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

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Date:	Fri, 01 Aug 2014 11:56:42 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Matt Wilson <msw@...ux.com>, David Vrabel <david.vrabel@...rix.com>
CC:	Matt Rushton <mvrushton@...il.com>, konrad.wilk@...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 08/01/2014 11:52 AM, Matt Wilson wrote:
> 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?
>


This appears to be happening on 32-bit dom0.

-boris
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