linux-kernel - Re: [RFC 0/8] Variable Order Page Cache

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Message-ID: <20070420084206.GO32602149@melbourne.sgi.com>
Date:	Fri, 20 Apr 2007 18:42:06 +1000
From:	David Chinner <dgc@....com>
To:	William Lee Irwin III <wli@...omorphy.com>
Cc:	Christoph Lameter <clameter@....com>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Paul Jackson <pj@....com>, Dave Chinner <dgc@....com>,
	Andi Kleen <ak@...e.de>
Subject: Re: [RFC 0/8] Variable Order Page Cache

On Thu, Apr 19, 2007 at 09:47:18PM -0700, William Lee Irwin III wrote:
> On Thu, Apr 19, 2007 at 09:35:04AM -0700, Christoph Lameter wrote:
> > This patchset modifies the core VM so that higher order page cache pages
> > become possible. The higher order page cache pages are compound pages
> > and can be handled in the same way as regular pages.
> > The order of the pages is determined by the order set up in the mapping
> > (struct address_space). By default the order is set to zero.
> > This means that higher order pages are optional. There is no attempt here
> > to generally change the page order of the page cache. 4K pages are effective
> > for small files.
> 
> Oh dear. Per-file pagesizes are foul. Better to fix up the pagecache's
> radix tree than to restrict it like this.

How is this restrictive? This opens up much goodness to filesystems.
I *want* to be able to use different radix tree index orders for
different inodes in a single filesystem. Not just for data, either;
XFS has metadata constructs larger than a page that mean we have to
carry our own buffer cache around.  Being able to use the page cache
directly for these metadata constructs would enable us to remove a
good chunk of code from XFS....

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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