linux-kernel - Re: [PATCH v2 05/10] DMA, CMA: support arbitrary bitmap granularity

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Date:	Thu, 12 Jun 2014 15:43:55 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	kvm@...r.kernel.org, linux-mm@...ck.org,
	Gleb Natapov <gleb@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alexander Graf <agraf@...e.de>, kvm-ppc@...r.kernel.org,
	linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 05/10] DMA, CMA: support arbitrary bitmap granularity

On Thu, Jun 12, 2014 at 03:06:10PM +0900, Minchan Kim wrote:
> On Thu, Jun 12, 2014 at 12:21:42PM +0900, Joonsoo Kim wrote:
> > ppc kvm's cma region management requires arbitrary bitmap granularity,
> > since they want to reserve very large memory and manage this region
> > with bitmap that one bit for several pages to reduce management overheads.
> > So support arbitrary bitmap granularity for following generalization.
> > 
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> > 
> > diff --git a/drivers/base/dma-contiguous.c b/drivers/base/dma-contiguous.c
> > index bc4c171..9bc9340 100644
> > --- a/drivers/base/dma-contiguous.c
> > +++ b/drivers/base/dma-contiguous.c
> > @@ -38,6 +38,7 @@ struct cma {
> >  	unsigned long	base_pfn;
> >  	unsigned long	count;
> >  	unsigned long	*bitmap;
> > +	int order_per_bit; /* Order of pages represented by one bit */
> 
> Hmm, I'm not sure it's good as *general* interface even though it covers
> existing usecases.
> 
> It forces a cma area should be handled by same size unit. Right?
> It's really important point for this patchset's motivation so I will stop
> review and wait other opinions.

If you pass 0 to order_per_bit, you can manage cma area in every
size(page unit) you want. If you pass certain number to order_per_bit,
you can allocate and release cma area in multiple of such page order.

I think that this is more general implementation than previous versions.

Thanks.
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