linux-kernel - Re: [RFC/query] kvm async_pf anon pined pages migration

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Message-ID: <20131008073914.GA3574@redhat.com>
Date:	Tue, 8 Oct 2013 10:39:14 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	chai wen <chaiw.fnst@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, pbonzini@...hat.com,
	tangchen@...fujitsu.com, Gu Zheng <guz.fnst@...fujitsu.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>
Subject: Re: [RFC/query] kvm async_pf anon pined pages migration

On Tue, Oct 08, 2013 at 02:58:22PM +0800, chai wen wrote:
> On 10/02/2013 12:04 AM, chaiwen wrote:
> >On 09/30/2013 08:51 PM, Gleb Natapov wrote:
> >>On Mon, Sep 30, 2013 at 06:03:07PM +0800, chai wen wrote:
> >>>Hi all
> >>>
> >>>Async page fault in kvm currently pin user pages via get_user_pages.
> >>>when doing page migration,the method can be found via
> >>>page->mmapping->a_ops->migratepage to offline old pages and migrate to
> >>>new pages. As to anonymous page there is no file mapping but a anon_vma.So
> >>>the migration will fall back to some *default* migration method.Anon pages
> >>>that have been pined in memory by some reasons could be failed in the migration
> >>>processing because of some reasons like ref-count checking.
> >>>(or I misunderstand some thing?)
> >>>
> >>>Now we want to make these anon pages in async_pf can be migrated, I try some
> >>>ways.But there are still many problems. The following is one that replaceing
> >>>the mapping of anon page arbitrarily and doing some thing based on it.
> >>>Kvm-based virtual machine can works on this patch,but have no experience of
> >>>offline pages because of the limitaion of resouces.I'll check it later.
> >>>
> >>>I don't know weather it is a right direction of this issue.
> >>>All comments/criticize are welcomed.
> >>The pinning is not mandatory and can (and probably should) be dropped, but
> >>pinning that is done by async page faults is short lived. What problems
> >>are you seeing that warrant the complexity of handling their migration?
> Hi Gleb
> 
> As to this issue, I still have some thing not very clear.
> If pages pinning is successfully holding (although not mandatory) by
> async page fault.
> And at the same time page migration happens because of memory
> hot-remove action.
> It has 120*hz timeout setting in common page offline processing,
> could it fail with
> these async_pf pined pages migration ?
> What's your opinion about this ?   If it may fail under this
> circumstance, should we do
> some thing on it ?
> 
120 seconds is more than enough time for pinning to go away, but as I
said the pinning is not even necessary. Patch to remove it is welcomed.

--
			Gleb.
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