By "page" I mean "spt page" which is a slab object. So "spt page"
> > AKA slab object will be available fo allocation immediately.
> 
> The object is reusable within that SLAB cache only, not the 
> entire system (therefore it does not prevent OOM condition).
> 
Since object is allocatable immediately by shadow paging code the number
of SLAB objects is bound by n_max_mmu_pages. If there is no enough
memory for n_max_mmu_pages OOM condition can happen anyway since shadow
paging code will usually have exactly n_max_mmu_pages allocated.

> OK, perhaps it is useful to use SLAB_DESTROY_BY_RCU, but throttling 
> is still necessary, as described in the RCU documentation.
> 
I do not see what should be throttled if we use SLAB_DESTROY_BY_RCU. RCU
comes into play only when SLAB cache is shrunk and it happens far from
kvm code.

--
			Gleb.
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