linux-kernel - Re: [PATCH][RF C/T/D] Unmapped page cache control

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Message-ID: <20100315104555.GD18054@balbir.in.ibm.com>
Date:	Mon, 15 Mar 2010 16:15:55 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	KVM development list <kvm@...r.kernel.org>,
	Rik van Riel <riel@...riel.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][RF C/T/D] Unmapped page cache control - via boot
 parameter

* Avi Kivity <avi@...hat.com> [2010-03-15 11:27:56]:

> >>>The knobs are for
> >>>
> >>>1. Selective enablement
> >>>2. Selective control of the % of unmapped pages
> >>An alternative path is to enable KSM for page cache.  Then we have
> >>direct read-only guest access to host page cache, without any guest
> >>modifications required.  That will be pretty difficult to achieve
> >>though - will need a readonly bit in the page cache radix tree, and
> >>teach all paths to honour it.
> >>
> >Yes, it is, I've taken a quick look. I am not sure if de-duplication
> >would be the best approach, may be dropping the page in the page cache
> >might be a good first step. Data consistency would be much easier to
> >maintain that way, as long as the guest is not writing frequently to
> >that page, we don't need the page cache in the host.
> 
> Trimming the host page cache should happen automatically under
> pressure.  Since the page is cached by the guest, it won't be
> re-read, so the host page is not frequently used and then dropped.
>

Yes, agreed, but dropping is easier than tagging cache as read-only
and getting everybody to understand read-only cached pages. 

-- 
	Three Cheers,
	Balbir
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