linux-kernel - Re: [RFC 4/8] Enhance fallback functions in libs to support higher order pages

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Date:	Fri, 20 Apr 2007 09:01:04 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Jens Axboe <jens.axboe@...cle.com>
cc:	Adam Litke <aglitke@...il.com>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <nickpiggin@...oo.com.au>, Andi Kleen <ak@...e.de>,
	Paul Jackson <pj@....com>, Dave Chinner <dgc@....com>
Subject: Re: [RFC 4/8] Enhance fallback functions in libs to support higher
  order pages

On Fri, 20 Apr 2007, Jens Axboe wrote:

> This works fine as long as you are in the submitter context, but once
> you pass the into the block layer, we don't have any way to find the
> address space (at least we don't want to). Would something like this be
> workable, name withstanding:
> 
> static unsigned long page_size(struct page *page)
> {
>         struct address_space *mapping;
>         int order = 0;
> 
>         mapping = page_mapping(page);
>         if (mapping)
>                 order = mapping->order;
> 
>         return PAGE_CACHE_SIZE << order;
> }

There is much simpler solution (possible with mm)

PAGE_SIZE << compound_order(page)

compound_order will return 0 for a non compound page.

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