linux-kernel - Re: [PATCH v3 11/11] KVM: MMU: improve write flooding detected

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Message-ID: <4E6F395A.5030609@redhat.com>
Date:	Tue, 13 Sep 2011 14:07:06 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 11/11] KVM: MMU: improve write flooding detected

On 08/30/2011 05:38 AM, Xiao Guangrong wrote:
> Detecting write-flooding does not work well, when we handle page written, if
> the last speculative spte is not accessed, we treat the page is
> write-flooding, however, we can speculative spte on many path, such as pte
> prefetch, page synced, that means the last speculative spte may be not point
> to the written page and the written page can be accessed via other sptes, so
> depends on the Accessed bit of the last speculative spte is not enough
>
> Instead of detected page accessed, we can detect whether the spte is accessed
> after it is written, if the spte is not accessed but it is written frequently,
> we treat is not a page table or it not used for a long time
>
>

The spte may not be accessed, but other sptes in the same page can be 
accessed.  An example is the fixmap area for kmap_atomic(), there will 
be a lot of pte writes but other sptes will be accessed without going 
through soft-mmu at all.

I think you have to read the parent_ptes->spte.accessed bits to be sure.

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

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