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

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Message-ID: <53DC3660.6050104@gmail.com>
Date:	Fri, 01 Aug 2014 17:52:48 -0700
From:	Matthew Rushton <mvrushton@...il.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Matt Wilson <msw@...ux.com>,
	David Vrabel <david.vrabel@...rix.com>
CC:	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 08/01/14 08:56, Boris Ostrovsky wrote:
> 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

It's unclear to me why the m2p update would be failing in this case (on 
first attempt) which causes the subsequent errors. I'll try to repro on 
a 32-bit dom0.

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