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

[James Bottomley <James.Bottomley@HansenPartnership.com>]

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