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 19:27:01 +0000
From:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Mel Gorman <mel@...net.ie>, 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, 2007-04-20 at 12:10 -0700, Christoph Lameter wrote:
> On Fri, 20 Apr 2007, Dave Kleikamp wrote:
> > Yeah.  I'm working on patches for storing file tails in buffers
> > allocated from the slab cache, and the tail will be represented by a
> > fake struct page.  (This is primarily for kernels with a larger page
> > size).  So my version of page_cache_size(page) may return a different
> > size for different pages belonging to the same mapping.  I'm in the
> > midst of cleaning up the patches and plan to post them to linux-mm by
> > Monday.
> 
> I am not sure what the point of that patchset would be in this context 
> given that this is about support for arbitrary page sizes. If the 
> filesystem wants it then it can reduce the page size for small files.
> 
> Different page sizes for one mapping may introduce high complexity into a 
> filesystem.

I'm trying to keep it from getting too complicated.

> And we can already represent different page size. We have compound pages 
> support in the kernel.

There are advantages to having a larger base page size, such as
increased TLB reach.  I'm specifically targetting kernels built with
CONFIG_PPC_64K_PAGES, but other architectures could benefit.

> page_cache_size(page) in terms of current code in mm is
> 
> PAGE_SIZE << compound_order(page)

I don't see page_cache_size() in linux-2.6.21-rc6-mm1.  Is there
something newer?

Anyway, my patches are at a proof-of-concept stage.  I look forward to
more discussion when I post the patch set.

Thanks,
Shaggy
-- 
David Kleikamp
IBM Linux Technology Center

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