linux-kernel - Re: [PATCH v2 1/1] block: blk-merge: don't merge the pages with non-contiguous descriptors

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Message-ID: <1358413880.2394.7.camel@dabdike.int.hansenpartnership.com>
Date:	Thu, 17 Jan 2013 09:11:20 +0000
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Subhash Jadavani <subhashj@...eaurora.org>,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-mmc@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	martin.petersen@...cle.com, asias@...hat.com,
	linux-arm-kernel@...ts.infradead.org,
	Russell King <linux@....linux.org.uk>
Subject: Re: [PATCH v2 1/1] block: blk-merge: don't merge the pages with
 non-contiguous descriptors

On Wed, 2013-01-16 at 15:18 -0800, Tejun Heo wrote:
> On Wed, Jan 16, 2013 at 10:32:35AM +0000, James Bottomley wrote:
> > diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> > index 6b2fb87..ab88c5b 100644
> > --- a/arch/arm/mm/dma-mapping.c
> > +++ b/arch/arm/mm/dma-mapping.c
> > @@ -809,7 +809,7 @@ static void dma_cache_maint_page(struct page *page, unsigned long offset,
> >  			op(vaddr, len, dir);
> >  		}
> >  		offset = 0;
> > -		page++;
> > +		page = pfn_to_page(page_to_pfn(page) + 1);
> 
> Probably page = nth_page(page, 1) is the better form.

It's the same thing.

I'd actually prefer page = pfn_to_page(page_to_pfn(page) + 1); because
it makes the code look like the hack it is.  The preferred form for all
iterators like this should be to iterate over the pfn instead of a
pointer into the page arrays, because that will always work correctly no
matter how many weird and wonderful memory schemes we come up with.

James


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