Re: [Xen-devel] [RFC PATCH] page_alloc: use first half of higher order chunks when halving

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Mar 26, 2014 at 05:15:08PM +0200, 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.

OK, but that call ends up using chunks from the SWIOTLB buffer which is
contingously allocated. That shouldn't be a problem?

> 
> --msw

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