linux-kernel - Re: [patch 10/21] buffer heads: Support slab defrag

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Date:	Tue, 20 May 2008 10:25:03 +1000
From:	David Chinner <dgc@....com>
To:	Christoph Lameter <clameter@....com>
Cc:	David Chinner <dgc@....com>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Mel Gorman <mel@...net.ie>, andi@...stfloor.org,
	Rik van Riel <riel@...hat.com>,
	Pekka Enberg <penberg@...helsinki.fi>, mpm@...enic.com
Subject: Re: [patch 10/21] buffer heads: Support slab defrag

On Mon, May 19, 2008 at 09:44:11AM -0700, Christoph Lameter wrote:
> On Mon, 19 May 2008, David Chinner wrote:
> 
> > Defragmentation is triggered as part of the usual memory reclaim
> > process. Which implies we've run out of free memory, correct?
> 
> Yes but we have already reclaimed some memory.
> 
> > > How do you trigger such a special writeout?
> > 
> > filemap_fdatawrite_range() perhaps?
> 
> Could you provide me such a patch? I would not know how much to writeout. 
> If we had such a method then we could also use that for the swap case 
> where we also write out single pages?

How hard is it? I don't have time right now to do this, but it's essentially:

	mapping = page->mapping;
	......
-	mapping->aops->writepage();
+	filemap_fdatawrite_range(mapping, start, end);

Where [start,end] span page->index and are is large enough
to get a substantial sized I/O to disk (say at least SWAP_CLUSTER_MAX
pages, preferrably larger for 4k page size machines).

Cheers,

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