linux-kernel - Re: [PATCH v2 2/2] xen/setup: Remap Xen Identity Mapped RAM

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Message-ID: <20140801155206.GA13865@u109add4315675089e695.ant.amazon.com>
Date:	Fri, 1 Aug 2014 08:52:08 -0700
From:	Matt Wilson <msw@...ux.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	Matt Rushton <mvrushton@...il.com>, konrad.wilk@...cle.com,
	boris.ostrovsky@...cle.com, msw@...zon.com,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	Matt Rushton <mrushton@...zon.com>
Subject: Re: [PATCH v2 2/2] xen/setup: Remap Xen Identity Mapped RAM

On Fri, Aug 01, 2014 at 03:52:28PM +0100, David Vrabel wrote:
> On 31/07/14 18:43, David Vrabel wrote:
> > On 20/07/14 01:01, Matt Rushton wrote:
> >> Instead of ballooning up and down dom0 memory this remaps the existing mfns
> >> that were replaced by the identity map. The reason for this is that the
> >> existing implementation ballooned memory up and and down which caused dom0
> >> to have discontiguous pages. In some cases this resulted in the use of bounce
> >> buffers which reduced network I/O performance significantly. This change will
> >> honor the existing order of the pages with the exception of some boundary
> >> conditions.
> >>
> >> To do this we need to update both the Linux p2m table and the Xen m2p table.
> >> Particular care must be taken when updating the p2m table since it's important
> >> to limit table memory consumption and reuse the existing leaf pages which get
> >> freed when an entire leaf page is set to the identity map. To implement this,
> >> mapping updates are grouped into blocks with table entries getting cached
> >> temporarily and then released.
> >>
> >> On my test system before:
> >> Total pages: 2105014
> >> Total contiguous: 1640635
> >>
> >> After:
> >> Total pages: 2105014
> >> Total contiguous: 2098904
> > 
> > Applied to devel/for-linus-3.17
> 
> Unfortunately, this produces too many WARNINGs on some boxes or
> with certain configurations.

Hi David,

Do you have more information about the systems or configurations that
showed a problem?

--msw
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