From mboxrd@z Thu Jan  1 00:00:00 1970
From: Matthew Rushton <mvrushton@gmail.com>
Subject: Re: [RFC PATCH] page_alloc: use first half of higher
 order chunks when halving
Date: Wed, 26 Mar 2014 08:59:04 -0700
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To: Matt Wilson <msw@linux.com>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Keir Fraser <keir@xen.org>, Jan Beulich <jbeulich@suse.com>, Andrew Cooper <andrew.cooper3@citrix.com>, Tim Deegan <tim@xen.org>, Matt Wilson <msw@amazon.com>, xen-devel@lists.xenproject.org
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On 03/26/14 08:15, Matt Wilson wrote:
> On Wed, Mar 26, 2014 at 11:08:01AM -0400, Konrad Rzeszutek Wilk wrote:
>> Could you elaborate a bit more on the use-case please?
>> My understanding is that most drivers use a scatter gather list - in which
>> case it does not matter if the underlaying MFNs in the PFNs spare are
>> not contingous.
>>
>> But I presume the issue you are hitting is with drivers doing dma_map_page
>> and the page is not 4KB but rather large (compound page). Is that the
>> problem you have observed?
> Drivers are using very large size arguments to dma_alloc_coherent()
> for things like RX and TX descriptor rings.
>
> --msw

It's the dma streaming api I've noticed the problem with, so 
dma_map_single(). Applicable swiotlb code would be 
xen_swiotlb_map_page() and range_straddles_page_boundary(). So yes for 
larger buffers it can cause bouncing.